- Email: [email protected]
Pigment distribution in the Caribbean sea: Observations from space
Prog. Oceanog. Vol. 23, pp. 23 - 64, 1989.
0079 - 6611/89 $0.00 + .50 © 1990 Pergamon Press pie,
Printed in Great Britain. All rights reserved.
Pigment distribution in the Caribbean Sea: Observations from space F. E. MOLL':~-KARoER", C. R. M c C t . A I N ' " , T. R. FISrIE~°'" , W . E. ESA.tAS'" a n d R. VARELA ....
"Horn Point Environmental Laboratories, University of Maryland, (Current Address and Affiliation: Department of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, EL 33701, U. S. A.) "NASA, Code 671, Goddard Space Flight Center, Greenbelt, MD 20771, U. S. A. ""Horn Point Environmental Laboratories, University of Maryland, Box 775 Cambridge, MD 21613, U. S. A_ .... Fundacion La Salle de Ciencias Naturales, Estacion de lnvestigaciones Marinas de Margarita, Apartado 144, Porlamar, Estado Nueva Esparta, Venezuela
Abstract - - The Caribbean is a semi-enclosed tropical sea which is generally considered oligotrophic, but that is influenced by nearly 20% of the annual discharge of the world's rivers (Amazon and Orinoco Rivers) and by seasonal upwelling along the southern margin. To investigate the role of these nutrient sources on the productivity of the region, we mapped the distribution of pigments in the eastern Caribbean (east of 80°W) using a series of Coastal Zone Color Scanner (C'ZCS) images collected between November 1978 and December 1982. Five additional images were examined for the period 1983-1986. The images revealed a seasonal cycle in the spatial structure of neax-surface pigment. During January-May, there were high pigment concentrations (> 0.5 mg m ~) along the continental margin (south of 14°N), where upwelling occurred. Very little pigment (< 0.2 mg m a) was found in the northern half of the Caribbean at this time. The fxequency of upwelling-related blooms decreased after July, but the seasonally-expanding plume of the Orinoco River dispersed pigment over a large area of the Caribbean (> 3x 10s kin2). This plume reached Puerto Rico around September-October and drifted westward, slowly losing its color signature. We estimate that the discharge of the Orinoco contributes 2-12% of the daily nitrogen requirements of the phytoplankton growing in the river plume, and leads to the fixation of 7-29x105 tons of carbon per year. The rest of the nitrogen demand appears to be met by nitrogen cycling. The large-scale (> 100 km) pigment distribution patterns in the Caribbean Sea seem to be conlxoUed by wind stress, flux of water through the basin, and river discharge. Westward advection of Atlantic water probably dominates the flow during the first half of the year, restricting the dispersal of blooms to the southern half of the Caribbean while flushing the central and northern portions. As the influx of Atlantic water decreases in the second half of the year, local Elonan transport driven by the trade winds b e c o m e s dominant and surface waters drift northwestward throughout the basin. The seasonal sequence of changes in pigment disu'ibution patterns was consistent from year to year except in 1980 and 1984, when wind conditions in the Caribbean were anomalous. Close scrutiny of the 4 years of CZCS images did not reveal any evidence of large-scale (> 300 kin) eastward-flowing currents in the cena'al Caribbean. This supports the view that previous observations of countercurrents were based on partial sampling of eddies of 100-250 km diameter.
23
24
MOLLr~R-KARGERet al.
CONTENTS I. 2.
3.
4.
5.
6. 7.
Introduction Site Description 2.1. Physical setting 2.2. Distribution of particles and phytoplanton 2.3. Where does the Amazon water go ? Methods 3.1. CZCS imagery 3.2. Regional studies 3.3. Possible sources of error 3.3.1. Calibration 3.3.2. Atmospheric effects 3.3.3. Sunglint 3.3.4. Turbidity 3.3.5. Floating blue-greens 3.3.6. Coccofithophores 3.3.7. Subsurface pigment maxima Results 4. I. Comparison with ship data 4.2. Spatial arrangement of pigments 4.3. Nutrient dynamics in the Orinoco River plume Discussion 5.1. Spatial structure of pigments and environmental forcing 5.2. Role of the Orinoco as a nutrient source Acknowledgements References
24 25 25 28 30 30 30 32 32 32 39 39 39 39 39 39 40 40 43 48 54 54 56 59 59
I. INTRODUCTION "Great indications are these that this is Paradise, because the place conforms to the opinion of the holy and sane theologians, and even the signs conform [sic], that I never read nor heard of so much fresh water entering inw the neighboring saline like this.... And if it did not originate in Paradise, this seems an even greater marvel, because I don' t believe that there is knowledge in the world of a river so large and so deep." - Christopher Columbus (From a letter sent to the King and Queen o f Spain on 18 October 1498from Hispaniola. Here Columbus described the Gulf of Paria, located between Trinidad and the mainland, and the effect of the discharge of the Orinoco River on this region)
The enormous amount of fresh water discharged into the ocean by the tropical rivers of South America helped Columbus determine that he had reached a new continent. But even though the combined discharge of the Amazon and Orinoco Rivers represents about 20% of the annual global river discharge, very little is known about the fate of this water and of particles associated with it (Moor~E, Snmva~yro and KEY, 1986; M0ULER-K~G~, McCt.A~ and RICHARDSON, 1988). Similarly, even though upwelling has been reported off northeast South America (RYrx-mR, Mm',rzEL and CORWIN, 1967; Mnz~r~, 1973; GraBs, 1980) and in the southern Caribbean (REDvn~.,.n, 1955; B~t~Sa~R and MAaGn.t~F, 1965; Ricanl~s, 1960; WOST, 1964; GORDON,1967; CO~a~DOR, 1976, 1979), it has not been possible to determine the magnitude or variability of the upwelling phenomenon. Land runoff and coastal upwelling supply abundant "new" nutrients (see DUGDALEand GOERInG, 1967) to surface waters near continents. Thus, continental margins around the globe support high concentrations of phytoplankton and productivity (about 8.4x109 tons C y'~ or 30% of the annual global marine primary production; W~..SH, 1988, 1977; KOnLENZ-MIsHI, m,
Pigmentdistributionin the CaribbeanSea
25
VOLKOVINSKYand K.nUANOVA, 1970; R~n'rmR, 1969). This is the base for harvestable fish production and also represents a possible sink for excess anthropogenic carbon in the atmosphere. But due to the wide range in scales at which processes operate in the ocean (STEm.E, 1978; ES~aAS, 1980), it has been difficult to obtain reliable estimates of the variability of phytoplankton near continental margins even after intensive sampling by ship (cf. WALSH,D,k-ruaLE and ESAJ.~S, 1987). These problems may be solved by complementing ship data with remote measurements of ocean color such as those obtained with the Coastal Zone Color Scanner (CZCS) (e.g. see YODFJ~, MCCLAIN,BLANTONand OEY, 1987; WALSH, DtE'rERLEand ESAIAS, 1987; BANSEand McCLAtN, 1986; ~ Z and McGowAN, 1986; Bgown, EVANS,BROWn,GORDON,S)~nTHand BAKER, 1985; MCCLAnq,PIETRAFESAand YODER, 1984). The CZCS allowed synoptic mapping of the concentration of pigments near the surface and provided an unprecedented capability to exanaine the surface circulation of the ocean. Even though the relationship between variance spectra of physical parameters and phytoplankton biomass is not constant (CAMPBELLand ESAIAS,1985; WILSON,OKLraoand ESAJ.AS1979; STEELEand HENDERSON, 1979; FASHAM,1978), blooms larger than approximately 2 km in diameter tend to maintain their coherence when phytoplankton growth exceeds losses due to turbulence and advective exchange (Die, tANand AaBoyr, 1988; MACKAS,D ~ and ABBOTt, 1985; CAMPBELL and ESAL~S, 1985; OKUBO, 1980; WROBLEWSraand O'BRIEN, 1976; KmRSTEADand SLOBODKIN, 1953). Thus, phytoplankton and their pigments can serve as visual tracers of the circulation (SvEJKOVSKV, 1988) in spite of their non-conservative behavior. Here we describe the distribution of pigments in surface waters of the eastern Caribbean Sea (Figs. l a and l b). Our goal was to develop a basic conceptual model of the distribution of phytoplankton biomass. In formulating a study plan, we identified key questions about the oceanography of the Caribbean. These questions are: (1) What is the impact of the Orinoco River on the Caribbean Sea? (2) Are there basin-scale countercurrents in the central Caribbean? (3) What is the variability in the distribution of pigments in the region? Below we provide a brief background description of the circulation and potential sources of particulates to the Caribbean Sea. Then we describe the methods used to process the satellite data. Finally, we discuss the distribution of pigments and evaluate the impact of the Amazon and Orinoco Rivers on the region. Of particular interest was the biological impact of the Orinoco on the Caribbean basin, which we evaluated using a budget for nitrogen in the river plume based on CZCS data. 2. SITEDESCRIFFION
2.1. Physical setting The meridional migration of the Intertropical Convergence Zone (ITCZ; HOP.EL,I'(OUSKYand KAGANO,1986) leads to seasonal variability in the circulation of tropical oceans thiough changes in the curl of the wind stress (KATz, 1987; BUSAI.ACCHIand PlcAtrr, 1983). Between January and April the ITCZ is at its southernmost position (0 ° to 50 S), and wind stress in the Caribbean is strong (ca. 0. I N m "2southwestward; ISEMERand HASSE, 1985). During this period, waters in the northern tropical Atlantic tend to move toward the west (R1cnAaDSONand McKr~, 1984) and flow through the Lesser Antilles into the Caribbean. Large flows may be expected in the basin at this time (MoRPaSONand NOWLIN,1982; I-IEBtrgNand RHODES,1987). In May, the ITCZ abruptly begins a northward migration ( P ~ E R and PACANOwsra, 1986), and it reaches its northernmost
26
MOLtJ~-KARGI~ et al.
(a)
-78
-74 I
22
-72 I
-70 I
-68 I
-66 I
-64I
-62 I
-60 I
-58 22
-
-
20
4~ 18
~ •
16
Caribbean
14-
-
G,<;,,
Guo
D<
~. /
a
(Po~to p~.d~,)
Orchilo Island
i
16
c~..... o %J~{-/////~ 7,'":":" Ilia riinlqu o ~~ ~_ U3 F/////2•
Marltarito Illond
I P*~*~'
1
Penlnllukl
18
Sea
Paroguario Peninsula Curocoo Gulf of Ven+zuelo I
-
-
".
/
,
--14-
~l't~doB
*
.
Gnlnelda
>.- .......
--12 --10
8
--
Colombio 6
i
I
-78
(b)
-80
I
-74
I
-78
I
-70
72
I
22
8
Yenezuelo I
-68
I
-66
I
-64
-74
-72
-70
-68
-66
-64
I
I
I
I
I
I
6
I
-62
-60
-6;
-58
-60
-58
I
•
22
20-PuerloRico
18-! ":
,,/,~,~~
1618
16-14 12--
12 r
10-8
-C i u d o d flollvor
80
(c)
I
I
I
i
I
78
-74
-72
-70
-68
12 65 6,4 V ~~ . . ~ ~ ~ I
/"
\
ir T
6,3
I
I
66
646,2 V
I
62
I
60
6
-58
61
----~-~.
10 FIG. la, b,c. The Caribbean Sea and study subregions. (a) Names of locatiorts mentioned in the text. (b) Crosshatched regions used to calculate time series of pigment means from individual CZCS scenes. Indicated are: wind stress stations (filled circles ) and sea level stations (filled triangles ); 1° x l°squares were used for estimates of sea-surface temperature (SST). Broken line shows approximate position of the shelf break. (c) Detail of the southeastern Caribbean showing 5 productivity regimes outlined by M ~ O A L ~ (1965). We estimated productivity in regions II-V using CZCS-derived chlorophyll concentrations in order to compare with the historical observations of MxRo.u.l~ (1965), Gii,~s (1972), and Mmo (unpublished) (see Table 2).
Pigmentdistributionin the CaribbeanSea
27
position at the southern edge of the Caribbean (6 ° to 10° N) in August. In October, the ITCZ starts its return to the southerly position. The presence of the ITCZ in the northern tropical Atlantic induces the lormation of the eastward-flowing North Equatorial Counter Current (NECC; BUSALACCHIand PICAUT, 1983; RICHARDSONand McKEE, 1984). Concurrently, water from the North Brazil Current and water discharged by the Amazon River are shunted east into the NECC (MOLLER-KARGER,McCLA1Nand RICHARDSON, 1988). Understanding this circulation pattern was necessary to address our question on the relative impact of the Amazon and the Orinoco on the Caribbean. While currents in the Caribbean south of 14°N are nearly zonal and swift (0.21-0.64 m s t westward), net water motion north of 14°N and near the Antilles is more sluggish (residual flow is 0.12-0.19 m s -I to the not-thwest; WILLIAMS,1986; KINDER,HEBURNand GREEN, 1985; KINDER, 1983; MOLINARI,SPtLLA2qE,BROOKS,ArWOODand DUCKETr, 1981). The average flow through the Caribbean is estimated as 20 to 30 Sv (Sv = Sverdrup = l06 m 3 s-l; KINDER, 1983; STALCUPand METCALF, 1972; GORDON, 1967; W0ST, 1964). The bulk of this flow enters through the three southernmost passes of the Lesser Antilles, but up to 12 Sv may enter into the northwestern Caribbean through Windward Passage (WORtHInGTON. 1976; ROEMMICH, 1981 ; NOF and OLSON, 1983). Unlortunately, there are not enough observations to quantify the annual cycle of the circulation. Our best estimates of seasonal variation are based on non-synoptic hydrographic sections (MoP,glSON and NOWLIN, 1982), current meter records (MAzEIKA, KINDER and BURNS, 1983), and numerical simulations (HEBURNand RHODES, 1987). Several investigators have inferred that countercurrents are part of the mean circulation of the Caribbean (flow > 0.4 m s q to the east near 140-15° N: FUKUOKA,1965; ATWOOD,1976; FEBRESORTEGAand HERRERA,1976; A~DRES, FEm~S and HEmO,ERa, 1979; MomodSONand NOWL~, 1982; DUNCAN, SCHI..ADOWand WILLIAMS,1982). However, the evidence for eastward currents may have been based on the partial sampling of eddies (KINDER,HEBUaNand GREEN, 1985). In an effort to shed some light on this question, we examined the CZCS satellite images for surface evidence of countercurrents. The migration of the ITCZ leads to a dry (Februmy-May) and a wet season (August-October) north of about 5°N (ETrva, LAMB and PoR'rls, 1987). These seasons are roughly in phase with changes observed in the surface salinity o fthe Caribbean. Salinity itmxima occur near Puerto Rico in April-May (about 36.5 psu; psu = practical salinity units). Minima occur in October-November (33.4 to 34.5 psu; FROELICn,ArWOOD and GmSE, 1978). But, despite the coherence in phase, rainfall has only a small direct effect on the surface salinity. Its effect is more indirect. FROELtCH, ATWOODand GIESE (1978) and MOm~,IsoNand NOWLIN(1982) measured high concentrations of silicate (3-51x M SiO4-Si) in the low-salinity water, which identified it as riverine in origin. The seasonal variation in rainfall strongly influences the discharge of tropical rivers. The three largest rivers affecting the Caribbean Sea are the Amazon, the Orinoco, and the Magdalena. The average discharge of the Amazon is 17.3x104 m 3 s"1 (Fig 2; range: from about 10x 104 m3s~ in November to about 23x 104 m 3s t in June; WUST, 1964; OLTMAN,1967; Stota, 1975; UNESCO, 1979). This is 4 to 5 times greater than the mean discharge of the Orinoco (Fig. 2; mean = 3.9x104 m 3 sl; range = lxl04 m 3 s-~ in March to about 7x104 m 3 s: in August; see also MEADE, NOPa~, PEREZ H~'~m'~eZ, MeJtA and PE~Z GODO¥, 1983). The Magdalena is the smallest of these three rivers (mean discharge of about 0.8 x 104 m 3sl; MIIaXMANand MZADE, 1983). Together these rivers account for about 20% of the total discharge of fresh water into the world's oceans. An additional, unknown amount of fresh water is supplied to the ocean by numerous small rivers draining northeastern South America. However, based on the large volume discharged by the Amazon and conceptual models of the circulation in the western tropical Atlantic developed prior to the
MOLLER-KARGERet al.
28
1980's, the primary source of the fresh water leading to the seasonal freshening of the Caribbean had been assumed to be the Amazon river. 2.2. Distribution o f particles and phytoplankton Particulates found in the Caribbean Sea are primarily derived from rivers or phytoplankton growing in the ocean. The Amazon and Orinoco Rivers contribute a large amount of particulates (Mo_LtMA~ and MEADE, 1983). Sediments from the Amazon are cast along the entire coast of northeastern South America and eventually reach the Caribbean (MEAoE, NORDIn, PEP,EZ HERNANDEZ, MF...qAand PEREZ GODOY, 1983; MILLIMAN,BtrrENKO, BARBOTand HEDBERa, 1982; EMMA, VAN Dim GAAST, MARTIN and TrloMAs, 1978; B~z~R, EOGIMA~, CARDER, K~TER and B~:rz~R, 1977; VAN ANDELand POSTMA, 1954). Most of the sediment presently discharged by the Amazon appears 1o move northwestward along the Guyanas, inshore of the 10 m isobath (cf. NtrrROUER, CtmT~ and DEMASTER, 1986; RINE and G~SBURG, 1985; WELLS and COLEMAN, 1981; DELFT HYDRAULJCS LABORATORY,1962). It is possible that little of the Amazon's sediment is carried offshore into the deep Atlantic, as opposed to a large fraction of Amazon water (MOLLERKAROEI~, McCLAIN and RlCaARDSON, 1988; see also BORSTAD, 1982a). The second important source of particulates is plankton. Previous to the CZCS era, there had been little information available with which to reconstruct the synoptic distribution ofphytoplankton in the Caribbean Sea. We knew that the eastern Caribbean (Fig.l) was more "turbid" and productive than other tropical marine ecosystems (cf. WrLKIZqSON, 1987), but the processes leading to this productivity remained largely undefined. The few measurements of nutrients available for the Caribbean Sea show that there is very little nitrate in the upper 50 m (<< 0.2 JJt,M NO3-N; ALBERTS, 1973; SELLm~R, 1981; OHER-OTEC STUDY, 1980; MOP,mSON and NOWL~, 240 220200-
Amazon 1 B(3160-
140~
120-
P
100-
i5
8O-
...........'
60-
....... ... ......... .....,.,............,...
4O2O0 Jon
I
I
I
Feb
Mar
Apr
Moy
Jun
dul
i
i
i
i
Aug
Sep
Oct
Nov
Dec
FIG.2. Mean + a standard deviation of the discharge of the Amazon (period 1969-1978; UNESCO, 1979) and OrinocoRivers (period 1979-1983; data courtesyofW. LBWlS;see also MEADEetal., 1983).
Pigment distribution in the Caribbean Sea
29
1982). Below about 140 m, nitrate concentrations are 2.5-10.0 ~M, and higher below 400 m (>251XM). Phosphate concentration is variable near the surface (0.0-0.06 p,MPO4-P), and increases with depth (0.5-1 ~M at 250 m). There does not appear to be an increase in nutrient concentration downstream of the Antilles, and the small increase in chlorophyll found in the wake of these islands appears to be caused mainly by export of cells rather than growth in situ (tthRORAVES, BRODYand BURm~OLD~, 1970). Appreciable island effects (cf. DOT¢and Ootnu, 1956; F~LDMAS, CLARKand HALPm~, 1984) could be observed in the CZCS data only in the southern Caribbean off Tobago and Margarita, in part because the nutricline is closer to the surface near the continental margin. Surface nutrients are more abundant near land masses. For example, nitrate-N concentrations > 2 IxMwere measured during 28-29 May 1975 off the Goajira Peninsula (Comua~R, 1976, 1979). Sea surface temperatures in this nutrient-rich plume were up to 2°C lower than waters 190 km further offshore (CoRREDOR,1976). Since these coastal waters are closer to the equator, the colder temperatures suggested the occurrence of upwelling. Upward warping of the pycno- and nutricline is readily visible in meridional transects of the Caribbean (see FROELICH, ATWOODand GIF_ZE, 1978; MORRISONand NOWL~, 1982). Similar to the distribution of nutrients, the lowest standing crops of phytoplankton have been observed in the western tropical Atlantic and in the western Caribbean (Table 1). In these T.~I..E I. Range of observed surface chlorophyll a concentration (Chla), primary productivity (P.P.), and Secchi depth in the Caribbean Sea. (NA = no estimate available.) (Secchi depths in parentheses courtesy of M. LL~ctsand N. Kupaso, Dalhousie University, Halifax, Nova Scotia (data fi-om NODC, US Department of Commerce)) Location Source (1)
(2)
(3)
(4)
Chla [rag m -3]
P.P. [mg C m ~ d "l]
Secchi [m]
Nortbem Atlantic E of Anti.lies (13-14°N) Barbados
0.05-0.35 0.05-0.27
50-150 139-380
(30~14) NA
Northern Caribbean W of Antilles, > 14°N
0.05-0.98
50-300
(25-30)
0.107-0.644
204-1060
(11-25)
0.28-2.4 0.5 -3
1200-2300 600-2500
4-26
0.04-0.4
150-250
(10-15)
Southern Caribbean Margarita Island Margarita to mainland, Gulf of Cariaco NW Colombia Western Caribbean
Source:
(1)
BOR~TAD, 1982b; KInD and S~,rD~, 1979; ~OP, AVES, BRODYand BU~mOLDm, 1970; BEERS, STEVEN and LEWIS, 1968; SAUO and NAKAMOTO,1973; BERMUDA BIOLOGICAL STATIOn, 1965.
(2)
HARORAV~,BRODYand Bugr.~ou)m, 1970; MoRms, SMrm and GLove, 1981 ; BALLmTm M~o~a.~, 1965; Bui~r~aou)m and ALMODOWa%1967; YOSHIOKA,OWENand I~sAm'a, 1985.
(3)
MAac/a.~, 1965; M_xaG/o_~, CmVmON and Ym~z, 1960; BALt.ES~:a and M.~GAL~g, 1965; MA,Zp~aO'm, 1967, 1970; MoRRis, Sm'rn and G t ~ v ~ , 1981; Cure., 1960; Rtc~u~.~s, 1960; Gn,~.s, 1972; Coaa.~oR, 1979.
(4)
S~,
1981; M.At.O~, 1971, 1973; BAIRD, 1973.
30
MOLLER-KAROERet al.
oligotrophic regions, nearly 90% of the primary productivity is due to nanoplankton, and chlorophyll a is only 20-50% of total pigment (MALONE, 1971; OHER-OTEC STUDY, 1980). The highest rates of photosynthesis have been measured near the continental margin (Tables 1 and 2), where abundant nutrients cause diatom- dominated blooms (between 200 and 400 g C m2 y~ near Margarita Island). The high productivity is reflected in the large fishing industry of these regions (G~ES, 1972, 1982; FAO, 1986). An important factor controlling phytoplankton abundances in the Caribbean is zooplankton. However, most zooplankton studies have been limited to coastal waters, where concentrations are over an order of magnitude larger than offshore (C~vicoN and MARCANO,1965; Gn,ms, 1982; YOSmOKA, OWENand PESArrrE, 1985). Similar to the distribution of phytoplankton, the largest numbers of zooplankton occur in the southern Caribbean (Table 3). The seasonal cycle in the abundance of zooplankton and phytoplankton in the northern Caribbean Sea is in phase with local changes in surface salinity (YosI-HOKA,OWENand PEsmcrE, 1985). High concentrations of both types of plankton occu~ between July and November, along with low surface salinities. We have known for some time that plankton concentrations in Amazon water drifting off South America are higher than those in typical western tropical Atlantic water (R'rrnER, Mm,rZEL and CoRwn~, 1967; CALm:and GRACE, 1967; HULBtmTand CORW~, 1969; KXDDand SANDER,1979; BORSTAD,1982b; DEUSER,MOLL~_x-KARGERand HmVa.EBEN, 1988). Accordingly, the seasonal increase of plankton in the northern Caribbean has been attributed to the dispersal of the Amazon's plume.
2.3. Where does the Amazon water go ?
Despite suggestions that the Amazon is responsible for the changes in salinity and plankton concentration in the Caribbean Sea, there are some pieces of conflicting information. The brackish water seen offshore in the Caribbean seems to have higher concentrations of silica than the low-salinity water drifting east of the Antilles (MooRE, SARMmm'Oand KEY, 1986). Also, it appears that the maxima in plankton biomass in the northern Caribbean (ca. 0.15-0.5 mg chlorophyll m-3; 0.15-0.28 ml zooplankton m-3; YOSrUOKA,OWZNand PESArCrE, 1985; OHEROTEC STUDy, 1980) are comparable to the highest concentrations observed near Barbados (ca. 0.15-0.2 mg chlorophyll m-t; 0.12 ml zooplankton m-3; CAL~ and GRACE,1967; KIDDand SArmER, 1979; BORSTAD,1982b). Since grazing, sinking, and mixing tend to reduce surface concentrations of nutrients and plankton in the absence of nutrient sources, it is hard to envision how these properties are conserved during the 880-krn transit between Barbados and Puerto Rico in periods > 1-3 months. Below we present conclusive evidence that the low salinities observed seasonally in the northern Caribbean are caused by the dispersal of Orinoco river water.
3. METHODS 3.1. CZCS imagery
The distribution of near-surface pigments in the Caribbean Sea was mapped using 159 CZCS images obtained between November 1978 and December 1982 (MCrLLER-KARGER,1988). Five additional images were examined for the period 1983-1986. Images were subsampled to 1/16th of their original resolution before processing. Pigment concentrations were derived from ratios
Pigment distribution in the Caribbean Sea
31
TABLE2. Summary of in situ primary productivity observations and primary productivity estimates based on CZCS data off Venezuela. Observed values were adapted from G~Es (1972), from data compiled by Mmo (unpublished). Data were grouped geographically according to MAROALBF(1965; Fig. lc). Estimated values ofprimaryproductivity werebased ontherelationshipproposed by EPPLEY, Sa'~WAR'r, ABBOT and HnVMAN (1985) : Primary Productivity [rag m "2 d'Z] = 1000 * SQRT (Chlorophyll), where we used Chlorophyll [mg m'3l = 0.5 * C7_,CS Pigments [rag m'3]. Grand mean values were obtained by weighing the mean productivity of a region obtained per CZCS scene by the number of observations. Regions were defined as in either MAROALEF(1965) or GINES(1972) (Fig. 1c). CZCS estimates from the Gulf of Venezuela were based only on the eastern half of the Gulf to avoid contamination by bottom reflection or resuspension, and do not include Lake MaracaibÜ. This area extended to 13°N to sample the entire shelf. (NA = not available)
Region Observations
Primary Productivity [rag C m 2 d"] CZCS estimates
Mean
Range
N
Mean
Range
N
I. G. Paria and Orinoco delta
341
30-750
14
NA
NA
NA
II. NE and E Margarita
357
100-750
24
746
199-1184
1549315
III. S of Margarita
505
100-1500
68
794
171-1223
286885
IV. W of Margarita
611
300-1100
18
555
256-1091
814378
V. N of shelf break
283
80-700
55
515
201-1021
4749130
Gulf of Venezuela and Lake Maracaibo
328
96-650
21
609
287-1146
1367974
TABLE3. Approximate range of zooplankton concentrations in the Caribbean Sea. Data from GL~ES (1982), CERVICONand MARCAr~O(1965), and YOSHIOKA,OWEN and I~SANTn (1985). (NA = not available) Region
Biomass [mg wet weight m -3]
Numbers [organisms m "3]
Northern Caribbean (January - May) (July - November)
(NA) (NA)
250 4OO
Near Margarita
(NA)
200-3000
Central Venezuela
30-300
100-2000
Gulf of Venezuela
30-400
90-2000
o f t h e blue (443 n m ) o r b l u e - g r e e n (520 n m ) w a t e r - l e a v i n g r a d i a n c e s to the g r e e n r a d i a n c e (550 n m ) , a c c o r d i n g to GORDON, CLAret, BROW'N, BROWN, EVANS a n d BRo~.Nrow, (1983a; s e e also GORDON, BROWN, BROVCN,EVAr~S a n d CLARK, 1983b; BAgAL~, McCLAZ~ a n d ~ O T r E - R a z z o t a , 1986). T h i s is a n e s t i m a t e o f the a v e r a g e p i g m e n t c o n c e n t r a t i o n in the first o p t i c a l d e p t h . P r e s u m a b l y , c h a n g e s in the d i s t r i b u t i o n p a t t e r n s r e f l e c t e d c o n d i t i o n s o v e r the E k r n a n d e p t h o f frictional r e s i s t a n c e . A t l o w c o n c e n t r a t i o n s (0.04 - 0.5 m g p i g m e n t m'3), the C Z C S s e n s e d p i g m e n t to d e p t h s o f a p p r o x i m a t e l y 1-10 m , a n d thus this a s s u m p t i o n m a y n o t b e u n r e a s o n a b l e . W e a l s o
32
MOLLER-KAK(3ERet al.
assumed that the surface distribution of pigrnents in river plumes reflected motion over the depth of the plume. We color-coded concentrations [mg m "3] in the images (cf. Plate 1). Blue represents low pigment concentrations and yellow and red indicate higher concentrations. Land was masked black and clouds and missing data, grey. Pigment scenes were remapped to a Universal Transverse Mercator (UTM) projection (bounds = 8°N, 21°N, 58°W, 78°W) and manually navigated to match a standard coastline. 3.2. Regional studies We wanted to examine differences between large-scale portions of the Caribbean Sea. For this purpose, up to three regional pigment means were obtained from each CZCS scene. The regions were def'med as follows (see Fig. lb): (a) Southern Caribbean [59x104 kmZ]: waters south of 14°N, from 78°W to the Lesser Antilles. The southern half of the Gulf of Venezuela was excluded to avoid bottom reflection and resuspended sediment. (b) Northem Caribbean [89x104 km2]: waters north of 14°N, from 78°W to the Antilles. (c) Southern Sargasso [50x104 km2]: waters from 14.4°N to21.4°N, 68°W to 57.8°W, east and northeast of the Antilles. Only samples larger than 1000 pixels (> 17x103 km 2) were accepted to minimize spatial aliasing. Standard deviations (sd) were computed but are not presented here because they are not appropriate for statistical comparisons due to the skewed and frequently bimodal distribution of pigments, and due to variation in sample size. 3.3. Possible sources of error
Previous results suggest that in low-pigment waters (0.08-1.5 mg m3), retrieved pigment concentrations are within 30-40% of in situ concentrations (GORDON,CLARK,Mtr~rr~.a and HovIs, 1980; GoRDoN,CLARK,BROWN,BROWNand Hows, 1982; GORDON,CLARK,Baow~, BROWN,EVANS and BROEr,~OW, 1983a). However, even though CZCS pigment values in areas affected by rivers may be realistic (YoD~, McCta~, Bt>acroN and OEY, 1987; B,~,~ta~, M c ~ and MAtaNOaWeRmZOLI, 1986; GORDON,CLARK, BROWN,BROWN,EVANSand BRom-~xow, 1983a), phytoplankton, suspended matter, and Gelbstoffe (yellow dissolved organic matter) are not correlated. Thus, the blue-green ratio of CZCS radiances becomes useless for quantifying chlorophyll concentration (FISHER, D O ~ and GRASSL, 1986). The following is a list of factors that may have affected CZCS-derived pigment concentrations in the Caribbean: 3.3.1. Calibration. One of the worst CZCS calibration problems occurred during JanuaryFebruary 1980 (R. EVANS, personal communication). Since the southern Sargasso (Fig. lb) typically supports low phytoplankton concentrations, we used it as a control region to monitor changes in background pigment levels or possible quality problems with the imagery. It appears that problems due to calibration drift during the study period were ameliorated by interactive selection of epsilon coefficients. BAr~SEand McCLAn~ (1986) also derived their atmospheric correction coefficients interactively. The fact that BANSE and McCLAIN (1986) observed an offshore increase in pigment concentrations in January-February 1980 in the Arabian Sea, when concentrations in the southern Sargasso decreased (Fig. 5c), suggests that the CZCS was able to detect real trends in pigment concentration.
PLATE I. March 1979. Composite of 3 CZCS scenes of the Caribbean Sea in March 1979. Color bar shows pigment concentrations (approximatdy chlorophyll a plus phaeopigment and Odbstoffe) in [rag m'~]. Longitude values are given on a 360 degree basis, starting at Greenwich and increasing eastward. The cenlral Caribbean shows low pigment concenlra6ous, while the southeastern Caribbean had high conccmtratious (> 0.5 mg m'3). Pigments in the southeaste~'n Caribbean appeared elongated toward the west, indicating the direction of the circulation. Patches of high pigment also ocoatred off the Paraguana and Goa'~a Peninsulas (see Fig. la for geographiclocations).
I
i
I
..fJ
I
.L~[
_," " ,!.,
i
I
I
l
I
I
,[~L~L
"
'~
I~80
s
c.
f~
NASA - GSFC
,JAN
.
...-
L
I~AT': 2. January 1980. CZCS composite of 7 overviews of the Caribbean Sea in January 1980. High concentrations occurred immediately to the southeast of the Caribbean. This indicated northwestward movement of Amazon water alc~ng the coast of Guyana and the presence of an anticyclone south of Barbados. High pigments in the southeastern Caribbean indicated the presen~ of upwelling and the plume of the Orinoco River, Pigments were low in the no~hm'n Caribbean.
i
. . . . . . . . . . . . . . . . . . . . . .
.
"'%
%/
"%
g.
%
NASA - GSFC
~
I~.ATE 3. March 1980. CZCS composite of 7 overviews of the Caribbean Sea in March 1980. Tropical Atlantic and Amazon water (concenu'afiom of alx~ut 0.3 mg m -3)entered the Caribbean south of about 15 °N. High pigment concentrafiom (> 0.5 mg m -3) occun:ed only near the continental maxgi~. Westward elongation of the patches along the coast indicated strong westward motion of the watt. Low pigment concenn'ations associated with North Atlantic water occupied the northeastern Caribbean.
.4
I
I
I
~I . I. . ~_1 . . . . .I
•
I
I
I
I
JNN NASA
1982 - GSI:'E:
0 o.
~
I~ArB 4. January 1982. CZCS composite of 9 overviews of the Caribbean Sea in January 1982. High pigment concentratiom (> 0.5 mg m -3) occtm'ed near the contincntaJ margin. Theze were indications of anticyclonic flow south of Barbados. Pigment patches with about 0.3 mg m 3 southwest of Puerto Rico were associated with Orinoco water discharged during the previous season. We infea'red that a jet of low-pigment Atlantic waters entered the Caribbean between 15 ° and 17°N (near Guadeloupe and Dominic.a) and continued westward thxough the central Caribbean, isolating pigment patches in the northern and southern Caribbean (see Fig. la for geographic locations).
.I. .I . .I . .I
•
L
i
i
¢
f
i
i
~
-
i
i
i
i
i
i
i
°
"';
0
NASA - GSFC
OCT 1 9 7 9
-"
11
l:'t,~l"E5. October 1979. CZCS composite of 9 overviews of the Caribbean Sea in October 1979.' The Orinoco plume occupied the eastern Caribbean and flowed northwestward past Puerto Rico. A small plume veering offshore near 10" N, 59 ° W was evidence of an anticyclonic eddy east of Trinidad.
i
i
.
....................
i /
." -
.
t~J
.
.
.
JJ
.
.
,
.
.
,~
.
.
.
~a
.
J
.
.
.
.
. -
i i f / f
e,i,
'-.
~,"
-
o .
•
[a"
'rrl~
-
t~
--,
0
NASA - QSFC
JUN 1980
.
t
PLATE6. June 1980. CZCS composite of 2 overviews of the Caribbean Sea in June 1980. Only 1 scene covered a significant portion of the study region, and it showed htighpigment concentrafiom in the central and northeasternCaribbean. A composite for July 1980 showed similar high concentrations far offshore in the westea-n half of the study area.
.
,.
PJ.ATE7. September 1980. CZCS composite of 5 overviews of the Caribbean Sea in September 1980. The plume of the Orinoco occupied a large portion of the study region. The genexal features observed in this composite persisted until December 1980, but pigment concentrations decreased with time.
PLAr~ 8. November 1981. CZCS composite of 7 overviews of the Caribbean Sea in November 1981. The Orinoco plume bifurcated in the southeastern basin. Low pigment concentrations occurred outside of the Caribbean and in the western Caribbean.
PLATE 9. Single-orbit CZCS scene showing pigment concentrations on November 24, 1984. The Orinoco plume appeared to disperse eastward into the interior Adantic. The Orinoco plume joined the northern extreme of the retloflecfion of the North Brazil Current, traced here by pigments associated with the dispersal of Amazon River water. Due to CZCS sensor decay, concentrations in this image are unreliable.
Pigment distribution in the Caribbean Sea
39
3.3.2. Atmospheric effects. Artifacts in derived products due to thin clouds and cloud edges were minimized by manual adjustment of land/cloud flag thresholds based on the 750 nm band of each scene. The atmosphere may also contribute sands and aerosols originating in Africa (PROSPERO,N ~ s and UFa~tATSU,1987; PROSPEROand C~LSON, 1972; PROSPERO,BONA'rrI,Srn~mtT and CARLSON,1970; CARLSONand PROSPERO,1972; C~au~m~, STEWARD,Bzl-Z~a~and PROSPERO, 1986a). Images of atmospherically-corrected CZCS band 4 (670 rim) were compared with the pigment images to ensure that patterns in the pigment field were not correlated with aerosol patterns. Scenes where aerosols caused artifacts in the pigment fields were discarded. Water-leaving radiance scenes were individually checked for accuracy using theoretical radiance limits for water with low and high pigment concentrations (GORDONand CLARK,1981). Selected scenes were also processed to normalized water radiances to verify agreement with values given in GORDON,CLARK,BROWN,BROWN,EVANSand BROEr,a 0.05 for the product-moment correlation coefficient with n = 130 orbits; Sog.m~and Roma=, 1981), suggesting that no artificial trends due to biased atmospheric correction coefficients occurred. 3.3.3. Sunglint. Many scenes contained sunglint, which appeared as a meridionallyelongated, bright spot in CZCS band 4 (sometimes > 1000 km long, 500 km wide). Sensor configuration and high sun angles over the northern hemisphere led to particularly frequent sunglint contamination during May-September periods. The glint pattern was masked in the pigment scenes by adjusting the land/cloud mask threshold based on CZCS band 5 (750 nm). 3.3.4. Turbidity. The large amount of sediment in coastal waters off northeastern South America poses a problem for the interpretation of CZCS imagery. However, the impact on derived pigment concentrations offshore in the Atlantic and Caribbean may be small, because clays and sands discharged tend to remain close to shore (i.e. within 30-100 km), where they are masked as land or clouds due to their high reflectance in CZCS band 5. 3.3.5. Floating blue-greens. Biases in the retrieved pigment concentrations in the Caribbean due to surface accumulation of Oscillatoria (see concerns of BANSEand McCt.A~, 1986) were unlikely. MARG~a.ZF(1965), CARPENTERand PrUCE(1977) and C~a,F.~rrm~ (1983) found that over 85% of the Oscillatoria were distributed uniformly in the upper 50 m, with peak densities (approximately 104 to 105 trichomes m -2) between the surface and 20 m. 3.3.6. Coccolithophores. MARGALEF(1965) measured concentrations of about 22 x 103 cells 1-1of Coccolithus huxleyi and C. pelagicus, which is 3 to 4 orders of magnitude less than concentrations measured in coccolithophore blooms off the Celtic Shelf (Hotz_zoA_~,VIOt~mR, ~otm, C~rt:s and CFtAMPAGr~-PHa~n,PE, 1983). B~'ZErt, EC~XMAN,CARDER, K.F.Sam_.~and BE'r-ZER (1977) also found that coccolithophores disappeared in waters affected by the Orinoco River plume. Thus, we did not expect artifacts due to coccolithophores. 3.3.7. Subsurface pigment maxima (cf Cvtza~, 1982). Near the continental shelf, pigments are generally uniformly distributed in the upper 50 m (M~GhtS~F, 1965; COmU~DOR,1979). But offshore, subsurface pigment maxima can be deeper than 10 attenuation depths (OHER-OTEC STUDY, 1980; MaRGXt.EF, 1965). If concentrations of pigment near the surface were low in offshore waters, the CZCS underestimated areal pigment values (Pt~rr and HERMAN,1983). If JPO 23:I-C
40
MOLLI~R-KARGERet al.
high surface concentrations occurred offshore, as when river plumes formed patches greater than 3 x 105 km 2 (see below), the subsurface phytoplankton community would be shaded and perhaps become unviable. We could not evaluate the importance of the subsurface chlorophyll maximum, but this needs to be addressed in future studies. 4. RESULTS 4.1. Comparison with ship data
We compared pigment estimates obtained from full-resolution CZCS images (1 km x 1 km pixels) with a limited number of ship-based observations. The CZCS underestimated the sum of surface in situ chlorophyll a and phaeopigments off Puerto Rico by a factor of about 0.61 in 1980 (Fig. 3, Table 4; data from the OHER-OTEC STUDY, 1980, courtesy of J.CoR~EDOR and P. YosmoKA). Comparisons in the southern Caribbean in 1986 showed more scatter, but CZCS data were on average within a factor of 0.92 of the surface in situ pigment concentration (Fig. 3, Table 4; data from the FUNDACIONLA SALLE). The CZCS overestimated two in situ observations of surface pigment concentration (bucket samples) in waters affected by the Orinoco (20 April 1986, Table 4). It was, however, not possible to evaluate small-scale (< l pixel) variability in the in situ concentrations nor its effects on the comparisons, primarily because of insufficient ship data. In general, there was good agreement between previous observations of phytoplankton pigment concentration (Table l) and concentrations obtained with the CZCS (Table 5).
/ / / / / I I /
/ / i /
I
/
E
/ / /
E
/ i i / / / /
÷
"5.
B
i /
/ "
N L)
/
•
1980
/ +
/ /
1986
/
/
-
-
-
1:1
J ( +_ s d )
/
1
0
i
I
I
I
2
In situ chlorophyll
I
4
+
phoeopigment
I 6
[rng
rn-31
FIG. 3. Comparisonbetween CZCS and ship observations made during OHER-OTEC cruises off Puerto Rico (January and August 1980; OHER-OTEC reports, 1980), and during a survey off Venezuela by the La Salle Foundation(Marchand April 1986). See also Table 4 for individual values and furtherexplanations.
Pigment distribution in the Caribbean Sea
41
TABLE 4. Comparison between CZCS and ship observations made during OHER-OTEC cruises off Puerto Rico (January and August 1980; OHER-OTEC reports, 1980), and during a survey off Venezuela by the La Salle Foundation (March and April 1986). CZCS values represent the average and standard deviation of values in 4 x 4 pixel boxes ( 4 km x 4 k m ) around ship positions [ mg m'3 ]
Ship date
CZCS date
Lat N (o)
Lon W (,)
Ship Ch
Ship Ph
Ship (Ch+
Ratio (Ch/
Ph)
Ch+Ph)
CZCS Pig.
C7_CS s.dev
1980
Jan28 Aug 1
Jan27 Aug l
Aug 2
Aug 1
17.96 17.94 18.04 17.81 17.74 17.93
65.86 65.78 65.55 65.55 65.88 65.88
0.13 0.10 0.11 0.09 0.08 0.13
0.01 0.27 0.32 0.31 0.27 0.30
0.143 0.373 0.442 0.413 0.356 0.431
0.910 0.273 0.257 0.230 0.224 0.301
0.14 0.212 0.301 0.251 0.253 0.39
0.02 0.03 0.1 0.03 0.01 0.09
12.34 12.16 12.18 12.00 11.99 11.68 11.52 11.38 11.77 11.48 11.30 11.63 11.80 11.82 11.65 11.68 11.50 10.73 10.83 10.35 10.05
70.38 70.54 70.89 70.91 70.71 70.32 70.25 70.40 70.53 70.52 70.65 70.65 70.83 71.17 71.45 71.02 70.82 63.50 63.15 62.45 62.05
0.16 0.28 0.53 0 0 0.6 0.75 0.93 0.46 0.14 0 0.34 0.09 0.1 0.16 0 1.34 5.45 0.21 0.5 1.03
0.34 0.2 0.13 0.91 0.36 0.43 0.47 0.39 0.29 0.92 2.06 0.87 0.16 0.04 0.29 0.76 0.56 1.46 0.09 0.21 1.22
0.5 0.48 0.66 0.91 0.36 1.03 1.22 1.32 0.75 1.06 2.06 1.21 0.25 0.14 0.45 0.76 1.9 6.91 0.3 0.71 2.25
0.32 0.583 0.803 0 0 0.582 0.614 0.704 0.613 0.132 0 0.280 0.36 0.714 0.355 0 0.705 0.788 0.7 0.704 0.457
0.51 0.35 0.41 0.43 0.38 0.66 2.75 1.64 0.74 0.63 2.99 0.81 0.38 0.51 0.86 0.86 0.74 6.22 0.38 3.25 4.47
0.05 0.02 0.02 0.07 0.03 0.03 0.66 0.57 0.t 0.04 0.1 0.07 0.04 0.04 0.09 0.09 0.09 0.65 0.04 2.1 0.21
1986
Marl5
Mar 16
Mar 16
* *
Mar 17
Apr 19 Apr 20
Apr 19 * *
Abbreviations are: Ch = Chlorophyll a; Ph = Phaeopigment; Pig. = chlorophyll plus phaeopigment and Gelbstoffe; s. dev = standard deviation of CZCS pigment concentration.* Indicates large influence of outflow f~omMaracalboLake(March 1986) oroutflow from the Orinoco River (April 1986). Note: 1980 insitu values are the optically-weighted mean concentrafion in the upper optical depth. 1986 data were surface samples. 1980 CZCS data were processed using corrections by GORDON, CLARK, BROWN, BROWN,EVANSand BROENKOW(1983a) and GORDON,BROWN,BROWN,EVANSand CLARK(1983b). 1986 scenes were processed using Sa'URM(1986).
With CZCS
c h l o r o p h y l l c o n c e n t r a t i o n d a t a as i n p u t , w e d e r i v e d e s t i m a t e s o f p r i m a r y
p r o d u c t i v i t y f o r s e l e c t e d r e g i o n s o f f V e n e z u e l a u s i n g t h e a l g o r i t h m p r o p o s e d b y Er,etzY, STEWART, A a a o a ' r a n d HEYMAN ( 19 85). T h e s e r e g i o n s w e r e o u t l i n e d f o l l o w i n g MAI~6AL~ ( 1 9 6 5 ) (cf. Fig. I C) w h o first d e f i n e d p r o d u c t i o n r e g i m e s o f f V e n e z u e l a u s i n g in situ 14 C o b s e r v a t i o n s ( T a b l e 2). F o r p u r p o s e s o f EPPLEY et al.'s a l g o r i t h m , w e a s s u m e d t h a t c h l o r o p h y l l c o n c e n t r a t i o n was 50% of the CZCS-derived pigment level. The correction was applied since only chlorophyll a is e m p i r i c a l l y r e l a t e d to v i a b l e p h y t o p l a n k t o n b i o m a s s a n d p r o d u c t i v i t y , w h i l e p h a e o p i g m e n t s
42
]V~0LLER-KARGIBRet al.
TARL1R5. Mean pigment concentrations, range, and coefficients of variation (CV) in the Caribbean Sea and southern Sargassoover the 4-year time series.The means were derived by weighing individual samples(scene.s)by the valid numberofpixels present.The range is based on extrema of regional mean concentrationsrather than on individualpixel values.The CVis the mean of all coefficientsof variation obtained from individual scenes. 'n' gives the number of scenes used in the computations ([days])and the sum total of pi.xelssampled in the series (LDixels]).Only those scenes with > 1000valid pixels per definedarea were used to derive these statistics.The range of valid concentrationswas arbitrarily fixed between 0.042 and 20.0 nag m -3
Area
Mean
Range
CV
n
(km 2)
(nag m-3)
(mg m-3)
(%)
(pix.elS)
S. Caribbean
586835
0.58
0.11-1.94
134
129 1412693
N. Caribbean
886470
0.24
0.08-1.08
119
144 2463044
S. Sargasso
497711
0.10
0.05-0.95
55
104 860615
Subregion
(days)
are a byproduct of grazing and senescence of the phytoplankton (J EFPREY, 1980; M~,o~_a:, 1965). As mentioned above, previous observations have indicated that there is a large but variable fraction of phaeopigment in Caribbean waters (Table 4; see also O H E R - O T E C Saa.rolr~, 1980; MALONE, 1971). The chlorophyll and phaeopigment measurements made during the OHEROTEC STUDY(1980) offPuerto Rico were extremely valuable because they provided an estimate of the fraction of viable pigment in the Orinoco plume. The values derived from E ~ L ~ ' S formulation do not compare very well with those compiled by MAROALEF(1965) and Gz~_.s (1972; see Table 2). In general, this simplistic (E~LEY, personal communication) formulation overestimated in situ productivity observations. The only exception was immediately downstream of the island of Margarita (area IV in Fig. 1 c). This could indicate, for example, that the in situ samples were collected during a bloom, while the CZCS averaged over a larger area and non-bloom periods. Elsewhere, the general trend toward higher productivity estimates m a y be the result of overestimating chlorophyll concentrations in, for example, large areas affected by river plumes (i.e. areas I, II and V in Fig. lc). In fact, we refrained from obtaining surrogate productivity estimates in area I because of the obviously large influence of the Orinoco on this region. It may also appear that EPPLEV,STEWART,ABBOTrand ~ (1985) formulation overestimates the productivity of oligotrophic oceanic waters (e.g. chlorophyll concentrations of 0.04 m g m 3 yield productivities of 200 m g C m "2d-l). On the other hand, we now know with certainty that the 14C technique underestimates productivity w h e n " c l e a n " methods are not used (PEn~SON, 1980). This has led to revision of our previous estimates of productivity in oceanic waters (cf. LAWS, DrruLLIO and REDALm, 1987; MARRA and HEIa,mMAr~, 1987). In any event, it is hard to draw conclusions from the comparisons in Table 2, since the in situ observations in the Caribbean are aliased in time and are not synoptic, while the CZCS sampled a variety of water masses with frequence.
Pigment distribution in the Caribbean Sea
4.2.
43
Spatial arrangement of pigments
Figure 4 shows how the number of valid pixels obtained per scene varied relative to the maximum number of possible pixels in an area. Complete spatial coverage of a region was rare due to changes in scan configuration between orbits, cloud cover, and frequent contamination by sun glint. 1979 was the year best sampled, and 1981 the worst. The region best sampled was the northern Caribbean, and the worst, the southern Sargasso, The highest and most variable concentrations occurred in the southern Caribbean (series mean = 0.58 mg m'3; Table 5). Moderate concentrations were observed in the northern Caribbean (mean = 0.24 mg m-3). In contrast, standing crop in the southern Sargasso Sea was low (mean = 0.10 mg m-3), even though patches of Amazon water drifting east and north of the Antilles caused some variability (Fig. 5c; cf. DEUSER,MOLLER-KARGERand HEMLEBEN,1988). Coefficients of variation (CV = standard deviation x 100/mean) were in the range of 50 to 150%, and increased with the 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0
,l,J,~,l,,, Jan 79
1 0.9 0.8 0.7 0,6 0.5' 0.4 0.3 0.2 0,1 0
'I r'~ Jan 80
'I
r't
'I'' Jan 81
I
I
ill
I
I
I
I
I
I
I
I
I
Jan 62
I
I
I
Jan 83
b
L
0"91 '1'''''1
''I''
don 79
Jan 80
Jan 79
Jan 80
I
'I'''''I'' Jan 81
I
I
I
I
I
I
I
I
Jan 82
I
I
I
I
I
r
I
Jan 83
1
O.B 0.7 0.6 o 0.5
0.4 0.3 0.2 0.1
o
'
'
I' Jan 81
' " I " ' " I
~''' Jan 82
i~l
II
Jan 8,.3
FIo. 4a,b,c. Area sampled pet CZCS scene (munber of pixels with valid data in the defined region) normalized to total area (maximum possible number of pixels sampled in a region) of each of three large-scale regions defined in the Caribbean Sea (see Fig. lb). (a) Southern Caribbean, (b) Northern Caribbean and (c) Southern Sargasso.
44
MOLLBR-KARGER et al.
2
,=
$ i£
t8 17 1 1.9 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 03 02 01 0
O
Observation
4--+
Mean
7
io
f
Southern Caribbean
Monthly
; •i
'l"r"l
....
TI .....
Jon 79
I .....
I .....
Joo 80
,I
r .....
I .....
Jon 81
r,'"'l'
JOn 82
Jon 83
L1
•
1
" "
Single Observation
b
4--+
Monthly Mean
Corlbbeon
0.9-
Northern
080.706 0.5 0.4
0.3 0.2 01
'1 . . . . . I ' " " 1 ' ~ " ~ 1 " ' " 1
Jon 79
'''''I'''''I'T'''I~''I~
Jon 8 0
don 82
don 81
o0'o
Observation
0.28 026
(0.95)
024 022
Jan 83
C
Monthly Mean
5ergesso
Sea
i
0.2
• m
•
E 018 016 -
•
0.14 -
•
•
:
•
o.12-
• •
'
°°°4
-~
0.0
°
° Jon 79
\7 4 / '~ "" .':"
"i Jart 80
ii
•
;~ "..
o,o~3/d.¢.~
il
•
I
.~"1 " I
~ Jan 81
Jon 82
Jan 83
FiG. 5a,b,c. Time series of regional and monthly pigment concentration means based on individual scenes of the Caribbean Basin (see Fig. lb). The regional means lumped effects of dyer discharge and of wind-driven upwelling. Monthly means were computed using individual scenes weighted by the numberof valid pixels. The areas of interest were: (a) SouthernCaribbean, (b) NorthernCaribbean, and (c) Southern Sargasso. The large mean concentration observed on August 26 1980 in the Southern Sargasso was caused by a patch of high concentrations associated with the northward dispersal of Amazon water (e.g. DSOSER,MOLLeR-IC~RO~Rand HEMLEBEN,1988).
Pigment distributionin the CaribbeanSea
45
mean. The mean CV for the series suggested that the distribution of pigments in the Caribbean was patchy (CV > 110%; Table 5), but more random in the Sargasso Sea (CV < 60%). The differences between regions were consistent throughout the series (Fig. 5). The time series o fregional concentration means showed large variability between consecutive samples (Fig. 5). This variability was particularly marked in the series obtained for the southern Caribbean (Fig. 5a). This short-term variance is to a great extent an artifact caused by spatial aliasing when sampling a large area with the CZCS. Figure 4 gave an indication of the fraction sampled during each satellite pass, but the actual geographical area sampled also varied from scene to scene. Thus, individual means frequently did not reflect conditions over the entire area of interest, and noise was introduced. We also examined time series of monthly mean concentrations (superimposed on the curves of Fig. 5). We found that monthly means based on monthly composites of CZCS images frequently overestimated the corresponding weighted mean. The best means were based on individual scenes weighted by the number of observations. Since the monthly means represent better spatial coverage of the individual subregions, the series of monthly means is more in accordance with the traditional view o flow variance of phytoplankton biomass in tropical oceans. Nevertheless, it is clear that near continental margins, traditional views of phytoplankton abundance in tropical oceans are flawed. Indeed, concentrations in these regions can be high and may appear to vary almost at random. The seasonal signal was somewhat muted in the series of means (Fig. 5), primarily because the regions were large (> 50x104 km 2) and variation introduced by patches that form seasonally was dampened (i.e. the largest patch was of the order of 3x104 km 2, compared with 5-9x 104 km 2 for the sample regions, see below). Also, the seasonal cycle was obscured by interannual variability. In contrast to the time series of regional means, dramatic seasonal changes in the spatial arrangement of pigments were revealed in the CZCS images. During January-May, concentrations > 0.5 mg m -3were seen almost exchisively south of 14°N (Plates 1-4). High pigment concentrations occurred along the coast, especially near capes and headlands off central Venezuela and eastern Colombia. Since these locations receive little or no river input, and since the sea surface temperature (SST) around these locations is always lower relative to SST in the central Caribbean (e.g. Fig. 6), this indicated that upwelling of nutrient-rich waters occurred near the coast. Also, the plumes of the Amazon and Orinoco could be seen entering the Caribbean and extending westward along the coast of Venezuela (Plates 1-4). In contrast, between June and December, concentrations > 0.5 mg m 3 occurred in a patch that spread west and northwestward over an area of up to 30x 104 km 2 in the eastern Caribbean (Plates 5-8). Over scales larger than about 500 km, features were usually elongated to the west or northwest, which reflected the dominant direction of water movement. At smaller scales, there were eddy-like features which in addition showed southward and eastward motions, as inferred by examining scenes collected during consecutive days (M~YL~R-KhRGER, 1988). The largest eddy-like motions detected were about 250 km in diameter. To determine the cause for the variability observed in the distribution of pigments, we examined time series of several other parameters. As previous studies had shown (H~,EPa~ and FEB~s-OR'mGA, 1975a), we found that the monthly mean wind stress in September (0.04-0.06 N m "2) is about 1/2 the stress observed in April (0.07-0.15 N m-2; e.g. Fig. 7). Also, September winds were rotated slightly clockwise (5-20 degrees) relative to April winds. The theoretical Ekman transport reflects these changes (Fig. 7). SST also showed a fairly regular seasonal cycle (Fig. 6), as did coastal sea level (M~tJ~R-K~owR, 1988). Clearly, the seasonal cycle in sea level is
46
MOLLER-KAROBR et al. 30 29 28 27 26
%-,
25 =' o
...........
i ...........
i ...........
i ...........
i ...........
i ...........
i ...........
i ...........
~. . . . . . . . . . .
i ..........
i ...........
r ...........
i. . . . . . . . . . .
i. . . . . . . . . . .
i ...........
i. . . . . . . . . . .
i. . . . . . . . . . .
J ...........
i ..........
29
~. 28 E 27
~ z6 ~
2s
~ 24
..........
29
28 27 26 25 24
Jan 7 9
Jon 8 0
J t ; n 81
Jan
82
Jan
83
dan
84
Jan
85
Jan
86
Jan
87
Jon
88
1.6
0.40 -0.4 -0.8
-1.2
l
1.2
o
E
o
0.8
',......i
0.4
e
0
illll ...........Ai ii
3 --0.4 -0.8 X -1.2 eE-1.6 1.2 0.8 0.4 0 -0.4
-2
...........
Jan 79
i ...........
Jan 80
i ...........
Jan 81
F ...........
Jan 82
I ...........
Jon 83
I ...........
Jan 84
I ...........
Jan 85
I ...........
don 86
I . . . . . . . .
Jcn 87
I
......
Jan 88
Fie. 6a,b,c. g-year seriesof monthly mean sea surfacetcmp~'atute (SST) and monthly SST anomalies in 4°x4 ° regions o f the central Caribbean Sea and the continental margin (see Fig. lb for location of series). Monthly means in each 4°x4 ° region were based on unclassified monthly temperatttre observations from merchant and naval vessels compiled in 1 °x l ° squares by Fleet Numerical Weather Center (Monterey, CA) and weighted by the number of observations in each 1 °xl ° box. Mean annual cycles were calculated by averaging monthly means across years. Anomalies are the difference betweenindividual monthly means and the mean annual cycle. The areas were centered on: (a) Central Caribbean: 4°x4 ° region centered on 14°N, 67°W. (b) Central Venezuela: 4°x4 ° region centered on II°N, 67°W. (c) Eastern Vcnczucla:4°x4 ° region centered on 1 I°N, 64°W.
Pigment disuibudon in the Caribbean Sea
-80
47
-76
-74
-72
-70
-68
-66
-64
-62
-60
-58
-76
-74
--72
-70
--68
--66
--64
--62
--60
--58
18-
--BO
Q.
-80
-76
April
-74
I
1979
-72
-70
-68
Tau o.1 N m-2 <---
-66
-64
Ekmon t r o n s p o r t 5.0x103 Kg rrr'le-1 <
-62
-60
-58
16
16,
14--
,,
12
10
8
-80
-76
-74
-72
-70
b. September 1979
-68
-66
-64
TQu 0.1 Nm -2
-62
-60
-58
Ekman t r a n s p o r t 5.0x10~ Kg m-ls-1 <
Flo. 7. Monthly mean wind stress and Ekman ~'ansport in the Caribbean Sea based on 1000 mb FGGE IIIB ( gridded ) wind speed (First Global GARP Experiment (FC.,-GE) data). (a) April 1979, (b) September 1979.
48
MOLLER-KARGERet al.
produced by seasonal changes in temperature through steric effects. However, variability in coastal sea level is accentuated by fluctuations in wind stress and Ekman transport. We expand on the significance of these changes in the Discussion. CZCS data covering the western tropical Atlantic shows that the Amazon discharge flows along the continent toward the northwest during February-May periods (MOLLER-KARGER, McCLAIN and RaCH.AgDSON,1988; DEUS~, MOLLER-KARcERand HEMLEBEr~,1988). We inferred that a large fraction of this water entered the Caribbean, while some is entrained eastward off Trinidad tracing an ephemeral anticyclonic eddy (lifespan of 1-3 months, cf. Plates 2 and 4). Part of the Orinoco discharge may also become entrained in this eddy, while the remainder is carried into the Caribbean. But the Orinoco's discharge at this time is 6 to 7 times less than during AugustOctober (Fig. 2). In contrast, between July and November, the bulk of the Amazon water is carried eastward by the NECC. This water may be entrained northward into the North Equatorial Current (NEC), and reach the Caribbean after extensive mixing with Atlantic water. The images clearly showed a plume of high pigment concentrations that occupied the eastern Caribbean during this time. However, this plume originated at the mouth of the Orinoco River (Plates 5-9), rather than at the mouth of tbe Amazon. 4.3. Nutrient dynamics in the Orinoco River plume The CZCS data clearly show that the origin of the large pigment patch present in the eastern Caribbean during the second half of the year is the Orinoco River. Since at this time winds are weak (Fig. 7), sea surface temperature is high (Fig. 6), and the seasonal thermocline is well developed, it is unlikely that this seasonal increase in chlorophyll concentration is due to upward mixing of nutrient-rich deep water. It is likely that a large fraction of the pigments seen originating in the Orinoco represent viable phytoplankton, similar to what is observed in the Amazon's discharge (cf. WOOD, 1966; RYrrmR, ME~ZELand CORW~, 1967; HtJLatmT and CORWI~, 1969; DEMASTER,KNAPPand Nrrr~otmR, 1983; DUST:~, unpublished data). High concentrations of chlorophyll have been observed in surface "bucket samples" taken in April 1986 in the Gulf of Paria, directly north of the Orinoco delta (Table 4). Also, as pointed out above, the surface salinity minima seen during October-November in waters south of Puerto Rico coincide with maxima in surface chlorophyll (YosrnoKA, OWEr~and PESArcrE, 1985). Since the Orinoco is a point source of nutrients, concentrations of phytoplankton should decrease with distance from the mouth as nutrients are consumed and exported into adjacent water. To gain some insight into how nutrients are lost from the Orinoco's plume, we examined transects along the plume's axis and built a simple trophic model based on histograms of the distribution of CZCS-derived pigments. Individual images showed that the plume can have several branches, and that these may maintain coherence for periods of weeks to months. Our transects started south of Trinidad, continued east of Trinidad, and roughly followed the plume's axis to the end of the longest branch. Figure 8 shows pigments along transects obtained on 27 September 1979 and on 9 October 1979, when the Orinoco plume was not excessively obstructed from view by clouds or missing data. Due to movement of the plume, these transects are not spatially congruent. A filter was applied to each transect to replace both cloud pixels and pixels exceeding 7 mg m -3 with a value of 0. Based on historical data (Table 1), we did not expect values as high as 7 mg m -3 to occur offshore, even in the river plumes. We removed such values because we considered them noise associated with
Pigmentdistributionin the CaribbeanSea
• 6
=---4
~
o
1.1
5
E
49
T
1111111
.......
[
•
27 Sep '79 o
9 Oct '79
o...... ,,vo
~
Exponential
4
I"-. 1
o
qll I ~ .
E
II 2
i
i
" .
~'I
t I
~ " / i l l l ...... ?JlJ Ii
it
I i~ ILl i I 1- I
""
I ill rl F'-. ~I I~ I "~'-..
Ill
1
0
o'2
oi,
01s
C[housonds) Distance [kin]
;
,2
,,
FxG.8. Transectsof the Orinocoplumein the Caribbeanon 27 September 1979and 9 October 1979.Thestraightlinerepresentsdilutionby mixingof a conservativeproperty.The exponential decay modelwas obtainedusingan exponentialcurvefit to the transectdata of October 1979: Pigments [mg m"3]= 5.34 × e('°'°'5~×'~=°tn,y,lL cloud margins. The pixels excluded represented < 5% of the number of pixels sampled per transect. Note, however, that we did not exclude these extremes from the series of regional means shown in Fig. 5 or Table 5. The transects were approximately 1000-1400 km long, which is the typical length of the plume between September and November (cf. Plate 5). Plume waters frequently traced some eddy-like features, which appeared in the transects as depressions in the pigments concentrations (e.g. see transect for 27 September 1979 in Fig. 8). Since the eddies were not stationary, their position in the monthly CZCS composites was blurred. Within each transect, pigments decreased along the axis of the plume with distance from the river mouth. To obtain an estimate of the rate at which pigments decreased, we fit a curve to the transect data. Distance from the river mouth was converted to time by assuming that particles in the plume moved at an average speed of 0.3 m s -t (ca. 26 km d -1) in the direction of the plume axis. We tracked plume features in September and October 1979, and obtained velocities of 0.2- 0.4 m s -1, which agreed with previous estimates of the mean west-northwestward motion in the region (KINDER, HEBLrRn and G ~ N , 1985; Kn,~ER, 1983; WLLL,~vtS, 1986; MOLINneJ, SPILLANE,BROOKS,ATWOODand DucKm'r, 1981). The least squares curve that best fit the 9 October 1979 data was an exponential decay model: Pigment = 5.34 x e C-°.°61× t~), n =163, r 2= 0.51,where pigments are in [mg m "a] and time is in [days]. This suggested that pigments in the plume had a half life of approximately 11 days. On 27 September 1979, eddies with lower pigment concentration (plume water surrounding clearer oceanic water) were observed about 200-300 km and 600-700 km downstream of the plume's origin (Fig. 8). Assuming a mean plume velocity of 0.3 m s 1, the exponential curve fit for this date produced a flatter curve than for 9 October (i.e. a lower intercept and a smaller decay constant; curve not shown):
50
MOLLER-KARGERet al.
Pigment = 2.5 x e (*'°34 × "~'), n = 214, r2= 0.29, or a half-life of 20 days. This decay rate is probably an underestimate due to the effect of the eddies. Pigments disappeared from the O r i n o c o ' s plume faster than by conservative dilution in a onedimensional model. Exponential decay models fit the data much better than a straight line (Fig. 8), and the decay rate obtained from the transects included losses due to 3 dimensional dispersion in open waters, sinking, and grazing of phytoplankton cells. This implied that the plume is a significant source of organic matter which contributes to surface food chains and offshore sedimentation in the eastern Caribbean. We then used the C Z C S images to develop atrophic model of the plume. The m o d e l was based on a series of nominal conversions between pigment and nitrogen concentrations, and between pigment concentration and productivity (see Table 6a). This enabled us to compute estimates of average primary productivity and nitrogen uptake in the plume, DO~DAUS and GOERINO'S (1967) " f ' ratio, turnover times of freshwater, particulate carbon and nitrogen, and the average number of times that a nitrogen atom is recycled before export (Table 6c). We present 3 case studies based on histograms obtained during high discharge (27 September 1979, 9 October 1979, and 8 N o v e m b e r 1981 ). As in the case of the transects, we selected scenes in which the Orinoco plume was not excessively obstructed from view by large clouds or missing data. We excluded pixels in upwelling regions and defined the river plume as all C Z C S pixels with concentrations between 0.25 and 10 mg pigment m 3 (Fig. 9; Table 6b). In order to obtain the best estimate of the area of the plume and of the total biomass contained in it, non-valid pixels (clouds) were artificially assigned a pigment concentration. The value given to a particular cloud pixel was randomly selected from valid pixels outside the cloud's perimeter in a window centered at the pixel to be filled. This window expanded dynamically until an arbitrary threshold of 25 valid pixels was reached. Typically, this represented an area of about 425 km 2 around the periphery of the cloud. About 25% of the pixels in the plume had to be filled in this fashion for the cases examined. All calculations in the model requiring an input of chlorophyll used a value o f 50% o f the
TAe LE6. Model of the nitrogen and carbon turnover in the Orinoco River plume based on the pigment distribution observed with the CZCS (Fig. 9). The model was run as an electronic spreadsheet on a personal computer. The table contains three parts, as follows: (a) Variables: definition of parameters, thresholds, conversion factors, and model formulas, (b) Model input parameters and (c) Model results TASLE6a. Variables: Salinity end members [psu]: S(river) = 0 psu, S(ocean) = 36 psu, which is North Atlantic Surface water, S(plume) = 34.5 psu, salinity observed in the fiver plume. z(S(plume)) = 20 m. Is the depth of the 34.5 psu layer, which was visually estimated from transects presented by FROELICH,A'rwooo and GmSE,(1978). z qu~..... (0 psu) = z(S(plume)) * (1 - (S(plume)/S(ocean) ), this is the equivalent depth of flesh water needed to yield z(S(,plume))when mixed with S(ocean). This was estimated assuming linear (conservative) mixing of the salinity end members given above. N(min NO3-N + NI-I4-N) and N(max NOj-N + NH+-N) are the minimum and maximum Orinoco nitrogen concentration in [ I.J,M]: These are discharge-weighted average concentrations of nitrogen in Orinoco river water. Low and high turnover rates were estimated based on dissolved inorganic nitrogen (nitrate + anunonium) and on total nitrogen concentration in the fiver (dissolved inorganic + dissolved organic + particulate nitrogen), respectively (from LEwisand SAUr~DEES,1988; see also HAMILTONand LEWIS, 1987).
Pigment distribution in the Caribbean Sea
TABLE 6.
cont.
D = Orinoco discharge rate [m' s"]: Approximate discharge rate for the month previous to the date for which each plume histo/vam was obtained. Th(rain) and Th(max) are minimum and maximum threshold values [rag m3] for pixels defining the fiver plume. Ch = chlorophyll concentration [rag chlorophyll ra3]. Ch/Pig = 0.5, assumed ratio of chlorophyll a to total pigment. This ratio was applied to all CZCS pigment values for calculating parameters relating to viable phytoplankton. C/Ch = 50, typical carbon to chlorophyll ratio (wt:wt) in a bloom after 18-20 days (see ANTIA, McALLISTER, PARSONS, SamPrtENs and S T R I C K L A N O , 1 9 6 3 ; P A R S O N S , T A K A H A S H I a l a d HARORAVE, 1977). C/N = 6, assumed carbon to nitrogen ratio (atom:atom; REDFIELD,KETCHUMarid RICHARDS, 1963; BANSZ, 1974; PARSONS, TAKArtASmand HARGRAVE,1977). Tp~z = Arithmetic sum of all the pixels in the river plume falling within the defined range of concentrations. At, [kin 2] = T.,. * 17.5 lma2/pixel (based on 1/16 full CZCS resolution, reraapped to UTM projection). "I]fis'is the total area covered by the plume. V tw [m3]= A [w * z ¢quivldent (0 psu) ~ or the area of plume times the equivalent fresh water depth is the equivalent fresh water volume. This was estimated assuming that the plume had a constant depth. Sch [mg Ch]= Sum(Ch * frequency), is the sum of chlorophyll over all pixels encompassing the river plume. The frequency is the number of occurrences of particular concentrations (Ch) in the population of plume pixels. Avech [mg ra'~]. = S Ch/F.x,., is the mean chlorophyll concentration, estimated, from. the plume histogram (Figure 9) and a constant chlorophyll to C-7_~S pigment raao (Ch/Pig). Tcn [rag] = 17.5x106 m s * Sch * z(S(phlme)), is the total amount of chlorophyll in the plume, obtained by integrating pigment concentrations over the area and depth of the plume' s low-salinity layer (20 m in this case). Pigments were assumed to be uniformly distributed in this layer. 17.5x 10 ~ m 2 is the area represented by one pixd. Av%s ling-at N m -2] = Avech * (C/Ch) * z(S(plume)) / (C:N * 12), is the mean particulate nitrogen concentration in the plume. Tpn [rag-at] = Tch * (C:Ch) / (C:N * 12), is total particulate nitrogen concentration in the plume. Ave c [mg C m "2] = Avecb * z(S(plume)) * (C/Ch), is the mean particulate carbon concentration in the plume.
T c [metric tons] = Tch * (C:Ch) x 10"9[ton/mg], is the total particulate carbon concentration in the plume. PP [g C In 2 d-J] = SQRT(Ch), is an estimate of primary productivity based on the phytoplank-ton chlorophyll according to EPPLEY,S'rEWART,ABBO'rTand I-I~YMAN,(1985). SQRT(x) stands for square root ofx. This formulation was obtained by EVPLEY based on data from both MOREL Case I and Case II waters (see Fig. 1 in EPPLEY,STEWART,ABBOtt and HEYMAN,1985). Ave.. r e [g C rn "2d'~] = SumfPP * frequency)/T plx 1 where the sum is taken over all pixels encompassing the plume. This is a rough estimate of the average primary productivity in the plume. Ave h° [rag-at N m -2 d "t] = Av%p / (12 * C/N), is the average absolute nitrogen uptake rate by phytoplankton. f = D • N * 60 * 60 * 2 4 / ( T .Inx * 17.5x10 6 ra2 * A v erlao . ) "~ whichistheOrinocoNfluxrelativetonitrogen uptake by phytoplankton. This quantity is equivalent to the 'f-ratio' (of. EPPLEYand PETERSON, 1979). t c [d] = Tch * (C/Ch) / (Tpj, * 17.5x10 * m z * Av%p), which is an estimate of the turnover time for particulate carbon, based on the formulation: (Bioraass / Primary productivity). t v [d] = VI,' / (D * 60 * 60 * 24), is the time needed to fill the equivalent volume of fresh water in the area occupied by the river plume. t~ [d] = Tn~ / (N * D * 60 * 60 * 24),is the time needed to balance the PN of the standing crop with nitrogen supplied by the Orinoco. r = (1 - f) / f, is the average number of times that a nitrogen atom derived from the Orinoco is recycled in the river plume, assuming that there are no other new nitrogen sources (cf. EPPLEY and PErERSON, 1979).
51
52
MOLt.e~-KAxom~ et al.
TAat~ 6b. Model input. Variables and constants are defined and explained in Table 6a. Salinity end members: S(river) = S(ocean) = S(plume) = z(S(plume)) = N(min NO3-N + NH4-N) = N(max NO3-N + NH4-N) =
0 psu, 36.0 psu, 34.5 psu 20 m 8.2 p,M-N 19.7 IxM-N + 13,2 ItM-N 32.9 ttM-N
(NO3 + N H ) (Total dissolved N, organic plusinorganic) (Particulate N)
(Total N)
D (Orinoco discharge rate): 60000 [m3 s-'] ( for 27 September 1979) 50000 [m~s -t] (for 9 October 1979) 40000 [m~ s "~] (for 8 November 1981) Pixel values defining the plume : Th(min) = 0.25 [rag pigment m "3] Th(max) = 10.0 [rag pigment m'31 Ch/Pig = 0.5 C/Ch = 50(weight:weight) C/N = 6(atom:atom)
TABLE6C. Model results. Bracketed quantities are based on low and high nitrogen input by the Orinoco (see Table 6b). Results were based on histograms of the pigment distribution (Fig 10). Each date was run using a different discharge rate (Table 6b). Some quantities were rounded for presentation in the table Dme Or~t
1979 27 Sep 4677
1979 9 Oct 4843
1981 8 Nov 15360
T~ A~ [kin 2] z r~,~(O psu) V~, [m j
13046 228305 0.83 1.9E+l 1
16573 290028 0.83 2.4E+11
17475 305813 0.83 2.5E+11
So [mg Ch] AV%h [nag m "3] Tc~ [mg]
7560 0.58 2.6E+12
9500 0.57 3.3E+12
5560 0.32 1.9E+12
Ave~ [mg-at N m -2] T~ [rag-at] Avec [rag C m"2] T c [metric tons]
8.04 1.8E+12 579 1.3E+05
7.96 2.3E+12 573 1.7E+05
4.42 1.4E+12 318 9.7E+04
0.60 8.32
0.58 8.08
0.23 3.14
[0.02 - 0.091
[0.02 - 0.06]
[0.03 - 0.12]
0.97
0.98
1.41
Aveve [g C m"2d"l] Ave~ [rag-at N m -~d"] f
t c (B/PP) [d]
t,,
[d]
37
56
74
t~
[d]
[11 -43]
[16 -65]
[12 -48]
[10 -49]
[16 -65]
[7 -331
Pigmentdistributionin the CaribbeanSea
¢00i i
Oceanic concentrations
.........
27 September 1979 9 October 1979
Orinoco plume 300
--;ore of Orinoco
53
8 November 1981
plume
at maturity o
LL
200
1 O0
0
--
[
I 1
"1 2
. . . . . . . . . . . . . . . . . 3
Pigments [rag
m--3]
Fla. 9. Histogramsshowingthe concentrationofpigmentsin the Orinocoplume.The histogramswere obtainedfrom three CZCS scenesduringthe period of high riverdischarge.Indicatedare the ranges of CZCS-derivedpigmentconcen~ationsusedto separateoceanicwatersfromthoserepresentingthe river plume (range = 0.25 - 10 mg pigmentsm'3). Each frequencycountrepresents approximately 17.5 km z. Valueshigher than4 mg rn"3were onlya smallfractionof the totalarea and were omitted from this figure. Frequenciesrepresentingoceanic waters (concentrations< 0.25 mg pigmentm .3) were omittedbecausethe area usedto constructthe histogramswas tailoredto the individualplumes. Thus, the number of pixels with oceanic pigment concentrationswas arbitrary, making any comparisonbetweenhistograms at eoncentxations< 0.25 mg rn'3meaningless. CZCS-derived pigment level. As discussed above, this correction was necessary since phaeopigments are a byproduct of grazing and senescence of the phytoplankton (Jm~azY, 1980; MAROAU~F,1965). Also, river plumes are classified as Case II waters (MORELand PRmtm, 1977), in which there may be a large concentration of Gelbstoffe which does not covary with phytoplankton. Typically this leads to an overestimate of phytoplankton concentration (see BAKERand Srcn'rI-L1982; CARDER,STEWARD,PAULand VARGO, 1986b). Unfortunately there is no data on the concentration of Gelbstoffe in Caribbean waters nor in the Orinoco's plume, and we could not correct for this effect. The model presented in Table 6 suggested that the nitrogen discharged by the Orinoco was sufficient to support the observed concentrations o f pigments. During the period of high discharge (September-November; Fig. 2), the average concentration of surrogate chlorophyll in the Orinoco river plume was about 0.5 mg m -3. We estimated that the standing stock of phytoplankton in the plume was approximately l x l 0 ~ metric tons carbon during this mature stage. Using the productivity estimates based on EPPt~V, STEWARTand ABBOaW(1985), it appeared that the turnover time (biomass/primary productivity) was roughly one day. Assuming that the Orinoco is the only source of "new" nitrogen to the bloom observed within the perimeter of the plume, we obtained " f ' ratios (DU6DALEand GO~.tNG, 1967) and recycling factors Cr"; EvPt.m: and P~rv.zSON, 1979) close to those typically observed in the deep, oligotrophic sea. This implied that nitrogen was recycled many times before being exported (Table 6c). Assuming that the turnover rate for carbon (t c, Table 6) also reflects the amount of time that
54
MOLLBR-KARoERet al.
it took to recycle ni~ogen, it appears that the time needed to fill the estimated volume occupied by fresh water in the plume at a given discharge rate (tv) was roughly equivalent to the average number of cycles undergone by nitrogen (i.e. t v ~ t c ×r; Table 6c). Also, the time needed to fill this equivalent volume of fresh water was approximately equal to the time needed to balance the surrogate particulate nitrogen with N discharged by the river (Table 6). Furthermore, if we assume that nitrogen disappeared from the plume at the decay rate obtained earlier from our transects (cf. Fig. 8), nitrogen input by the Orinoco decreased to trace levels (i.e. I% of the average input) in roughly the time needed for the plume to cover the area observed in Plate 5, 7, or 8, namely a • little over 1 month. The above estimates are independent except for the rate of discharge of the Orinoco. Assuming that the estimates of primary productivity based on the empirical formulation of E~LEV, STEWARTand ABBcrrr (I 985; Table 6c) are representative of each of the months for which we had a plume sample, we estimated that the Orinoco contributed about 11 x 106 ton C to the Caribbean during the approximately 90 days in which the plume was well developed. Assuming that all nitrogen discharged by the Orinoco is taken up by algae to fix carbon, however, only 29 x 105 ton C may be considered "new" productivity over 90 days (at an approximate discharge rate of 5 x 104 m 3 s'l). Only the new production is available for export, and this amounts to an annual export of between 7 x l0 s and 29 x l0 s ton C due to the Orinoco alone. We believe that our estimates of productivity and turnover rates in the river plume are not adversely affected by the high light levels typical of tropical oceanic environments. Even though the plume is maintained in the upper part of the water column, vertical mixing within the plume would provide a mechanism by which few cells are photoinhibited. In fact, photoinhihition seems to be an artifact observed only during extended incubation of phytoplankton in clear bottles at surface light intensities. Results of productivity experiments in which realistic in situ fight histories are simulated do not show indications of photoinhibition (e.g. ~ , 1980; YOD~ and BJsHoP, 1985). However, to our knowledge, in situ productivity measurements have never been carried out in the Orinoco River plume. Clearly, these questions need to be addressed in the future. Finally, we realize that the relationship between chlorophyll and productivity presented by E~LEY, STEWARTand ABBo'rr (1985) is simplistic and has a wide margin of error (E~LEY, personal communication). Our earlier comparisons with historical data from the Caribbean region suggest that this approach leads to overestimates of regional productivity. However, this is a convenient formulation for its simplicity, and it allows exploration of the lower trophic dynamics of large patches of phytoplankton based on a limited number of parameters. More sophisticated methods are now being developed (e.g. PLATr and SATnVF~RANATn, 1988). 5. DISCUSSION
5.1. Spatial structure of pigments and environmental forcing The large-scale (> 100 km) distribution of pigments in the Caribbean Sea seems to be controlled by wind stress, flux of water through the basin, and river discharge. Wind plays an important role by forcing the surface circulation in the open tropical Atlantic (BusAt,ACCm and PIcAtrr, 1983) and regulating Ekman drift within the Caribbean (cf. Fig. 7). The wind field clearly is upwelling-favorable for most of the year in the southern Caribbean, but an area of Ekman convergence occurs in the northern Caribbean. The changes in SST observed in the Caribbean are the result of several processes acting in concert. During the first half of the year, cool surface water from the subtropical Atlantic invades
Pigment distribution in the Caribbean Sea
55
the upper layers of the Caribbean and the stronger winds erode the seasonal thermocline formed during the previous summer. Along the coast, the seasonal intensification of the trade winds leads to upwelling (I~Drnm.J~, 1955; MARGntJW,Cm~VIOONand Ym,Ez, 1960; MARGAUW,1965; BALLESTm~ and MAgGAJ.~F,1965; Ricnam~s, 1960; W0ST, 1964; GORDON,1967; ~ and Fm3gES-ORTEOA, 1975a, 1975b; COmU~DOR,1976, 1979). Isotherms may be raised as much as 90-175 m during February-June periods, enough to bring Subtropical Underwater and Antarctic Intermediate Water to the euphotic zone (I~RRm~ and FEBRES-ORTEGA,1975a; KINDER,HEBtnU~and GREEN, 1985). Thus, SST along the continental margin is always lower than SST in the central Caribbean (cf. Fig. 6). The higher nutrient concentration characteristic of this deeper water (MoRR~SONand NowLtn, 1982) leads to the blooms observed in these regions. In contrast, little or no coastal upwelling may take place during July-November periods (MARGALEF,1965; HEggEgAand FEBRES-ORTE6A,1975a). Blooms near the coast become increasingly rare, in particular in September and October. This appears to be the combined result of reduced wind stress, reduced turbulence along the coast, formation or strengthening of a seasonal thermocline, and deepening of the permanent thermocline. Monthly mean wind stress between July and November is < 0.07 N m "z, and sea level and SST are higher than their means, or ca. +0.1 m and 29 ° C, respectively (MOLLER-KARGER,1988). In spite of the strong westward winds early in the year, under which we expected evidence of a strong northward Ekman transport throughout the region, pigments remained confined to the southern Caribbean. As mentioned above, it has been inferred that the flux of water through the Caribbean is larger during the first half of the year than during the second half (MoR~JSONand NOWLIN,1982; I-~Btnunand RHODES,1987). We propose that the jet of Atlantic water that moves westward through the Caribbean during the first part of the yearpresses the plumes and upwelled water against the coast, despite local Ekman forcing driven by the trade winds. In contrast, when the trade wind and the inflow of Atlantic water relax during the second half of the year, the circulation switches to a mode dominated by local Ekman transport, and a large pigment patch spreads across the Caribbean toward Puerto Rico. The change in the circulation is traced by the discharge of the Orinoco, which can cross the Caribbean and reach the vicinity of Puerto Rico in approximately 1 month. River plumes in the northern hemisphere typically turn to the right of their mouths (BEARDSLEYand HART,1978; CHAO and BOXCOLrRT,1986). But the Orinoco's plume turns to the left as it leaves the delta. This indicates that accelerations other than those due to the rotation of the earth are strong. Because of the decrease in the Coriolis parameter (f) toward the equator, the Ekman depth of frictional resistance in the tropics increases to depths which encompass the plumes of rivers (typically 20 m or greater). Thus, river plume dispersal is tightly coupled to variation in the wind. The movement of Orinoco water is probably controlled by the large-scale (> 1000 km) drift of the North Equatorial Current against the coast and island chain, as modified by local Ekman drift. Orinoco waters that reach the northern Caribbean are rapidly diluted by mixing or removed when Atlantic water moves westward early in the year. This westward flow may show interannual variation, as suggested by the different dispersal patterns traced by the plume in different years. For example, the absence of pigment north o f 14°N in March 1979 (Plate 1), as opposed to January and March 1980 (Plates 2 and 3), implied that there was stronger westward flow in early 1979 compared with early 1980. Flow during the second half of the year also varies. For example, pigment patches during the second half of 1979 (Plate 5) were more elongated to the northnorthwest than during the second half of 1980 and 1981 (Plates 7 and 8), which indicated stronger westward flow in the second half of the latter years. As mentioned in the introduction, measurements of dynamic height in the Caribbean Sea have OPO ~3:l-O
56
MOLLER-KARGERet al.
led to inferences of one or two currents flowing eastward near the surface (FtrloJOr,~, 1965; ATWOOD, 1976; F ~ e . s - O R ~ o A and ~ R A , 1976; Arml~,s, FEm~JES and H.mu~lu~,, 1979; MOm~ISON and NOWLIN, 1982; DUNCAN, Scrm.~x~ow and Wmt.tAMS, 1982). However, the 4-year series of CZCS images did not provide any evidence for near-surface, basin-scale countercurrents. Southward and eastward motions at scales of about 250 km occurred in association with eddies such as those observed in the model of I-IEBtrl~, Kn,rOF_X,A ~ and HtntLmrRT (1982). ThUS, as pointed out by Ka,mER, Hm3trRN and GREEN (I 985), the evidence for eastward flows may have been an artifact caused by non-synoptic mapping of the dynamic topography. Dramatic examples of the importance of the wind in the dispersal of tropical river water were observed in 1980 and 1984. Of the years for which we examined wind stress and coastal sea level ( 1979-1981 ), the strongest westward stress (daily mean stronger than - 0.18 N m2) and lowest sea level (daily mean < -0.2 m) occurred during March-May 1980 (MC.~£R-K~GER, 1988). Pigment levels in the southern Caribbean increased simultaneously (cf. Plate 3). High winds persisted until after June 1980, possibly causing a premature northward dispersal of pigments which had formed along the continental margin, e.g. in the plume of the Orinoco (cf. Plate 6, Fig. 5b). However, it was not possible to make clear inferences about the origin of the high pigments in the northern Caribbean in June 1980 since only a few CZCS images were available, and these did not cover the southern Caribbean nor Atlantic waters immediately east of the Antilles. There was no other comparable event during the time series. In November 1984, the Orinoco plume was observed drifting northeastward, extending over 1000 km into the northern Atlantic (Plate 9). This was the only instance in which this situation was observed. In fact, during the first half of November 1984, wind stress in the southern Caribbean (Isla Orchila, Venezuela) was about 0.020 N m -2 to the NNE (Cagigal Observatory records, Venezuela). Presumably, the Ekman drift induced by these winds, combined with residual flow in the western tropical Atlantic, explain the northeastward elongation of the plume. In mid-November, winds became westward again, but unfortunately there were no additional CZCS data available to examine the plume's behavior. This event supported inferences that a phenomenon analogous to that of El Nino occurred in the Atlantic in 1984 (see special edition of Nature, vol. 322, pp. 236-253, 1986).
5.2. Role of the Orinoco as a nutrient source The Caribbean is biologically productive due to an abundance o f nutrients derived from river discharge and upwelling. But the rate at which "new" nutrients (see DUGDALEand GOERIN6, 1967) are supplied to surface waters of the Caribbean via these processes has remained unquantified. With the CZCS, we had the capability to measure the area over which a nutrient source had an impact on phytoplankton biomass, and thus we could refine nutrient budgets for marginal seas such as the Caribbean. An essential variable in simple models is the rate at which nutrients are supplied. The Orinoco is a point source for which we have the daily discharge rates and discharge-weighted nutrient concentration (L~,as and SAtrNDERS, 1988; see also HAMILTONand L~wcIs, 1987). In contrast, upwelling is a source which is spread out over a larger area. Unfortunately, we did not have information on either the upward velocities at upwelling sites, nor of the depth from which upwelled waters are drawn in the Caribbean, at the time these CZCS data were collected. Therefore, we were unable to estimate the amount of nutrient supplied to the euphotic zone via upwelling. We also did not have good information on the amount of nitrogen discharged by the
Pigmentdistributionin the CaribbeanSea
57
Amazon River, and could not separate the effect of inorganic suspended matter and Gelbstoffe from phytoplankton in the Amazon plume as it drifted along the coast toward the Caribbean during the first half of the year (see MOLLER-KARGF_~,1988). Transects along the axis of the Orinoco's plume showed that pigments decreased exponentially with distance from the delta (Fig. 8). We estimated that concentrations along the plume decreased with a half life of 10-20 days. This suggested that nutrients introduced at the mouth of the river were slowly lost from the plume, and that additional input of new nutrients was small. Losses included spreading and mixing, as well as grazing and sinking of phytoplankton. This simple model did not explain local (pixel-size) maxima seen along the transects, which may argue for noise in the CZCS data or localized nutrient additions. Alternatively, the noise may have been due to losses of particles by localized grazing. From an atmospheric point of view, the noise may have been due to small or thin clouds that escaped detection by the land and cloud-flag algorithm. Since there are no estimates of grazing or sinking in the Caribbean Sea, and because transects do not represent the 3-dimensional nature of the plume, inferences about the dynamics of nutrients based on individual transects were limited. We examined the potential effect of the Orinoco by estimating the phytoplankton crop contained in the plume and comparing it to the river's output of nitrogen. The plume experiences growing, mature, and senescent stages, which are reflected in the shape of the histograms of pigment concentration (Fig.9). Given the young age of the plume on 27 September 1979, the model based on this date is probably the least erroneous. At maturity (October), the plume developed a distinct core of high pigment concentrations, which vanished again as the plume entered the senescent stage (November). At later stages of development, our simple model does not reflect the true history of river discharge nor the dynamics of nutrients within the plume. The model suggested that the Orinoco provided 2-12% of the daily nitrogen requirements of the phytoplankton in the plume (Table 6c). We assumed that upwelling was small during the season of maximum discharge, and in particular, that it was negligible in the areas affected by the plume. This assumption may not hold off the northern coast of Trinidad, but we had no data to explore the hydrography of this region in more detail. We also assumed that there were no additional sources of new nutrient to the plume other than the river. These assumptions may lead to an underestimate of 'f' (see DUGDALEand GOEP,~G, 1967), and imply that the balance of the nitrogen required for daily productivity was provided by recycled nitrogen. 'f' may be further underestimated because it is a surrogate of the EPPLEY,STEWARTand ABBcrrr (1985) formulation for primary productivity (see Table 6a). The result is that the recycling factor 'r" may be grossly overestimated. We examined the importance of recycling by ftrst estimating the time needed to supply the fresh water contained in the body of the plume (Table 6b). The volume discharged during September 1979 was comparable to the estimated volume of the plume at the end of September (Table 6c). Also, the nitrogen discharged during this period was roughly equivalent to the estimated total particulate nitrogen (PN) in the plume (see time needed to balance the PN in the plume with discharged N, tn, in Table 6c). This suggested that little nitrogen had been lost by 27 September 1979. But as the plume expanded and aged, the time needed to balance the standing PN crop with nutrients provided by the river remained relatively constant (Table 6c). This reflected a decrease in average pigment concentrations (Fig. 9) and a loss of nutrients. A loss may be expected because the discharge, and consequently the nutrient input, decreased after October. Also, the plume was progressively diluted offshore with nutrient-poor oceanic water. Assuming that the amount of nutrient supplied by the Orinoco relative to the rate of nutrient
58
MOLt:aR-KARanRet al.
uptake by phytoplankton is an estimate of 'f'. we estimated that nitrogen atoms were recycled 765 times (recycling factor = r = (1-0/f; Table 6c; see EI~LWeand l:'zrv_asoN, 1979; EPPLZY, 1981; I-IARP,ISON,DOUGLAS,FALKOWSga,ROWEand VID~a., 1983). The value of 65 is probably unrealistic, since even in the central oceanic gyres nitrogen is cycled only about 25 times (EPrLL'Y and Pm ~SON, 1979). The lower values, however, are comparable to values found in productive areas such as the Mid-Atlantic Bight (H.~asoN, Doubt.as, F~a~Kowsi,a, Rowe and V ~ , 1983). This suggested that the plume exported a large amount of nutrients while supporting large standing crops. However, the lower values of "r" are probably underestimates, since not all of the total nitrogen discharged by the river is likely to be available to or utilized by the plankton. Clearly, in order to refine nutrient budgets such as the one described above, we need to obtain better data on the concentration and productivity of viable plankton in these river plumes. To take full advantage of the remote sensing technology available in the future, we also have to define the bio-optical characteristics of these waters. To expand our nutrient budgets, we would need to examine the effect of the material in the plume on the submarine light field, on which the phytoplmtkton forming the deep chlorophyll maximum depend. We also need better data on upwelling near the margins and, in general, on biological processes affecting phytoplankton communities in the region (i.e. productivity, sinking, and grazing). A large amount of inorganic and biogenic material is exported to deep waters near the continental margin. However, the amount oftbe production stimulated by nutrient addition at the margins that sinks below the thermocline is unknown. These fluxes help maintain the meridional gradient in properties observed in deep waters of the Caribbean (Mov,v,asoN and Nowi.~, 1982; ATWOOD,FROELICH,Pa..so~, BARCELONAand VmEN, 1979; SV'EVa)Rt.rI',JOHNSONand FLEMING,1942). Our heuristic results suggest that efficient recycling within the river plume delays transport of some carbon and other nutrients to deeper waters of the basin. Nevertheless, materials that sink may be trapped in the basin for periods of at least 50 to 800 years (R.zDFmt~, KzrcrrLrM and Racr-t~s, 1963; K n , ~ , I-IEBt~,Yand G~',~, 1985), and much longer if they are incorporated into sediments. This is long compared to the time scale of increase of CO 2 in the ocean-atmosphere system by fossil fuel burning, deforestation, and cement manufacturing (see ~ A ~ , 1988), and could provide a regional sink for atmospheric CO 2. If we assume that half of the carbon fixed within the plume in a year is trapped within the basin below the thermocline, then approximately 3 to 15 x 105 ton C are removed from the atmosphere. This is a small amount relative to the carbon released annually from the harvest and clearing of the global tropical forests (i.e. less than about 0.01%; see WOODWELL,WHrlq'AKER,REINERS, LIx_~s, D~wlcrn~ and BOTr,m, 1978), but has an important effect on the local geochemistry and carbon cycling through the regional food web. Remote sensors like the CZCS are the only tool capable of providing synoptic estimates of plankton abundance which may help us further understand possible sinks of carbon. In anticipation of interdisciplinary earth observation studies planned for the decade of 1990, we need to develop a comprehensive data base of in situ optical information for the continental margins of the world. This information must be related to phytoplankton biomass or productivity, and to the fate of humic and fulvic compounds, or clay particles, which absorb blue light similar to phytoplankton pigments (cf. DAvxs-CoL~Y and V~rr, 1987; BAKERand S~wr,a, 1982; Y~'crsc~I, 1960). Particular attention should be given to rivers since, even though they are responsible for less than 1% of the new productivity of the world's oceans (HARRXSON, 1980), they are important local nutrient sources and help trace the circulation of nearby oceans.
Pigment distribution in the Caribbean Sea
59
6. ACKNOWLEDGEMENTS The suggestions of TI~OMASMALONEand BILL BOICOURT(University of Maryland, UMCEES), WATSONGREOO (University of South Florida), and two anonymous reviewers for revising the manuscript are greatly appreciated. We thank JOHN SISSALA,MInE DOLINEand RICaARDSn,~ (Nimbus Operations office, General Electric) for their help in selecting the imagery. JUDY CnEN, MIKE DARZI and JIM FmBSZONE(General Sciences Co.) implemented the image analysis software (SEAPAK). OHER--OTEC project reports on chlorophyll observations off Puerto Rico were kindly provided by JOROECORREDOR and PAUL YOSmOKA (Department of Marine Sciences, University of Puerto Rico, Mayaguez). This work was supported by the Ocean Processes Branch at NASA Headquarters and by the NASA Graduate Student Researcher's Program at the Goddard Space Flight Center (Grant No. NGT 21-002-822). University of Maryland (UMCEES) publication No. 1983. 7. REFERENCES
ALBERrS,J. (1973) Reportof"DISCOVERER" cruise. Nutrient chemistry. Data Report S.C.O.R. Expedition, May 1970. Volume 1. University of California, SIC) Ref. 73-16, pp. L1-LI2. ANDRES, J. M., G. A. FEBRES and L. E. HERRERA(1979) Circulacion geostrofica fxente a Venezuela, Mar Caribe Suroriental. Boletin Instituto Oceanografico Universidad de Oriente, 18, 115-126. ANTXA,N. J., C. D. MCALLIS'rgR,T. IL PARSONS,K. STEPr~ENSand I. D. H. S'rRICKLAND(1963) Further measurements of primary production using a large-volume plastic sphere. Limnology and Oceanography, 8, 166-183. ATWOOD,D. K. (1976) La oceanografia regional con respecto a los problemas actuales y futuros de la contaminacion y de los recursos vivos - CARIBE. Intergovernmental Oceanographic Commission, FAO, Reunion internacional de Trabajo IOC/FAO/UNEP/IWMPCAR/8, Paris, 18 November 1976, pp. 1-36. AXWOOD,D. K., P. N. FROELXCH,M. E. PlrsoN, M. J. BARCELONAand J. L. VmE~ (1979) Deep silicate content as evidence of renewal processes in the Venezuela Basin, Caribbean Sea. Deep-Sea Research, 26A, 1179-1184. BAreD, I. E. (1973) Report of "DISCOVERER" cruise. Chlorophyll a and Phaeopigments. Data Report S.C.O.R. Expedition, May 1970. Volume 1. University of California, SIO Ref. 73-16, lap. C1-C25. BAKER, K. S. and IL C. SMTm (1982) Bio-optical classification and model of natttral waters. Limnology and Oceanography, 27,500-509. BALLESrER, A. and R. MAROALEF(1965) Produccion primaria. Memoria, Sociedad de Ciencias Naturales La Salle, 25, 209-221. BAtqSE, K. (1974) On the interpretation of data for the carbon-to-nitrogen ratio of phytoplank-ton. Limnology and Oceanography, 19, 695-699. BANSE, IC and C. R. McCLAxN(1986) Winter blooms of phytoplankton in the Arabian Sea as observed with the Coastal Zone Color Scanner. Marine Ecology - Progress Series. Inter-Research, 34, 201-211. BARALE, V., C. R. MCCLAtN and P. MALArqOrrE-RIzzoLI(1986) Space and time variability of the surface color field in the northern Adriatic Sea. Journal of GeophysicalResearch, 91:C11, 12957-12974. BEAROSLr~Y,R. C. and J. HART (1978) A simple theoretical model for the flow of an estuary onto a continental shelf. Journal of Geophysical Research, 83,873-883. BEERS,J. IL, D. M. STEVEr~and J. B. l.~wxs (1968) Primary productivity in the Caribbean Sea off Jamaica and the tropical North Atlantic off Barbados. Bulletin of Marine Science, 18, 86-104. BERMUDABIOLOOICALSTATION(1965) Primary productivity in the tropical North Atlantic offBarbados and the Caribbean Sea off Jamaica. Final Report submitted to the Biology Branch, Office of Navai Research, St. Georges's West, Bermuda, 130pp. B~'rz~R, P. 1L, D. W. EOOXMAr~,K. L. CARDER,D. R. IC~_SrERand S. B. BErZ~R (1977) Seasonal patterns in suspended calcium carbonate concentration during the dry and wet seasons in the eastern Caribbean. In: The Fate of Fossil Fuel CO: in the Oceans, N. R. ANDERSENand A. MALAHOPF,editors, Plenum Press, 63-79. B ORSTAD,G. A. (1982a) The influence of the meandering Guiana Current and Amazon River discharge on surface salinity near Barbados. Journal of Marine Research, 40,421-434. B ORSTAO,G. A. (1982b) The influence of the meandering Guiana Current on surface conditiom near Barbados - temporal variations of Trichodesmium (Cyanophyta) and other plankton. Journal of Marine Research, 40, 435-452. BRows, O. B., IL H. EVANS,J. W. BROWN,H. IL GORDON,R. C. SMIThand IC S. BAKSR(1985) Phytoplankton blooming off the U. S. east coast: a satellite description. Science, 229, 163-167. BURKHOLDER,P. R., L. M. BURKHOLDERand L. R. ALMODOVAR(1967) Carbon assimilation of marine flagellate blooms in nerilSc waters of southern Puerto Rico. Bulletin of Marine Science, 17, 1-15. BUSALACCMt, A. J. and J. PICAUT(1983) Seasonal variability from a model of the tropical Atlantic Ocean. Journal of Physical Oceanography, 13, 1564-1588.
MOLLER-KARQER
60
et al.
G. W. and G. D. GRICE (1967) Influence of the Amazon River outflow on the ecology of the western tropical Atlantic. Il. Zooplankton abundance, copepod distribution, with remarks on the fauna of low-sali& areas. Journal of Marine Research, 25,84-94. CAMPBELL, J. W. and W. E. E~AIAS (1985) Spatial patterns in temperature and chlorophyll on Nantucket Shoals from airborne remote sensing data, May 7-9, 198 1. Journal of Marine Research, 43, 139- 161. CARDER, K. L., R G. STEWARD. D. L. BETZER and J. M. PROSPERO (1986a) Dynamics and composition ofparticles from an aeolian input event to the Sargasso Sea. Journal of Geophysical Research, 91 Dl, 1055-66. CARDER, K. L., R. G. STEWARD, J. H. PAUL and G. A. VARGO (1986b)Relationships between chlorophyll and oceancolor constituentsastheyaffectremote-sensingretlectancemodels. LimnologyandOceanography,31(2).403413. CARLSON, T. N. and J. M. PROSPLIRO (1972) The large scale movement of Saharan air outbreaks over the north equatorial Atlantic. Journal of Applied Mereorology, 11.283-297. CARPENTER. E. J. (1983) Niuogen fixation by marine OsciUaroria (Trichodesmium) in the world’s oceans. In: Nitrogen in rhe Marine Environmenr, E. J. CARPENTER and D. G. CAPONE, editors, 65-103. CARPENTER, E. J. and C. C. PRICE (1977) Nitrogen fixation, distribution, and production of Oscillaroria (Trichadesmium) spp. in the western Sargasso and Caribbean Seas. Limnology and Oceanography, 22,60-72. CBRVIGON, F. and P. J. MARCANO (1965) Zaoplsnkton Memoria, Sociedad de Ciencias Naturales Lm Salle, 25,263-360. CHAO, S.-Y. and W.C. Bo1cou~~(1986)Gnsetofestuarineplumes. JournalofPhysicalOceanography, 16,2137-2149. CORREDOR, J. E. (1976) Aspects of phytoplardcton dynamics in the Caribbean Sea and adjacent regions. In: Cooperative Invesrigations of the Caribbean and Adjacent Regions (CIGAR) - II. Symposium on Progress in Marine Research in the Caribbean andAdjacent Regions, Caracas, 12-16 July 1976, FAO Fisheries Report NO. 200, 101-l 14. CORREDOR, J. E. (1979) Phytoplankton response to low level nutrient enrichment through upwelling in the Colmnbian Caribbean Basin. Deep-Sea Research, 26A. 731-741. CULLBN, J. J. (1982) The deep chlorophyll maximum: comparing vertical profiles of chlorophyll a. Canadian Journal of Fisheries and Aquatic Science, 39,79 1. CURL, H. (1960) Primary production measurements in the north coastal waters of South America. Deep-Sea Research,
CALEP,
7. 183-189.
DAVIS-C•
LL~Y,R. I. and W.N. V~~~(1987)Absorbtionoflight by yellow substanceintieshwaterlakes. Limnologyand Oceanography, 32,416425. DELPTHYDRAULICS LABORATORY (1962) Demerara coastal investigation: Report on siltation of Demerara Bar Channel and coastal erosion in British Guiana. The Nerherlands Del&, v. l-3,240 pp. DBMASTFZR, D. J., G. B. KNAPP and C. A. NITTROUHR (1983) Biological uptake and accumulation of silica on the Amazon continental shelf. Geochimica et Cosmochimica Acta, 47, 1713-1723. DENMAN, K. L. and M. R. ABBOTT (1988) Time evolution of surface chlorophyll patterns from cross-spectrum analysis of satellite color images. JOWMI of Geophysical Research, 93 C6,6789-6798. DEUSER. W. G.. F. E. M~~LLIX-LARGER and Ch. H~MLEBEN (1988) Temporal variations of psrticle fluxes in the deep subtropical and mopical North Atlantic: Eulerianversus Lagrangian effects. Journal of GeophysicalResearch, 93 C6.6857-6862.
DOTY, M. S. and N. OOURI (1956) The island mass effect. J. Conreilpenn. inl. E.xpIor. Mer., 22, 33-37. DUGDALE, R. C. and J. J. GOHUNG (1967) Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography. 12.685695. DUNCAN, C. P., S. G. SCHLADOW and W. G. WILLIAMS (1982) Surface currents near the Greater and Lesser Antilles, Int. Hydrog. Rev., Monaco., LIX. 2,67-78. EISMA, D., S. J. VAN DIZR GAAST, J. M. MARTIN and A. J. THOMAS (1978) Suspended matter and bottom deposits of the Grinoco delta: Turbidity, mineralogy, and elementary composition Netherlands Journal ofSea Research, 12, 224-25 1. EPPLEY,R W. (1981)Autotrophicproductionofparticulatematter. In:AnalysisofmarineEcosystems,A. R. LONGHURST, editor, 343-361. EPPLEY, R. W. and B. J. P~TIZRSON (1979) Particulate organic matter 5ux and planktonic new production in the deep ocean. Nature, 282, 677-680. EPPLEY. R. W.. E. STEWART, M. R. ABBOTT and U. HEYMAN, (1985) Estimating ocean primary production from satellite chlorophyll. Introduction to regional differences and statistics for the Southern California Bight. Journal of Plankton Research, 7.57-70. ESAIAS. W. E. (1980) Remote sensing of oceanic phytoplankton: present capabilities and future goals. In: Primary Productivity in rhe Sea, P. G. FALKOWSKI, editor, Plenum, New York, 321-337. Erra~,P. C., P. J. LAMB andD.H. Poar1s(l987)Heatand heshwaterbudgets ofthecaribbean Seawithrevisedestimates for the cenb-al american seas. Journal of Physical Oceanography, 17, 1232-1248. FAO (1986) Yearbook of Fishery Statistics. Catches and Landings. 1984. Vol. 58, Food and Agriculture Organization of the United Nations, Rome, 89-91. FASHAM, M. J. R. (1978) The application of some stochastic processes to the study of plankton patchiness. In: Spatial Pattern in Plankton Communiries. J. H. STEELE, editor, 131-156. FEBRES-GRTBGA, G. and L. E. HIZRREIRA (1976) Caribbean Sea circulation and water mass transports near the Lesser k’Uilles. Boledn Institute Oceanografico Universidadde Orienre, Cumana, Venezuela, lS,83-96.
Pigment disffibution in the Caribbean Sea
61
Fm.OMAN,G.C., D. CLARKand D. I-Ixu,m~ (1984) Satellite color observations of the phytoplankton distribution in the eastern equatorial Pacific during the 1982-83 El Nino. Science, 226, 1069-71. Ftsrn~, J., R. D o m u ~ and H. GR.~SSL(1986) Factor analysis of mdtispectral radiances over coastal and open ocean water based on radiative transfer calculations. Applied Optics, 25, 446-456. FRom.lc~, P. N., D. IC As'WOODand G. S. GIr~B(1978) The influence of Amazon River water on surface salinity and dissolved silicate concentration in the Caribbean Sea. Deep-Sea Research, 25, 735-744. Foruor,~, J. (1965) Condiciones meteorologicas e hidrograficas de los mares adyacentes a Venezuela. 1962-1963. Mernoria, Sociedad de Ciencias Naturales La Salle, XXV, 11-38. GraBs, IL J. (1980) Wind-controlled coastal upwelling in the western equatorial Atlantic. Deep-Sea Research, 27A, 857866. GmEs, H. (1972) Carta Pesquera de Venezuela (1): Areas del Nororiente y Guayana. I-ino. GmF.s,editor, Monografia No. 16, Fundacion La SaUe de Ciencias Naturales, Caracas, Venezuela. 328 pp. Gnx,v.s, H. (1982) Curta Pesquera de Venezuela (2): Areas Central y Occidental. Hno. Gl~.S, editor, Monografla No. 27, Ftmdacion La Salle de Ciencias Naturales, Caracas, Venezuela. 227 pp. GORDON,A. L. (1967) Circulation of the Caribbean Sea. Journal of Geophysical Research, 72, 6207-6223. G omaor~,H. R. and D. K. C ~ (1981) Clear water radiances for atmospheric correction of Coastal Zone Color Scanner Imagery. Applied Optics, 20, 4175--4180. GORDOr4,H.R., D. K. CLARK,J. L. M ~ and W. A. Hoxas (1980) Phyloplankton pigments derived from the Nimbus7 CZCS: comparisons with surface measurements. Science, 210, 63-66. GoRooN, H. R., D. K. CLARK, J. W. BROWN,O. B. BROWNand R. H. EVANS(1982) Satellite measurement of the phytoplankton pigment concentration in the surface waters of a warm core GttlfStream ring. Journal ofMarine Research, 40, 491-502. GORDON,H. R., D. K. CLARK,J. W. BROWN,O. B. BROWN,R. H. EVANSand W. W. Baom,n~ow (1983a) Phytoplankton pigment concentrations in the Middle Atlantic Bight: Comparison of ship determinations and CZCS estimates. Applied Optics, 22, 20-35. GoaooN, H. R., J. W. BROWN,O. B. BROW~, R. H. Ew~Nsand D. K. CLARr (1983b) Nimbus 7 CZCS: reduction of its radiomea-ic sensitivity with time. Applied Optics, 22, 3929-3931. HAMm'ros, S. K. and W. M. LEwis., Jr. (1987) Causes of seasonality in the chemistry of a lake on the Orinoco River floodplain, Venezuela. Limnology and Oceanography, 32, 1277-1290. I-[gacP.xv~, P. E., R. W. B~ODYand P. R. BtntKnOLD~ (1970) A study of phytoplankton in the Lesser Antilles region. Bulletin of Marine Science, 20, 331-349. t-IAmtlSON,W. G. (1980) Nutrient regeneration and primary production m the sea. In: Primary Productivity in the Sea, P. G. F~a.rowsKJ,editor, Plenum Press, 433-460. HARRISON,W. G., D. DOUGLAS,P. FAtXOWSKX,G. ROWEand J. VmAL(1983) Summer nutrient dynamics of the middle Atlantic bight: nitrogen uptake and regeneration. Journal of Plankton Research, 5, 539-556. I'-Ir:auva~,G. W. and R. C. RHODES(1987) Numerical simulation of wind forced seasonal and interannual variability of the transport through the Caribbean Sea. EOS, Vol. 68, No. 16, Abstract O31A-07, p. 337. I4a~tn~, G. W., T. H. KINDER,J. H. Ata.m'~m and H. E. Hta~LBta~a"(1982) A numerical model of eddy generation in the southeastern Caribbean Sea. In: Hydrodynamics ofSemi-EnclosedSeaa, J. C. J. Nmou-t.(ed.), Elsevier, 299328. ~ , L. E. and G. FmR.eS43RrEOA (1975a) Procesos de surgencia y de renovacion de aguas en la Fosa de Cariaco, Mar Caribe. Boletin Instituto Oceanografico de la Universidad de Oriente, 14, 31-44. ~ , L. E. and G. FmaVaES-ORaaZ~A(1975b) Kinematics of the wind-generated velocity field in the surface waters off eastern Venezuela in the Caribbean Sea. Boletin Instituto Oceanografico de la Universidad de Oriente, 19, 165-186. HOLLIES, P. M., M. VIOLLma,D. S. HAm3ou~,P. CAMVS and M. C~Ac~r~-Prm.t~p~ (1983) Satellite and ship studies of coccolithophore production along a continental shelf edge. Nature, 304, 339-342. HOI~L, J. D., V. E. KousKYand M. KAOAr~O(1986) Atmospheric conditions in the Atlantic sector during 1983 and 1984. Nature, 322, 248-251. HtJLat~T, E. M. and N. CoRwIr~(1969) Influence of the Amazon River outflow on the ecology of the western tropical Atlantic. HI. The planktonic flora between the Amazon River and the Windward Islands. Journal of Marine Research, 27, 55-72. I s E ~ , H. J. and L. HAssr~(1985) The Bunker Climate Atlas of the North Atlantic Ocean. Volume 1: Observations. Springer-Verlag. 218 lap. Jm~.Ev, S. W. (1980) Algal pigment systems. In: Primary Productivity in the Sea, P. G. FAt~owsra, editor, Plenum Press, 33-58. KATZ, E. J. (1987) Seasonal response of the sea surface to the wind in the equatorial Atlantic. Journal of Geophysical Research, 92 C2, 1885-1893. Kn~D,R. and F. S ~ (1979) Influence of Amazon River discharge on the marine production system offBarbados, West Indies. Journal of Marine Research, 37,669-681. Kn~SrV.AD, H. and L. B. SLOBODr.m(1953) The size of water masses containing plankton blooms. Journal of Marine Research, 12, 141-147. Kn,ro~, T. H. (1983) Shallow currents in the Caribbean Sea and Gulf of Mexico as observed with satellite-tracked drifters.
62
MOLL~R-KAg~ et al.
Bulletin of Marine Science, 33,239-246.
KINDER,T. H., G. W. H~BURNand A. W. GREEN(1985) Some aspects of the Caribbean circulation. Marine Geology, 68, 25-52. KOBLEr~z-MIsnKe,O. J., V. V. VOLKOVICSKY and J. G. IG~BAr~OVA(1970) Plankton primary production of the world ocean. In: Scientific Exploration of the Southern Pacific, W. S. WOOSTER,editor, National Academy of Science, Washington, DC, 183-193. LAWS, E. A., G. R. DITULLIOand D. G. REDALJE(1987) High phytoplankton growth and production rates in the North Pacific subtropical gyre. Limnology and Oceanography, 32,905-918. LEwxs, JR., W. M. and J. F. SAUNDERS,III (1988, in press) Concentration and transport of dissolved and suspended substances in the Orinoco River. Biogeochemistry. MACKAS,D. L., K. L. DENStANand M. R. A~ aoT'r (1985 ) Plankton patchiness: biology in the physical vernacular. Bulletin of Marine Science, 37,652-674. IVlxLor~E,T. C. ( 197 I) The relative importance ofnannoplankton and netplankton as primary producers in tropical oceanic and neritic phytoplankton communities. Limnology and Oceanography, 16,633~539. MALONE, T. C. (1973) Report of "DISCOVERER" cruise. Nanoplankton (<22) and net plankton (>22) chlorophyll concentrations in mg m -~. Data Report S.C.O.1L Expedition, May 1970. Volume 1. University of California, SIO Ref. 73-16, p. El-E7. IVIARGALEF,R. F., CERVIGONand G. YEPEZT (1960) Exploracion preliminar de las caracteristicas hidrograficas y de la distribucion del fitoplancton en el area de la Isla Margarita (Venezuela). Meraorias Sociedad de Ciencias Naturales La Salle, Caracas, Venezuela, 20, 211-221. MAROALEF,R. (1965) Composicion y distribucion del fitoplancton. Memorias. Sociedad de Ciencias Naturales La Salle, XXV, 141-205. MARRA,J. (1980) Vertical mixing and primary production. In: Primary Productivity in the Sea, P. G. FALKOWSKt(editor), Plenum Publishing Co., New York, 121-137. MARRA,J. and K. R. I"I~tNEMAr~N(1987) Primary production in the North Pacific central gyre: Some new measurements based on x4C.Deep-Sea Research, 34, 18-21. MAZEIKA,P. A. (1973) Circulation of water masses east of the lesser Antilles. Deutsche Hydrographische Zeitschrifi, 2, 49 -73. MAZEIKA,P. A., T. H. KINDERand D. A. BURNS(1983) Measurements of subtidal flow in the Lesser Antilles passages. Journal of Geophysical Research, 88 C7, 4483-4488. MAZPARROTE, S. (1967) Observaciones preliminares sobre la composicion y aspectos del "Turbio" o marea roja en las costas orientales de Venezuela. Memorias Sociedad Ciencias Naturales La Salle, 76, 37-45. MXZPARROTE,S. (1970) Composicion del Turbio o Marea Roja en las costas orientales de Venezuela. Memorias Sociedad Ciencias Naturales La Salle, 86, 102-121. MCCLAIN, C. 1~, L. J. PIETRAF.ESA&/ld J. A. YODER(1984) Observations of Gulf Stream-induced and wind-driven upwelling in the Georgia Bight using ocean color and infrared imagery. JournalofGeophysicalResearch, 89 C3, 3705-3723. MEAD.~,R. H., C. F. NORDIN,Jr., D. PER~ZHZRNANDEZ,A. M~IA B. and J. M. PEREZGODOY(1983) Sediment and water discharge in Rio Orinoco, Venezuela and Colombia. In: Proceedings of the Second International Symposium on River Sedimentation, 11-16 October, 1983, Nanjing, China. Water Resources and Electric Power Press, Beijing, China, 1134-1144. MXLLIMAN,J. D. and R. H. M~ADE(1983) World-delivery of river sediment to the oceans. The Journal of Geology, 91, 1-21. MILLIMAN,J. D., J. BU'fENKO,J. P. BARBOTand J. I-I~DBERG(1982) Depositional patterns of modern Orinoco/Amazon muds on the northern Venezuelan shelf. Journal of Marine Research, 40,643-657. MIRO, M. (Unpublished, 1969) Productividad primaria y pigmentos fotosintetizadores del Caribe Sur, Venezuela. Manuscript archived at Em MAR,Fundacion La Salle, Ptmta de Piedras, Isla Margarita, Estado Nueva Esparta, Venezuela. MOLINARI,R. L., M. SPILLANE,I. BROOKS,D. ATWOODand C. DUCKE'fT(1981) Surface curlent in the Caribbean Sea as deduced form Lagrangian observations. Journal of Geophysical Research, 86 C7, 6537-6542. Mooa~, W. S., J. L. SARMmNrOand R. M. K~Y (1986) Tracing the Amazon component of surface Atlantic water using 228Ra, salinity and silica. Journal of GeophysicalResearch, 91 C2, 2574-2580. MOREL, A. and L. PRmUR(1977) Analysis of variations in ocean color. Limnology and Oceanography. 22, 709-722. MORRIs,I., A. E. SMTXHand H. E. GLOVER(1981) Products of photosynthesisin phytoplankton offthe Orinoco River and in the Caribbean Sea. Limnology and Oceanography, 26, 1034-1044. MORRISOr~,J. M. and W. D. NOWLIN,Jr. (1982) General distributions of water masses within the eastern Caribbean Sea during winter of 1972 and fall of 1983. Journal of Geophysical Research, 87 C6, 4207-4229. MOLLER-KAR~ER,F. E. (1988) Pigment variability in the Caribbean Sea: A study using the Coastal Zone Color Scanner. Ph.D. Dissertation, Horn Point Environmental Laboratories, University of Maryland, 218 pp. MOLLER-KARonR,F. E., C. R. McCLAINand P. L. RaCHARDSON(1988) The dispersal of the Amazon's water. Nature, 333, 56-59. NITTROUER,C. A., T. B. CURTINand D. J. DEMASTI~R( 1986) Concenlxation and flux of suspended sediment on the Amazon continental shelf. Continental Shelf Research, 6, 151-174.
Pigment distribution in the Caribbean Sea
63
NOF, D. and D. B. OLsorq(1983) On the flow through broad gaps with application to the Windward Passage. Journal of Physical Oceanography, 13, 1940-1956. OHER-OTEC S.rUDY (1980) Data Report (compiled and edited by J. M. Lopez). Center for Energy and Environment Research. University of Puerto Rico. U.S. Department of Energy. Bimonthly cruise reports, 1980. 80 pp. OKOBO, A. (1980) Diffusion and ecologicalproblems: mathematical models. Springer-Verlag, New York, 254 pp. OLTMAN,R. E. (1967)Reconnaissance investigations of the discharge and water quality of the Amazon. Atas. Syrup. Biota Amazonica 3 (Limnologia), 163-185. PARSONS,T. P,., M. TAKAHASHIand B. HARGRAVE(1977) Biological Oceanographic Processes. 2nd Edition, Pergamon International Library, 332 pp. PELAEZ, J. and J. A. McGOWAN (1986) Phytoplankton pigment patterns in the California Current as determined by satellite. Limnology and Oceanography. 31(5), 927-950. PE.rERSON,B. J. (1980) Aquatic primary productivity and the )4C-CO2method: a history of the productivity problem. Ann. Rev. Ecol. Syst. 11,359-385. PHILANDER)S. G. H. and R. C. PACANOWSKI(1986) A model of the seasonal cycle in the ta'opical Atlantic. Journal of Geophysical Research, 91 C12, 14192-14206. PLA'r'r T. and A. W. HERMAN(1983) Remote sensing ofphytoplanktonin thesea: surface-layer chlorophyll as an estimate of water-column chlorophyll and priniary production. International Journal of Remote Sensing, 4, 343-351. PLA.rr, T. and S. SATHYENDRANA'rH(1988) Oceanic primary production: estimation by remote sensing at local and regional scales. Science, 241, 1613-1620. PROSWRO,J. M., R. T. NEEs and M. UEraATStJ(1987) Deposition rate of particulate and dissolved aluminum derived from Sahara dust in precipitation at Miami, Florida. EOS, 68, 46, p. 1602. PROSPnRO,J. M. E. BONA.r.rl,C. SHUI3ER.rand T. N. CARLSON(1970) Dust in the Caribbean atmosphere traced to African dust storm. Earth Planetary Science Letters, 9, 287 293. PROSPERO, J. M. and T. N. CARLSON(1972) Vertical and aerial distribution of Saha.ran dust over the western equatorial Adantic Ocean. Journal of Geophysical Research, 77, 5255-5265. RAMANATHAN,V. (1988) The greenhouse theory of climate change: a test by an inadvertent global experiment. Science, 240,257-364. REd,toLD, A. C. (1955) The hydrography of the Gulf of Venezuela. Deep Sea Research, Supplement, 3. REDFmLD,A. C., B. H. K~TCHUMand F. A. POCHAROS(1963) The influence of orgamsms on the composition of sea water. In: The Sea. M. N. HILL,editor, Chapter U, interscience, New York, 26-77. RICHARDS,F. A. (1960) Some chemical and hydrographic observations along the north coast of South America. I. Cabo Tres Plmtas to Curacao, including the Cariaco Trench and the Gulf of Cariaco. Deep-Sea Research, 7, 163182. RICHARDSON,P. L. and T. K~ MCKEE(1984) Average seasonal variation of the Atlantic equatorial currents from historical shipdrifts. Journal of Physical Oceanography, 14, 1226-1238. RINE, J. M. and R. N. GINSBURG(1985) Depositional facies of a mud shoreface in Suriname, South America - A mud analogue to sandy, shalJow-marine deposits. Journal of Sedimentary Petrology, 55, 633-652. ROEMMICH, D. (1981) Circulation of the Caribbean Sea: a well-resolved inverse problem. Journal of Geophysical Research, 86 C9, 7993-8005. RV.rnER, J. H. (1969) Photosynthesis and fish production in the sea. Science, 166, 72-76. Rv .rnER,J. H., D. W. MENZ~'Land N. CORWIN(1967) Influence of the Amazon River outflow on the ecology of the western tropical Atlantic. I. Hydrography and nutrient chemistry. Journal of Marine Research, 25, 69-83. SAIJO, Y. and N. NAKAMOTO(1973) Report of "DISCOVERER" cruise. In situ photosynthesis. Data Report S.C.O.R. Expedition, May 1970. Volume i. University of California, SIO Ref. 73-16, p. A1-AI5. SELLNER, K. G. (1981) Primary productivity and the flux of dissolved organic matter in several marine environments. Marine Biology, 65, 101-112. SIOLLH. (1975)Tropical river: the Amazon. In: River Ecology. B. A. WRE'r'rON,editor, BlackweU Scientific Publication, London, 18,461-488. SMITH, R. C. and K. S. BAKER (1978) The bid-optical state of ocean waters and remote sensing. Limnology and Oceanography, 23,247-259. SOKAL,R.R. and F. J. ROrtLE(1981) Biometry. Second edition, W. H. Freeman and Co., 859 pp. STALCUP,M. G. and W. G. MaeTCALF(1972) Current measurements in the passages of the Lesser Antilles. Journal of Geophysical Research, 77, 1032-1049. STEELE, J. H. (1978) Some comments on planktnn patches. In: SpatialPattern in Plankton Communities. J. H. STEELE, editor, Plenum, New York, 1-20. STEELE, J. H. and E. W. HENDERSON(1979) Spatial patterns in North Sea Plankton. Deep-Sea Research, 26, 955-963. STORM, B. (1986) Correction of the sensor degradation of the Coastal Zone Color Scanner on Nimbus-'/. ESA/EARSeL Symposium on Europe from Space, Lyngby, DK, 25-28 June 1986. ESA SP-258, December 1986. SVERJKOVSKY,J. (1988) Sea surface flow estimation from Advanced Very High Resolution Radiometer and Coastal Zone Color Scanner satellite imagery: a verification study. Journal of Geophysical Research, 93 C6, 6735-6743. SVERDRtrP, H. U., M. V. JOHNSONand R. H. FLEMING(1942) The oceans: their physics, chemistry andgeneral biology. Prentice-Hall, 1087 pp.
64
MOLLER-KARGERet al.
UNESCO (1979) Discharge of selected rivers of the world. Volume I11 (part rlI). Mean monthly and exta-eme dischaxges (1972-1975), p. 54-70. VAN AND~L,T. H. and H. POSTMA(1954) Recent sediments of the Gulf ofParia. Vehr. der K. Nederl. Adad. Van Wetensch., Afd. Natuurkunde, Eerste Reeks, 20, 1-244. WALSa, J. J. (1988) On the nature of continental shelves. Academic Press, 508 pp. WALSH,J. J. (1977) A biological sketchbook for an eastern boundary current. In: The Sea, Volume 6, E. D. Go-Dsm~, I. N. McCAvE, J. J. O'BamN and J. H. S~sLS, editors, Wiley, New York, 923-968. WALSH,J. J., D. A. DI~'rmu.E and W. E. ESAIAS(1987) Satellite detection ofphytoplankton export fi'om the mid-Atlantic Bight during the 1979 spring bloom. Deep-Sea Research, 34, 675-703. W~LLS,J. T. and J. M. COLEMAr~(1981) Physical processes and fine-grained sediment dynamics, coast of Surinam, South America. Journal of Sedimentary Petrology, 51, 1053-1063. WtLKINSON, C. R. (1987) Interocean differences in size and nutrition of coral reef sponge populations. Science, 236, 1654-1657. Wtt.LIAMS, W. G. (1986) Lagrangian measurements of surface circulation near Puerto Rico. Journal of Physical Oceanography, 16, 1715-1719. WILSON,R. E., A. OKUBOand W. E. ESAIAS(1979) A note on time-dependent speclra for chlorophyll variance. Journal of Marine Research, 37,485-491. WOOD, E. J. F. (1966) A phytoplankton study of the Amazon region. Bulletin of Marine Science, 16, 102-123. WOODWELL,G. M., R. H. WHITTAKER,W. A. R~lr4EaS, G. E. LIKBr~S,C. C. DELWICHEand D. B. BOTKIN.1978. The biota and the world carbon budget. Science, 199, 141-146. WORTmrqG'rON,L. V. (1976) On the NorthAtlantic circulation. Johns Hopkins University Press, Baltimore, Md., 120pp. Wr~OBLBWSKI,J. S. and J. O'BRmN (1976) A spatial model of phytoplankton patchiness. Marine Biology, 35, 161-175. WUST, G. (1964) Stratification and circulation in the Antillean-Caribbean basins. Part 1. Spreading and mixing of the water types. Columbia University Press, New York, 201 pp. YErqTSCr~,C. S. (1960) The influence of phytoplankton pigments on the colour of sea water. Deep-Sea Research, 7, 1-9. YODER, J. A. and S. S. BISHOP (1985) Effects of mixing-induced i~adiance fluctuations on photosynthesis of natural assemblages of coastal phytoplankton. Marine Biology, 90, 87-93. YODEg, J. A., C. R. McCLAIN, J. O. BLANTONand L.-Y. OEr (1987) Spatial scales in CZCS-chlorophyll imagery of the southeastern U.S. continental shelf. Limnology and Oceanography, 32,929-941. YOSHIOKA,P. M., G. P. Owr~N and D. PESAi'qTE(1985) Spatial and temporal variations in Caribbean zoopla.n_kton near Puerto Rico. Journal of Plankton Research, 7,733-751.
0079 - 6611/89 $0.00 + .50 © 1990 Pergamon Press pie,
Printed in Great Britain. All rights reserved.
Pigment distribution in the Caribbean Sea: Observations from space F. E. MOLL':~-KARoER", C. R. M c C t . A I N ' " , T. R. FISrIE~°'" , W . E. ESA.tAS'" a n d R. VARELA ....
"Horn Point Environmental Laboratories, University of Maryland, (Current Address and Affiliation: Department of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, EL 33701, U. S. A.) "NASA, Code 671, Goddard Space Flight Center, Greenbelt, MD 20771, U. S. A. ""Horn Point Environmental Laboratories, University of Maryland, Box 775 Cambridge, MD 21613, U. S. A_ .... Fundacion La Salle de Ciencias Naturales, Estacion de lnvestigaciones Marinas de Margarita, Apartado 144, Porlamar, Estado Nueva Esparta, Venezuela
Abstract - - The Caribbean is a semi-enclosed tropical sea which is generally considered oligotrophic, but that is influenced by nearly 20% of the annual discharge of the world's rivers (Amazon and Orinoco Rivers) and by seasonal upwelling along the southern margin. To investigate the role of these nutrient sources on the productivity of the region, we mapped the distribution of pigments in the eastern Caribbean (east of 80°W) using a series of Coastal Zone Color Scanner (C'ZCS) images collected between November 1978 and December 1982. Five additional images were examined for the period 1983-1986. The images revealed a seasonal cycle in the spatial structure of neax-surface pigment. During January-May, there were high pigment concentrations (> 0.5 mg m ~) along the continental margin (south of 14°N), where upwelling occurred. Very little pigment (< 0.2 mg m a) was found in the northern half of the Caribbean at this time. The fxequency of upwelling-related blooms decreased after July, but the seasonally-expanding plume of the Orinoco River dispersed pigment over a large area of the Caribbean (> 3x 10s kin2). This plume reached Puerto Rico around September-October and drifted westward, slowly losing its color signature. We estimate that the discharge of the Orinoco contributes 2-12% of the daily nitrogen requirements of the phytoplankton growing in the river plume, and leads to the fixation of 7-29x105 tons of carbon per year. The rest of the nitrogen demand appears to be met by nitrogen cycling. The large-scale (> 100 km) pigment distribution patterns in the Caribbean Sea seem to be conlxoUed by wind stress, flux of water through the basin, and river discharge. Westward advection of Atlantic water probably dominates the flow during the first half of the year, restricting the dispersal of blooms to the southern half of the Caribbean while flushing the central and northern portions. As the influx of Atlantic water decreases in the second half of the year, local Elonan transport driven by the trade winds b e c o m e s dominant and surface waters drift northwestward throughout the basin. The seasonal sequence of changes in pigment disu'ibution patterns was consistent from year to year except in 1980 and 1984, when wind conditions in the Caribbean were anomalous. Close scrutiny of the 4 years of CZCS images did not reveal any evidence of large-scale (> 300 kin) eastward-flowing currents in the cena'al Caribbean. This supports the view that previous observations of countercurrents were based on partial sampling of eddies of 100-250 km diameter.
23
24
MOLLr~R-KARGERet al.
CONTENTS I. 2.
3.
4.
5.
6. 7.
Introduction Site Description 2.1. Physical setting 2.2. Distribution of particles and phytoplanton 2.3. Where does the Amazon water go ? Methods 3.1. CZCS imagery 3.2. Regional studies 3.3. Possible sources of error 3.3.1. Calibration 3.3.2. Atmospheric effects 3.3.3. Sunglint 3.3.4. Turbidity 3.3.5. Floating blue-greens 3.3.6. Coccofithophores 3.3.7. Subsurface pigment maxima Results 4. I. Comparison with ship data 4.2. Spatial arrangement of pigments 4.3. Nutrient dynamics in the Orinoco River plume Discussion 5.1. Spatial structure of pigments and environmental forcing 5.2. Role of the Orinoco as a nutrient source Acknowledgements References
24 25 25 28 30 30 30 32 32 32 39 39 39 39 39 39 40 40 43 48 54 54 56 59 59
I. INTRODUCTION "Great indications are these that this is Paradise, because the place conforms to the opinion of the holy and sane theologians, and even the signs conform [sic], that I never read nor heard of so much fresh water entering inw the neighboring saline like this.... And if it did not originate in Paradise, this seems an even greater marvel, because I don' t believe that there is knowledge in the world of a river so large and so deep." - Christopher Columbus (From a letter sent to the King and Queen o f Spain on 18 October 1498from Hispaniola. Here Columbus described the Gulf of Paria, located between Trinidad and the mainland, and the effect of the discharge of the Orinoco River on this region)
The enormous amount of fresh water discharged into the ocean by the tropical rivers of South America helped Columbus determine that he had reached a new continent. But even though the combined discharge of the Amazon and Orinoco Rivers represents about 20% of the annual global river discharge, very little is known about the fate of this water and of particles associated with it (Moor~E, Snmva~yro and KEY, 1986; M0ULER-K~G~, McCt.A~ and RICHARDSON, 1988). Similarly, even though upwelling has been reported off northeast South America (RYrx-mR, Mm',rzEL and CORWIN, 1967; Mnz~r~, 1973; GraBs, 1980) and in the southern Caribbean (REDvn~.,.n, 1955; B~t~Sa~R and MAaGn.t~F, 1965; Ricanl~s, 1960; WOST, 1964; GORDON,1967; CO~a~DOR, 1976, 1979), it has not been possible to determine the magnitude or variability of the upwelling phenomenon. Land runoff and coastal upwelling supply abundant "new" nutrients (see DUGDALEand GOERInG, 1967) to surface waters near continents. Thus, continental margins around the globe support high concentrations of phytoplankton and productivity (about 8.4x109 tons C y'~ or 30% of the annual global marine primary production; W~..SH, 1988, 1977; KOnLENZ-MIsHI, m,
Pigmentdistributionin the CaribbeanSea
25
VOLKOVINSKYand K.nUANOVA, 1970; R~n'rmR, 1969). This is the base for harvestable fish production and also represents a possible sink for excess anthropogenic carbon in the atmosphere. But due to the wide range in scales at which processes operate in the ocean (STEm.E, 1978; ES~aAS, 1980), it has been difficult to obtain reliable estimates of the variability of phytoplankton near continental margins even after intensive sampling by ship (cf. WALSH,D,k-ruaLE and ESAJ.~S, 1987). These problems may be solved by complementing ship data with remote measurements of ocean color such as those obtained with the Coastal Zone Color Scanner (CZCS) (e.g. see YODFJ~, MCCLAIN,BLANTONand OEY, 1987; WALSH, DtE'rERLEand ESAIAS, 1987; BANSEand McCLAtN, 1986; ~ Z and McGowAN, 1986; Bgown, EVANS,BROWn,GORDON,S)~nTHand BAKER, 1985; MCCLAnq,PIETRAFESAand YODER, 1984). The CZCS allowed synoptic mapping of the concentration of pigments near the surface and provided an unprecedented capability to exanaine the surface circulation of the ocean. Even though the relationship between variance spectra of physical parameters and phytoplankton biomass is not constant (CAMPBELLand ESAIAS,1985; WILSON,OKLraoand ESAJ.AS1979; STEELEand HENDERSON, 1979; FASHAM,1978), blooms larger than approximately 2 km in diameter tend to maintain their coherence when phytoplankton growth exceeds losses due to turbulence and advective exchange (Die, tANand AaBoyr, 1988; MACKAS,D ~ and ABBOTt, 1985; CAMPBELL and ESAL~S, 1985; OKUBO, 1980; WROBLEWSraand O'BRIEN, 1976; KmRSTEADand SLOBODKIN, 1953). Thus, phytoplankton and their pigments can serve as visual tracers of the circulation (SvEJKOVSKV, 1988) in spite of their non-conservative behavior. Here we describe the distribution of pigments in surface waters of the eastern Caribbean Sea (Figs. l a and l b). Our goal was to develop a basic conceptual model of the distribution of phytoplankton biomass. In formulating a study plan, we identified key questions about the oceanography of the Caribbean. These questions are: (1) What is the impact of the Orinoco River on the Caribbean Sea? (2) Are there basin-scale countercurrents in the central Caribbean? (3) What is the variability in the distribution of pigments in the region? Below we provide a brief background description of the circulation and potential sources of particulates to the Caribbean Sea. Then we describe the methods used to process the satellite data. Finally, we discuss the distribution of pigments and evaluate the impact of the Amazon and Orinoco Rivers on the region. Of particular interest was the biological impact of the Orinoco on the Caribbean basin, which we evaluated using a budget for nitrogen in the river plume based on CZCS data. 2. SITEDESCRIFFION
2.1. Physical setting The meridional migration of the Intertropical Convergence Zone (ITCZ; HOP.EL,I'(OUSKYand KAGANO,1986) leads to seasonal variability in the circulation of tropical oceans thiough changes in the curl of the wind stress (KATz, 1987; BUSAI.ACCHIand PlcAtrr, 1983). Between January and April the ITCZ is at its southernmost position (0 ° to 50 S), and wind stress in the Caribbean is strong (ca. 0. I N m "2southwestward; ISEMERand HASSE, 1985). During this period, waters in the northern tropical Atlantic tend to move toward the west (R1cnAaDSONand McKr~, 1984) and flow through the Lesser Antilles into the Caribbean. Large flows may be expected in the basin at this time (MoRPaSONand NOWLIN,1982; I-IEBtrgNand RHODES,1987). In May, the ITCZ abruptly begins a northward migration ( P ~ E R and PACANOwsra, 1986), and it reaches its northernmost
26
MOLtJ~-KARGI~ et al.
(a)
-78
-74 I
22
-72 I
-70 I
-68 I
-66 I
-64I
-62 I
-60 I
-58 22
-
-
20
4~ 18
~ •
16
Caribbean
14-
-
G,<;,,
Guo
D<
~. /
a
(Po~to p~.d~,)
Orchilo Island
i
16
c~..... o %J~{-/////~ 7,'":":" Ilia riinlqu o ~~ ~_ U3 F/////2•
Marltarito Illond
I P*~*~'
1
Penlnllukl
18
Sea
Paroguario Peninsula Curocoo Gulf of Ven+zuelo I
-
-
".
/
,
--14-
~l't~doB
*
.
Gnlnelda
>.- .......
--12 --10
8
--
Colombio 6
i
I
-78
(b)
-80
I
-74
I
-78
I
-70
72
I
22
8
Yenezuelo I
-68
I
-66
I
-64
-74
-72
-70
-68
-66
-64
I
I
I
I
I
I
6
I
-62
-60
-6;
-58
-60
-58
I
•
22
20-PuerloRico
18-! ":
,,/,~,~~
1618
16-14 12--
12 r
10-8
-C i u d o d flollvor
80
(c)
I
I
I
i
I
78
-74
-72
-70
-68
12 65 6,4 V ~~ . . ~ ~ ~ I
/"
\
ir T
6,3
I
I
66
646,2 V
I
62
I
60
6
-58
61
----~-~.
10 FIG. la, b,c. The Caribbean Sea and study subregions. (a) Names of locatiorts mentioned in the text. (b) Crosshatched regions used to calculate time series of pigment means from individual CZCS scenes. Indicated are: wind stress stations (filled circles ) and sea level stations (filled triangles ); 1° x l°squares were used for estimates of sea-surface temperature (SST). Broken line shows approximate position of the shelf break. (c) Detail of the southeastern Caribbean showing 5 productivity regimes outlined by M ~ O A L ~ (1965). We estimated productivity in regions II-V using CZCS-derived chlorophyll concentrations in order to compare with the historical observations of MxRo.u.l~ (1965), Gii,~s (1972), and Mmo (unpublished) (see Table 2).
Pigmentdistributionin the CaribbeanSea
27
position at the southern edge of the Caribbean (6 ° to 10° N) in August. In October, the ITCZ starts its return to the southerly position. The presence of the ITCZ in the northern tropical Atlantic induces the lormation of the eastward-flowing North Equatorial Counter Current (NECC; BUSALACCHIand PICAUT, 1983; RICHARDSONand McKEE, 1984). Concurrently, water from the North Brazil Current and water discharged by the Amazon River are shunted east into the NECC (MOLLER-KARGER,McCLA1Nand RICHARDSON, 1988). Understanding this circulation pattern was necessary to address our question on the relative impact of the Amazon and the Orinoco on the Caribbean. While currents in the Caribbean south of 14°N are nearly zonal and swift (0.21-0.64 m s t westward), net water motion north of 14°N and near the Antilles is more sluggish (residual flow is 0.12-0.19 m s -I to the not-thwest; WILLIAMS,1986; KINDER,HEBURNand GREEN, 1985; KINDER, 1983; MOLINARI,SPtLLA2qE,BROOKS,ArWOODand DUCKETr, 1981). The average flow through the Caribbean is estimated as 20 to 30 Sv (Sv = Sverdrup = l06 m 3 s-l; KINDER, 1983; STALCUPand METCALF, 1972; GORDON, 1967; W0ST, 1964). The bulk of this flow enters through the three southernmost passes of the Lesser Antilles, but up to 12 Sv may enter into the northwestern Caribbean through Windward Passage (WORtHInGTON. 1976; ROEMMICH, 1981 ; NOF and OLSON, 1983). Unlortunately, there are not enough observations to quantify the annual cycle of the circulation. Our best estimates of seasonal variation are based on non-synoptic hydrographic sections (MoP,glSON and NOWLIN, 1982), current meter records (MAzEIKA, KINDER and BURNS, 1983), and numerical simulations (HEBURNand RHODES, 1987). Several investigators have inferred that countercurrents are part of the mean circulation of the Caribbean (flow > 0.4 m s q to the east near 140-15° N: FUKUOKA,1965; ATWOOD,1976; FEBRESORTEGAand HERRERA,1976; A~DRES, FEm~S and HEmO,ERa, 1979; MomodSONand NOWL~, 1982; DUNCAN, SCHI..ADOWand WILLIAMS,1982). However, the evidence for eastward currents may have been based on the partial sampling of eddies (KINDER,HEBUaNand GREEN, 1985). In an effort to shed some light on this question, we examined the CZCS satellite images for surface evidence of countercurrents. The migration of the ITCZ leads to a dry (Februmy-May) and a wet season (August-October) north of about 5°N (ETrva, LAMB and PoR'rls, 1987). These seasons are roughly in phase with changes observed in the surface salinity o fthe Caribbean. Salinity itmxima occur near Puerto Rico in April-May (about 36.5 psu; psu = practical salinity units). Minima occur in October-November (33.4 to 34.5 psu; FROELICn,ArWOOD and GmSE, 1978). But, despite the coherence in phase, rainfall has only a small direct effect on the surface salinity. Its effect is more indirect. FROELtCH, ATWOODand GIESE (1978) and MOm~,IsoNand NOWLIN(1982) measured high concentrations of silicate (3-51x M SiO4-Si) in the low-salinity water, which identified it as riverine in origin. The seasonal variation in rainfall strongly influences the discharge of tropical rivers. The three largest rivers affecting the Caribbean Sea are the Amazon, the Orinoco, and the Magdalena. The average discharge of the Amazon is 17.3x104 m 3 s"1 (Fig 2; range: from about 10x 104 m3s~ in November to about 23x 104 m 3s t in June; WUST, 1964; OLTMAN,1967; Stota, 1975; UNESCO, 1979). This is 4 to 5 times greater than the mean discharge of the Orinoco (Fig. 2; mean = 3.9x104 m 3 sl; range = lxl04 m 3 s-~ in March to about 7x104 m 3 s: in August; see also MEADE, NOPa~, PEREZ H~'~m'~eZ, MeJtA and PE~Z GODO¥, 1983). The Magdalena is the smallest of these three rivers (mean discharge of about 0.8 x 104 m 3sl; MIIaXMANand MZADE, 1983). Together these rivers account for about 20% of the total discharge of fresh water into the world's oceans. An additional, unknown amount of fresh water is supplied to the ocean by numerous small rivers draining northeastern South America. However, based on the large volume discharged by the Amazon and conceptual models of the circulation in the western tropical Atlantic developed prior to the
MOLLER-KARGERet al.
28
1980's, the primary source of the fresh water leading to the seasonal freshening of the Caribbean had been assumed to be the Amazon river. 2.2. Distribution o f particles and phytoplankton Particulates found in the Caribbean Sea are primarily derived from rivers or phytoplankton growing in the ocean. The Amazon and Orinoco Rivers contribute a large amount of particulates (Mo_LtMA~ and MEADE, 1983). Sediments from the Amazon are cast along the entire coast of northeastern South America and eventually reach the Caribbean (MEAoE, NORDIn, PEP,EZ HERNANDEZ, MF...qAand PEREZ GODOY, 1983; MILLIMAN,BtrrENKO, BARBOTand HEDBERa, 1982; EMMA, VAN Dim GAAST, MARTIN and TrloMAs, 1978; B~z~R, EOGIMA~, CARDER, K~TER and B~:rz~R, 1977; VAN ANDELand POSTMA, 1954). Most of the sediment presently discharged by the Amazon appears 1o move northwestward along the Guyanas, inshore of the 10 m isobath (cf. NtrrROUER, CtmT~ and DEMASTER, 1986; RINE and G~SBURG, 1985; WELLS and COLEMAN, 1981; DELFT HYDRAULJCS LABORATORY,1962). It is possible that little of the Amazon's sediment is carried offshore into the deep Atlantic, as opposed to a large fraction of Amazon water (MOLLERKAROEI~, McCLAIN and RlCaARDSON, 1988; see also BORSTAD, 1982a). The second important source of particulates is plankton. Previous to the CZCS era, there had been little information available with which to reconstruct the synoptic distribution ofphytoplankton in the Caribbean Sea. We knew that the eastern Caribbean (Fig.l) was more "turbid" and productive than other tropical marine ecosystems (cf. WrLKIZqSON, 1987), but the processes leading to this productivity remained largely undefined. The few measurements of nutrients available for the Caribbean Sea show that there is very little nitrate in the upper 50 m (<< 0.2 JJt,M NO3-N; ALBERTS, 1973; SELLm~R, 1981; OHER-OTEC STUDY, 1980; MOP,mSON and NOWL~, 240 220200-
Amazon 1 B(3160-
140~
120-
P
100-
i5
8O-
...........'
60-
....... ... ......... .....,.,............,...
4O2O0 Jon
I
I
I
Feb
Mar
Apr
Moy
Jun
dul
i
i
i
i
Aug
Sep
Oct
Nov
Dec
FIG.2. Mean + a standard deviation of the discharge of the Amazon (period 1969-1978; UNESCO, 1979) and OrinocoRivers (period 1979-1983; data courtesyofW. LBWlS;see also MEADEetal., 1983).
Pigment distribution in the Caribbean Sea
29
1982). Below about 140 m, nitrate concentrations are 2.5-10.0 ~M, and higher below 400 m (>251XM). Phosphate concentration is variable near the surface (0.0-0.06 p,MPO4-P), and increases with depth (0.5-1 ~M at 250 m). There does not appear to be an increase in nutrient concentration downstream of the Antilles, and the small increase in chlorophyll found in the wake of these islands appears to be caused mainly by export of cells rather than growth in situ (tthRORAVES, BRODYand BURm~OLD~, 1970). Appreciable island effects (cf. DOT¢and Ootnu, 1956; F~LDMAS, CLARKand HALPm~, 1984) could be observed in the CZCS data only in the southern Caribbean off Tobago and Margarita, in part because the nutricline is closer to the surface near the continental margin. Surface nutrients are more abundant near land masses. For example, nitrate-N concentrations > 2 IxMwere measured during 28-29 May 1975 off the Goajira Peninsula (Comua~R, 1976, 1979). Sea surface temperatures in this nutrient-rich plume were up to 2°C lower than waters 190 km further offshore (CoRREDOR,1976). Since these coastal waters are closer to the equator, the colder temperatures suggested the occurrence of upwelling. Upward warping of the pycno- and nutricline is readily visible in meridional transects of the Caribbean (see FROELICH, ATWOODand GIF_ZE, 1978; MORRISONand NOWL~, 1982). Similar to the distribution of nutrients, the lowest standing crops of phytoplankton have been observed in the western tropical Atlantic and in the western Caribbean (Table 1). In these T.~I..E I. Range of observed surface chlorophyll a concentration (Chla), primary productivity (P.P.), and Secchi depth in the Caribbean Sea. (NA = no estimate available.) (Secchi depths in parentheses courtesy of M. LL~ctsand N. Kupaso, Dalhousie University, Halifax, Nova Scotia (data fi-om NODC, US Department of Commerce)) Location Source (1)
(2)
(3)
(4)
Chla [rag m -3]
P.P. [mg C m ~ d "l]
Secchi [m]
Nortbem Atlantic E of Anti.lies (13-14°N) Barbados
0.05-0.35 0.05-0.27
50-150 139-380
(30~14) NA
Northern Caribbean W of Antilles, > 14°N
0.05-0.98
50-300
(25-30)
0.107-0.644
204-1060
(11-25)
0.28-2.4 0.5 -3
1200-2300 600-2500
4-26
0.04-0.4
150-250
(10-15)
Southern Caribbean Margarita Island Margarita to mainland, Gulf of Cariaco NW Colombia Western Caribbean
Source:
(1)
BOR~TAD, 1982b; KInD and S~,rD~, 1979; ~OP, AVES, BRODYand BU~mOLDm, 1970; BEERS, STEVEN and LEWIS, 1968; SAUO and NAKAMOTO,1973; BERMUDA BIOLOGICAL STATIOn, 1965.
(2)
HARORAV~,BRODYand Bugr.~ou)m, 1970; MoRms, SMrm and GLove, 1981 ; BALLmTm M~o~a.~, 1965; Bui~r~aou)m and ALMODOWa%1967; YOSHIOKA,OWENand I~sAm'a, 1985.
(3)
MAac/a.~, 1965; M_xaG/o_~, CmVmON and Ym~z, 1960; BALt.ES~:a and M.~GAL~g, 1965; MA,Zp~aO'm, 1967, 1970; MoRRis, Sm'rn and G t ~ v ~ , 1981; Cure., 1960; Rtc~u~.~s, 1960; Gn,~.s, 1972; Coaa.~oR, 1979.
(4)
S~,
1981; M.At.O~, 1971, 1973; BAIRD, 1973.
30
MOLLER-KAROERet al.
oligotrophic regions, nearly 90% of the primary productivity is due to nanoplankton, and chlorophyll a is only 20-50% of total pigment (MALONE, 1971; OHER-OTEC STUDY, 1980). The highest rates of photosynthesis have been measured near the continental margin (Tables 1 and 2), where abundant nutrients cause diatom- dominated blooms (between 200 and 400 g C m2 y~ near Margarita Island). The high productivity is reflected in the large fishing industry of these regions (G~ES, 1972, 1982; FAO, 1986). An important factor controlling phytoplankton abundances in the Caribbean is zooplankton. However, most zooplankton studies have been limited to coastal waters, where concentrations are over an order of magnitude larger than offshore (C~vicoN and MARCANO,1965; Gn,ms, 1982; YOSmOKA, OWENand PESArrrE, 1985). Similar to the distribution of phytoplankton, the largest numbers of zooplankton occur in the southern Caribbean (Table 3). The seasonal cycle in the abundance of zooplankton and phytoplankton in the northern Caribbean Sea is in phase with local changes in surface salinity (YosI-HOKA,OWENand PEsmcrE, 1985). High concentrations of both types of plankton occu~ between July and November, along with low surface salinities. We have known for some time that plankton concentrations in Amazon water drifting off South America are higher than those in typical western tropical Atlantic water (R'rrnER, Mm,rZEL and CoRwn~, 1967; CALm:and GRACE, 1967; HULBtmTand CORW~, 1969; KXDDand SANDER,1979; BORSTAD,1982b; DEUSER,MOLL~_x-KARGERand HmVa.EBEN, 1988). Accordingly, the seasonal increase of plankton in the northern Caribbean has been attributed to the dispersal of the Amazon's plume.
2.3. Where does the Amazon water go ?
Despite suggestions that the Amazon is responsible for the changes in salinity and plankton concentration in the Caribbean Sea, there are some pieces of conflicting information. The brackish water seen offshore in the Caribbean seems to have higher concentrations of silica than the low-salinity water drifting east of the Antilles (MooRE, SARMmm'Oand KEY, 1986). Also, it appears that the maxima in plankton biomass in the northern Caribbean (ca. 0.15-0.5 mg chlorophyll m-3; 0.15-0.28 ml zooplankton m-3; YOSrUOKA,OWZNand PESArCrE, 1985; OHEROTEC STUDy, 1980) are comparable to the highest concentrations observed near Barbados (ca. 0.15-0.2 mg chlorophyll m-t; 0.12 ml zooplankton m-3; CAL~ and GRACE,1967; KIDDand SArmER, 1979; BORSTAD,1982b). Since grazing, sinking, and mixing tend to reduce surface concentrations of nutrients and plankton in the absence of nutrient sources, it is hard to envision how these properties are conserved during the 880-krn transit between Barbados and Puerto Rico in periods > 1-3 months. Below we present conclusive evidence that the low salinities observed seasonally in the northern Caribbean are caused by the dispersal of Orinoco river water.
3. METHODS 3.1. CZCS imagery
The distribution of near-surface pigments in the Caribbean Sea was mapped using 159 CZCS images obtained between November 1978 and December 1982 (MCrLLER-KARGER,1988). Five additional images were examined for the period 1983-1986. Images were subsampled to 1/16th of their original resolution before processing. Pigment concentrations were derived from ratios
Pigment distribution in the Caribbean Sea
31
TABLE2. Summary of in situ primary productivity observations and primary productivity estimates based on CZCS data off Venezuela. Observed values were adapted from G~Es (1972), from data compiled by Mmo (unpublished). Data were grouped geographically according to MAROALBF(1965; Fig. lc). Estimated values ofprimaryproductivity werebased ontherelationshipproposed by EPPLEY, Sa'~WAR'r, ABBOT and HnVMAN (1985) : Primary Productivity [rag m "2 d'Z] = 1000 * SQRT (Chlorophyll), where we used Chlorophyll [mg m'3l = 0.5 * C7_,CS Pigments [rag m'3]. Grand mean values were obtained by weighing the mean productivity of a region obtained per CZCS scene by the number of observations. Regions were defined as in either MAROALEF(1965) or GINES(1972) (Fig. 1c). CZCS estimates from the Gulf of Venezuela were based only on the eastern half of the Gulf to avoid contamination by bottom reflection or resuspension, and do not include Lake MaracaibÜ. This area extended to 13°N to sample the entire shelf. (NA = not available)
Region Observations
Primary Productivity [rag C m 2 d"] CZCS estimates
Mean
Range
N
Mean
Range
N
I. G. Paria and Orinoco delta
341
30-750
14
NA
NA
NA
II. NE and E Margarita
357
100-750
24
746
199-1184
1549315
III. S of Margarita
505
100-1500
68
794
171-1223
286885
IV. W of Margarita
611
300-1100
18
555
256-1091
814378
V. N of shelf break
283
80-700
55
515
201-1021
4749130
Gulf of Venezuela and Lake Maracaibo
328
96-650
21
609
287-1146
1367974
TABLE3. Approximate range of zooplankton concentrations in the Caribbean Sea. Data from GL~ES (1982), CERVICONand MARCAr~O(1965), and YOSHIOKA,OWEN and I~SANTn (1985). (NA = not available) Region
Biomass [mg wet weight m -3]
Numbers [organisms m "3]
Northern Caribbean (January - May) (July - November)
(NA) (NA)
250 4OO
Near Margarita
(NA)
200-3000
Central Venezuela
30-300
100-2000
Gulf of Venezuela
30-400
90-2000
o f t h e blue (443 n m ) o r b l u e - g r e e n (520 n m ) w a t e r - l e a v i n g r a d i a n c e s to the g r e e n r a d i a n c e (550 n m ) , a c c o r d i n g to GORDON, CLAret, BROW'N, BROWN, EVANS a n d BRo~.Nrow, (1983a; s e e also GORDON, BROWN, BROVCN,EVAr~S a n d CLARK, 1983b; BAgAL~, McCLAZ~ a n d ~ O T r E - R a z z o t a , 1986). T h i s is a n e s t i m a t e o f the a v e r a g e p i g m e n t c o n c e n t r a t i o n in the first o p t i c a l d e p t h . P r e s u m a b l y , c h a n g e s in the d i s t r i b u t i o n p a t t e r n s r e f l e c t e d c o n d i t i o n s o v e r the E k r n a n d e p t h o f frictional r e s i s t a n c e . A t l o w c o n c e n t r a t i o n s (0.04 - 0.5 m g p i g m e n t m'3), the C Z C S s e n s e d p i g m e n t to d e p t h s o f a p p r o x i m a t e l y 1-10 m , a n d thus this a s s u m p t i o n m a y n o t b e u n r e a s o n a b l e . W e a l s o
32
MOLLER-KAK(3ERet al.
assumed that the surface distribution of pigrnents in river plumes reflected motion over the depth of the plume. We color-coded concentrations [mg m "3] in the images (cf. Plate 1). Blue represents low pigment concentrations and yellow and red indicate higher concentrations. Land was masked black and clouds and missing data, grey. Pigment scenes were remapped to a Universal Transverse Mercator (UTM) projection (bounds = 8°N, 21°N, 58°W, 78°W) and manually navigated to match a standard coastline. 3.2. Regional studies We wanted to examine differences between large-scale portions of the Caribbean Sea. For this purpose, up to three regional pigment means were obtained from each CZCS scene. The regions were def'med as follows (see Fig. lb): (a) Southern Caribbean [59x104 kmZ]: waters south of 14°N, from 78°W to the Lesser Antilles. The southern half of the Gulf of Venezuela was excluded to avoid bottom reflection and resuspended sediment. (b) Northem Caribbean [89x104 km2]: waters north of 14°N, from 78°W to the Antilles. (c) Southern Sargasso [50x104 km2]: waters from 14.4°N to21.4°N, 68°W to 57.8°W, east and northeast of the Antilles. Only samples larger than 1000 pixels (> 17x103 km 2) were accepted to minimize spatial aliasing. Standard deviations (sd) were computed but are not presented here because they are not appropriate for statistical comparisons due to the skewed and frequently bimodal distribution of pigments, and due to variation in sample size. 3.3. Possible sources of error
Previous results suggest that in low-pigment waters (0.08-1.5 mg m3), retrieved pigment concentrations are within 30-40% of in situ concentrations (GORDON,CLARK,Mtr~rr~.a and HovIs, 1980; GoRDoN,CLARK,BROWN,BROWNand Hows, 1982; GORDON,CLARK,Baow~, BROWN,EVANS and BROEr,~OW, 1983a). However, even though CZCS pigment values in areas affected by rivers may be realistic (YoD~, McCta~, Bt>acroN and OEY, 1987; B,~,~ta~, M c ~ and MAtaNOaWeRmZOLI, 1986; GORDON,CLARK, BROWN,BROWN,EVANSand BRom-~xow, 1983a), phytoplankton, suspended matter, and Gelbstoffe (yellow dissolved organic matter) are not correlated. Thus, the blue-green ratio of CZCS radiances becomes useless for quantifying chlorophyll concentration (FISHER, D O ~ and GRASSL, 1986). The following is a list of factors that may have affected CZCS-derived pigment concentrations in the Caribbean: 3.3.1. Calibration. One of the worst CZCS calibration problems occurred during JanuaryFebruary 1980 (R. EVANS, personal communication). Since the southern Sargasso (Fig. lb) typically supports low phytoplankton concentrations, we used it as a control region to monitor changes in background pigment levels or possible quality problems with the imagery. It appears that problems due to calibration drift during the study period were ameliorated by interactive selection of epsilon coefficients. BAr~SEand McCLAn~ (1986) also derived their atmospheric correction coefficients interactively. The fact that BANSE and McCLAIN (1986) observed an offshore increase in pigment concentrations in January-February 1980 in the Arabian Sea, when concentrations in the southern Sargasso decreased (Fig. 5c), suggests that the CZCS was able to detect real trends in pigment concentration.
PLATE I. March 1979. Composite of 3 CZCS scenes of the Caribbean Sea in March 1979. Color bar shows pigment concentrations (approximatdy chlorophyll a plus phaeopigment and Odbstoffe) in [rag m'~]. Longitude values are given on a 360 degree basis, starting at Greenwich and increasing eastward. The cenlral Caribbean shows low pigment concenlra6ous, while the southeastern Caribbean had high conccmtratious (> 0.5 mg m'3). Pigments in the southeaste~'n Caribbean appeared elongated toward the west, indicating the direction of the circulation. Patches of high pigment also ocoatred off the Paraguana and Goa'~a Peninsulas (see Fig. la for geographiclocations).
I
i
I
..fJ
I
.L~[
_," " ,!.,
i
I
I
l
I
I
,[~L~L
"
'~
I~80
s
c.
f~
NASA - GSFC
,JAN
.
...-
L
I~AT': 2. January 1980. CZCS composite of 7 overviews of the Caribbean Sea in January 1980. High concentrations occurred immediately to the southeast of the Caribbean. This indicated northwestward movement of Amazon water alc~ng the coast of Guyana and the presence of an anticyclone south of Barbados. High pigments in the southeastern Caribbean indicated the presen~ of upwelling and the plume of the Orinoco River, Pigments were low in the no~hm'n Caribbean.
i
. . . . . . . . . . . . . . . . . . . . . .
.
"'%
%/
"%
g.
%
NASA - GSFC
~
I~.ATE 3. March 1980. CZCS composite of 7 overviews of the Caribbean Sea in March 1980. Tropical Atlantic and Amazon water (concenu'afiom of alx~ut 0.3 mg m -3)entered the Caribbean south of about 15 °N. High pigment concentrafiom (> 0.5 mg m -3) occun:ed only near the continental maxgi~. Westward elongation of the patches along the coast indicated strong westward motion of the watt. Low pigment concenn'ations associated with North Atlantic water occupied the northeastern Caribbean.
.4
I
I
I
~I . I. . ~_1 . . . . .I
•
I
I
I
I
JNN NASA
1982 - GSI:'E:
0 o.
~
I~ArB 4. January 1982. CZCS composite of 9 overviews of the Caribbean Sea in January 1982. High pigment concentratiom (> 0.5 mg m -3) occtm'ed near the contincntaJ margin. Theze were indications of anticyclonic flow south of Barbados. Pigment patches with about 0.3 mg m 3 southwest of Puerto Rico were associated with Orinoco water discharged during the previous season. We infea'red that a jet of low-pigment Atlantic waters entered the Caribbean between 15 ° and 17°N (near Guadeloupe and Dominic.a) and continued westward thxough the central Caribbean, isolating pigment patches in the northern and southern Caribbean (see Fig. la for geographic locations).
.I. .I . .I . .I
•
L
i
i
¢
f
i
i
~
-
i
i
i
i
i
i
i
°
"';
0
NASA - GSFC
OCT 1 9 7 9
-"
11
l:'t,~l"E5. October 1979. CZCS composite of 9 overviews of the Caribbean Sea in October 1979.' The Orinoco plume occupied the eastern Caribbean and flowed northwestward past Puerto Rico. A small plume veering offshore near 10" N, 59 ° W was evidence of an anticyclonic eddy east of Trinidad.
i
i
.
....................
i /
." -
.
t~J
.
.
.
JJ
.
.
,
.
.
,~
.
.
.
~a
.
J
.
.
.
.
. -
i i f / f
e,i,
'-.
~,"
-
o .
•
[a"
'rrl~
-
t~
--,
0
NASA - QSFC
JUN 1980
.
t
PLATE6. June 1980. CZCS composite of 2 overviews of the Caribbean Sea in June 1980. Only 1 scene covered a significant portion of the study region, and it showed htighpigment concentrafiom in the central and northeasternCaribbean. A composite for July 1980 showed similar high concentrations far offshore in the westea-n half of the study area.
.
,.
PJ.ATE7. September 1980. CZCS composite of 5 overviews of the Caribbean Sea in September 1980. The plume of the Orinoco occupied a large portion of the study region. The genexal features observed in this composite persisted until December 1980, but pigment concentrations decreased with time.
PLAr~ 8. November 1981. CZCS composite of 7 overviews of the Caribbean Sea in November 1981. The Orinoco plume bifurcated in the southeastern basin. Low pigment concentrations occurred outside of the Caribbean and in the western Caribbean.
PLATE 9. Single-orbit CZCS scene showing pigment concentrations on November 24, 1984. The Orinoco plume appeared to disperse eastward into the interior Adantic. The Orinoco plume joined the northern extreme of the retloflecfion of the North Brazil Current, traced here by pigments associated with the dispersal of Amazon River water. Due to CZCS sensor decay, concentrations in this image are unreliable.
Pigment distribution in the Caribbean Sea
39
3.3.2. Atmospheric effects. Artifacts in derived products due to thin clouds and cloud edges were minimized by manual adjustment of land/cloud flag thresholds based on the 750 nm band of each scene. The atmosphere may also contribute sands and aerosols originating in Africa (PROSPERO,N ~ s and UFa~tATSU,1987; PROSPEROand C~LSON, 1972; PROSPERO,BONA'rrI,Srn~mtT and CARLSON,1970; CARLSONand PROSPERO,1972; C~au~m~, STEWARD,Bzl-Z~a~and PROSPERO, 1986a). Images of atmospherically-corrected CZCS band 4 (670 rim) were compared with the pigment images to ensure that patterns in the pigment field were not correlated with aerosol patterns. Scenes where aerosols caused artifacts in the pigment fields were discarded. Water-leaving radiance scenes were individually checked for accuracy using theoretical radiance limits for water with low and high pigment concentrations (GORDONand CLARK,1981). Selected scenes were also processed to normalized water radiances to verify agreement with values given in GORDON,CLARK,BROWN,BROWN,EVANSand BROEr,a
40
MOLLI~R-KARGERet al.
high surface concentrations occurred offshore, as when river plumes formed patches greater than 3 x 105 km 2 (see below), the subsurface phytoplankton community would be shaded and perhaps become unviable. We could not evaluate the importance of the subsurface chlorophyll maximum, but this needs to be addressed in future studies. 4. RESULTS 4.1. Comparison with ship data
We compared pigment estimates obtained from full-resolution CZCS images (1 km x 1 km pixels) with a limited number of ship-based observations. The CZCS underestimated the sum of surface in situ chlorophyll a and phaeopigments off Puerto Rico by a factor of about 0.61 in 1980 (Fig. 3, Table 4; data from the OHER-OTEC STUDY, 1980, courtesy of J.CoR~EDOR and P. YosmoKA). Comparisons in the southern Caribbean in 1986 showed more scatter, but CZCS data were on average within a factor of 0.92 of the surface in situ pigment concentration (Fig. 3, Table 4; data from the FUNDACIONLA SALLE). The CZCS overestimated two in situ observations of surface pigment concentration (bucket samples) in waters affected by the Orinoco (20 April 1986, Table 4). It was, however, not possible to evaluate small-scale (< l pixel) variability in the in situ concentrations nor its effects on the comparisons, primarily because of insufficient ship data. In general, there was good agreement between previous observations of phytoplankton pigment concentration (Table l) and concentrations obtained with the CZCS (Table 5).
/ / / / / I I /
/ / i /
I
/
E
/ / /
E
/ i i / / / /
÷
"5.
B
i /
/ "
N L)
/
•
1980
/ +
/ /
1986
/
/
-
-
-
1:1
J ( +_ s d )
/
1
0
i
I
I
I
2
In situ chlorophyll
I
4
+
phoeopigment
I 6
[rng
rn-31
FIG. 3. Comparisonbetween CZCS and ship observations made during OHER-OTEC cruises off Puerto Rico (January and August 1980; OHER-OTEC reports, 1980), and during a survey off Venezuela by the La Salle Foundation(Marchand April 1986). See also Table 4 for individual values and furtherexplanations.
Pigment distribution in the Caribbean Sea
41
TABLE 4. Comparison between CZCS and ship observations made during OHER-OTEC cruises off Puerto Rico (January and August 1980; OHER-OTEC reports, 1980), and during a survey off Venezuela by the La Salle Foundation (March and April 1986). CZCS values represent the average and standard deviation of values in 4 x 4 pixel boxes ( 4 km x 4 k m ) around ship positions [ mg m'3 ]
Ship date
CZCS date
Lat N (o)
Lon W (,)
Ship Ch
Ship Ph
Ship (Ch+
Ratio (Ch/
Ph)
Ch+Ph)
CZCS Pig.
C7_CS s.dev
1980
Jan28 Aug 1
Jan27 Aug l
Aug 2
Aug 1
17.96 17.94 18.04 17.81 17.74 17.93
65.86 65.78 65.55 65.55 65.88 65.88
0.13 0.10 0.11 0.09 0.08 0.13
0.01 0.27 0.32 0.31 0.27 0.30
0.143 0.373 0.442 0.413 0.356 0.431
0.910 0.273 0.257 0.230 0.224 0.301
0.14 0.212 0.301 0.251 0.253 0.39
0.02 0.03 0.1 0.03 0.01 0.09
12.34 12.16 12.18 12.00 11.99 11.68 11.52 11.38 11.77 11.48 11.30 11.63 11.80 11.82 11.65 11.68 11.50 10.73 10.83 10.35 10.05
70.38 70.54 70.89 70.91 70.71 70.32 70.25 70.40 70.53 70.52 70.65 70.65 70.83 71.17 71.45 71.02 70.82 63.50 63.15 62.45 62.05
0.16 0.28 0.53 0 0 0.6 0.75 0.93 0.46 0.14 0 0.34 0.09 0.1 0.16 0 1.34 5.45 0.21 0.5 1.03
0.34 0.2 0.13 0.91 0.36 0.43 0.47 0.39 0.29 0.92 2.06 0.87 0.16 0.04 0.29 0.76 0.56 1.46 0.09 0.21 1.22
0.5 0.48 0.66 0.91 0.36 1.03 1.22 1.32 0.75 1.06 2.06 1.21 0.25 0.14 0.45 0.76 1.9 6.91 0.3 0.71 2.25
0.32 0.583 0.803 0 0 0.582 0.614 0.704 0.613 0.132 0 0.280 0.36 0.714 0.355 0 0.705 0.788 0.7 0.704 0.457
0.51 0.35 0.41 0.43 0.38 0.66 2.75 1.64 0.74 0.63 2.99 0.81 0.38 0.51 0.86 0.86 0.74 6.22 0.38 3.25 4.47
0.05 0.02 0.02 0.07 0.03 0.03 0.66 0.57 0.t 0.04 0.1 0.07 0.04 0.04 0.09 0.09 0.09 0.65 0.04 2.1 0.21
1986
Marl5
Mar 16
Mar 16
* *
Mar 17
Apr 19 Apr 20
Apr 19 * *
Abbreviations are: Ch = Chlorophyll a; Ph = Phaeopigment; Pig. = chlorophyll plus phaeopigment and Gelbstoffe; s. dev = standard deviation of CZCS pigment concentration.* Indicates large influence of outflow f~omMaracalboLake(March 1986) oroutflow from the Orinoco River (April 1986). Note: 1980 insitu values are the optically-weighted mean concentrafion in the upper optical depth. 1986 data were surface samples. 1980 CZCS data were processed using corrections by GORDON, CLARK, BROWN, BROWN,EVANSand BROENKOW(1983a) and GORDON,BROWN,BROWN,EVANSand CLARK(1983b). 1986 scenes were processed using Sa'URM(1986).
With CZCS
c h l o r o p h y l l c o n c e n t r a t i o n d a t a as i n p u t , w e d e r i v e d e s t i m a t e s o f p r i m a r y
p r o d u c t i v i t y f o r s e l e c t e d r e g i o n s o f f V e n e z u e l a u s i n g t h e a l g o r i t h m p r o p o s e d b y Er,etzY, STEWART, A a a o a ' r a n d HEYMAN ( 19 85). T h e s e r e g i o n s w e r e o u t l i n e d f o l l o w i n g MAI~6AL~ ( 1 9 6 5 ) (cf. Fig. I C) w h o first d e f i n e d p r o d u c t i o n r e g i m e s o f f V e n e z u e l a u s i n g in situ 14 C o b s e r v a t i o n s ( T a b l e 2). F o r p u r p o s e s o f EPPLEY et al.'s a l g o r i t h m , w e a s s u m e d t h a t c h l o r o p h y l l c o n c e n t r a t i o n was 50% of the CZCS-derived pigment level. The correction was applied since only chlorophyll a is e m p i r i c a l l y r e l a t e d to v i a b l e p h y t o p l a n k t o n b i o m a s s a n d p r o d u c t i v i t y , w h i l e p h a e o p i g m e n t s
42
]V~0LLER-KARGIBRet al.
TARL1R5. Mean pigment concentrations, range, and coefficients of variation (CV) in the Caribbean Sea and southern Sargassoover the 4-year time series.The means were derived by weighing individual samples(scene.s)by the valid numberofpixels present.The range is based on extrema of regional mean concentrationsrather than on individualpixel values.The CVis the mean of all coefficientsof variation obtained from individual scenes. 'n' gives the number of scenes used in the computations ([days])and the sum total of pi.xelssampled in the series (LDixels]).Only those scenes with > 1000valid pixels per definedarea were used to derive these statistics.The range of valid concentrationswas arbitrarily fixed between 0.042 and 20.0 nag m -3
Area
Mean
Range
CV
n
(km 2)
(nag m-3)
(mg m-3)
(%)
(pix.elS)
S. Caribbean
586835
0.58
0.11-1.94
134
129 1412693
N. Caribbean
886470
0.24
0.08-1.08
119
144 2463044
S. Sargasso
497711
0.10
0.05-0.95
55
104 860615
Subregion
(days)
are a byproduct of grazing and senescence of the phytoplankton (J EFPREY, 1980; M~,o~_a:, 1965). As mentioned above, previous observations have indicated that there is a large but variable fraction of phaeopigment in Caribbean waters (Table 4; see also O H E R - O T E C Saa.rolr~, 1980; MALONE, 1971). The chlorophyll and phaeopigment measurements made during the OHEROTEC STUDY(1980) offPuerto Rico were extremely valuable because they provided an estimate of the fraction of viable pigment in the Orinoco plume. The values derived from E ~ L ~ ' S formulation do not compare very well with those compiled by MAROALEF(1965) and Gz~_.s (1972; see Table 2). In general, this simplistic (E~LEY, personal communication) formulation overestimated in situ productivity observations. The only exception was immediately downstream of the island of Margarita (area IV in Fig. 1 c). This could indicate, for example, that the in situ samples were collected during a bloom, while the CZCS averaged over a larger area and non-bloom periods. Elsewhere, the general trend toward higher productivity estimates m a y be the result of overestimating chlorophyll concentrations in, for example, large areas affected by river plumes (i.e. areas I, II and V in Fig. lc). In fact, we refrained from obtaining surrogate productivity estimates in area I because of the obviously large influence of the Orinoco on this region. It may also appear that EPPLEV,STEWART,ABBOTrand ~ (1985) formulation overestimates the productivity of oligotrophic oceanic waters (e.g. chlorophyll concentrations of 0.04 m g m 3 yield productivities of 200 m g C m "2d-l). On the other hand, we now know with certainty that the 14C technique underestimates productivity w h e n " c l e a n " methods are not used (PEn~SON, 1980). This has led to revision of our previous estimates of productivity in oceanic waters (cf. LAWS, DrruLLIO and REDALm, 1987; MARRA and HEIa,mMAr~, 1987). In any event, it is hard to draw conclusions from the comparisons in Table 2, since the in situ observations in the Caribbean are aliased in time and are not synoptic, while the CZCS sampled a variety of water masses with frequence.
Pigment distribution in the Caribbean Sea
4.2.
43
Spatial arrangement of pigments
Figure 4 shows how the number of valid pixels obtained per scene varied relative to the maximum number of possible pixels in an area. Complete spatial coverage of a region was rare due to changes in scan configuration between orbits, cloud cover, and frequent contamination by sun glint. 1979 was the year best sampled, and 1981 the worst. The region best sampled was the northern Caribbean, and the worst, the southern Sargasso, The highest and most variable concentrations occurred in the southern Caribbean (series mean = 0.58 mg m'3; Table 5). Moderate concentrations were observed in the northern Caribbean (mean = 0.24 mg m-3). In contrast, standing crop in the southern Sargasso Sea was low (mean = 0.10 mg m-3), even though patches of Amazon water drifting east and north of the Antilles caused some variability (Fig. 5c; cf. DEUSER,MOLLER-KARGERand HEMLEBEN,1988). Coefficients of variation (CV = standard deviation x 100/mean) were in the range of 50 to 150%, and increased with the 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0
,l,J,~,l,,, Jan 79
1 0.9 0.8 0.7 0,6 0.5' 0.4 0.3 0.2 0,1 0
'I r'~ Jan 80
'I
r't
'I'' Jan 81
I
I
ill
I
I
I
I
I
I
I
I
I
Jan 62
I
I
I
Jan 83
b
L
0"91 '1'''''1
''I''
don 79
Jan 80
Jan 79
Jan 80
I
'I'''''I'' Jan 81
I
I
I
I
I
I
I
I
Jan 82
I
I
I
I
I
r
I
Jan 83
1
O.B 0.7 0.6 o 0.5
0.4 0.3 0.2 0.1
o
'
'
I' Jan 81
' " I " ' " I
~''' Jan 82
i~l
II
Jan 8,.3
FIo. 4a,b,c. Area sampled pet CZCS scene (munber of pixels with valid data in the defined region) normalized to total area (maximum possible number of pixels sampled in a region) of each of three large-scale regions defined in the Caribbean Sea (see Fig. lb). (a) Southern Caribbean, (b) Northern Caribbean and (c) Southern Sargasso.
44
MOLLBR-KARGER et al.
2
,=
$ i£
t8 17 1 1.9 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 03 02 01 0
O
Observation
4--+
Mean
7
io
f
Southern Caribbean
Monthly
; •i
'l"r"l
....
TI .....
Jon 79
I .....
I .....
Joo 80
,I
r .....
I .....
Jon 81
r,'"'l'
JOn 82
Jon 83
L1
•
1
" "
Single Observation
b
4--+
Monthly Mean
Corlbbeon
0.9-
Northern
080.706 0.5 0.4
0.3 0.2 01
'1 . . . . . I ' " " 1 ' ~ " ~ 1 " ' " 1
Jon 79
'''''I'''''I'T'''I~''I~
Jon 8 0
don 82
don 81
o0'o
Observation
0.28 026
(0.95)
024 022
Jan 83
C
Monthly Mean
5ergesso
Sea
i
0.2
• m
•
E 018 016 -
•
0.14 -
•
•
:
•
o.12-
• •
'
°°°4
-~
0.0
°
° Jon 79
\7 4 / '~ "" .':"
"i Jart 80
ii
•
;~ "..
o,o~3/d.¢.~
il
•
I
.~"1 " I
~ Jan 81
Jon 82
Jan 83
FiG. 5a,b,c. Time series of regional and monthly pigment concentration means based on individual scenes of the Caribbean Basin (see Fig. lb). The regional means lumped effects of dyer discharge and of wind-driven upwelling. Monthly means were computed using individual scenes weighted by the numberof valid pixels. The areas of interest were: (a) SouthernCaribbean, (b) NorthernCaribbean, and (c) Southern Sargasso. The large mean concentration observed on August 26 1980 in the Southern Sargasso was caused by a patch of high concentrations associated with the northward dispersal of Amazon water (e.g. DSOSER,MOLLeR-IC~RO~Rand HEMLEBEN,1988).
Pigment distributionin the CaribbeanSea
45
mean. The mean CV for the series suggested that the distribution of pigments in the Caribbean was patchy (CV > 110%; Table 5), but more random in the Sargasso Sea (CV < 60%). The differences between regions were consistent throughout the series (Fig. 5). The time series o fregional concentration means showed large variability between consecutive samples (Fig. 5). This variability was particularly marked in the series obtained for the southern Caribbean (Fig. 5a). This short-term variance is to a great extent an artifact caused by spatial aliasing when sampling a large area with the CZCS. Figure 4 gave an indication of the fraction sampled during each satellite pass, but the actual geographical area sampled also varied from scene to scene. Thus, individual means frequently did not reflect conditions over the entire area of interest, and noise was introduced. We also examined time series of monthly mean concentrations (superimposed on the curves of Fig. 5). We found that monthly means based on monthly composites of CZCS images frequently overestimated the corresponding weighted mean. The best means were based on individual scenes weighted by the number of observations. Since the monthly means represent better spatial coverage of the individual subregions, the series of monthly means is more in accordance with the traditional view o flow variance of phytoplankton biomass in tropical oceans. Nevertheless, it is clear that near continental margins, traditional views of phytoplankton abundance in tropical oceans are flawed. Indeed, concentrations in these regions can be high and may appear to vary almost at random. The seasonal signal was somewhat muted in the series of means (Fig. 5), primarily because the regions were large (> 50x104 km 2) and variation introduced by patches that form seasonally was dampened (i.e. the largest patch was of the order of 3x104 km 2, compared with 5-9x 104 km 2 for the sample regions, see below). Also, the seasonal cycle was obscured by interannual variability. In contrast to the time series of regional means, dramatic seasonal changes in the spatial arrangement of pigments were revealed in the CZCS images. During January-May, concentrations > 0.5 mg m -3were seen almost exchisively south of 14°N (Plates 1-4). High pigment concentrations occurred along the coast, especially near capes and headlands off central Venezuela and eastern Colombia. Since these locations receive little or no river input, and since the sea surface temperature (SST) around these locations is always lower relative to SST in the central Caribbean (e.g. Fig. 6), this indicated that upwelling of nutrient-rich waters occurred near the coast. Also, the plumes of the Amazon and Orinoco could be seen entering the Caribbean and extending westward along the coast of Venezuela (Plates 1-4). In contrast, between June and December, concentrations > 0.5 mg m 3 occurred in a patch that spread west and northwestward over an area of up to 30x 104 km 2 in the eastern Caribbean (Plates 5-8). Over scales larger than about 500 km, features were usually elongated to the west or northwest, which reflected the dominant direction of water movement. At smaller scales, there were eddy-like features which in addition showed southward and eastward motions, as inferred by examining scenes collected during consecutive days (M~YL~R-KhRGER, 1988). The largest eddy-like motions detected were about 250 km in diameter. To determine the cause for the variability observed in the distribution of pigments, we examined time series of several other parameters. As previous studies had shown (H~,EPa~ and FEB~s-OR'mGA, 1975a), we found that the monthly mean wind stress in September (0.04-0.06 N m "2) is about 1/2 the stress observed in April (0.07-0.15 N m-2; e.g. Fig. 7). Also, September winds were rotated slightly clockwise (5-20 degrees) relative to April winds. The theoretical Ekman transport reflects these changes (Fig. 7). SST also showed a fairly regular seasonal cycle (Fig. 6), as did coastal sea level (M~tJ~R-K~owR, 1988). Clearly, the seasonal cycle in sea level is
46
MOLLER-KAROBR et al. 30 29 28 27 26
%-,
25 =' o
...........
i ...........
i ...........
i ...........
i ...........
i ...........
i ...........
i ...........
~. . . . . . . . . . .
i ..........
i ...........
r ...........
i. . . . . . . . . . .
i. . . . . . . . . . .
i ...........
i. . . . . . . . . . .
i. . . . . . . . . . .
J ...........
i ..........
29
~. 28 E 27
~ z6 ~
2s
~ 24
..........
29
28 27 26 25 24
Jan 7 9
Jon 8 0
J t ; n 81
Jan
82
Jan
83
dan
84
Jan
85
Jan
86
Jan
87
Jon
88
1.6
0.40 -0.4 -0.8
-1.2
l
1.2
o
E
o
0.8
',......i
0.4
e
0
illll ...........Ai ii
3 --0.4 -0.8 X -1.2 eE-1.6 1.2 0.8 0.4 0 -0.4
-2
...........
Jan 79
i ...........
Jan 80
i ...........
Jan 81
F ...........
Jan 82
I ...........
Jon 83
I ...........
Jan 84
I ...........
Jan 85
I ...........
don 86
I . . . . . . . .
Jcn 87
I
......
Jan 88
Fie. 6a,b,c. g-year seriesof monthly mean sea surfacetcmp~'atute (SST) and monthly SST anomalies in 4°x4 ° regions o f the central Caribbean Sea and the continental margin (see Fig. lb for location of series). Monthly means in each 4°x4 ° region were based on unclassified monthly temperatttre observations from merchant and naval vessels compiled in 1 °x l ° squares by Fleet Numerical Weather Center (Monterey, CA) and weighted by the number of observations in each 1 °xl ° box. Mean annual cycles were calculated by averaging monthly means across years. Anomalies are the difference betweenindividual monthly means and the mean annual cycle. The areas were centered on: (a) Central Caribbean: 4°x4 ° region centered on 14°N, 67°W. (b) Central Venezuela: 4°x4 ° region centered on II°N, 67°W. (c) Eastern Vcnczucla:4°x4 ° region centered on 1 I°N, 64°W.
Pigment disuibudon in the Caribbean Sea
-80
47
-76
-74
-72
-70
-68
-66
-64
-62
-60
-58
-76
-74
--72
-70
--68
--66
--64
--62
--60
--58
18-
--BO
Q.
-80
-76
April
-74
I
1979
-72
-70
-68
Tau o.1 N m-2 <---
-66
-64
Ekmon t r o n s p o r t 5.0x103 Kg rrr'le-1 <
-62
-60
-58
16
16,
14--
,,
12
10
8
-80
-76
-74
-72
-70
b. September 1979
-68
-66
-64
TQu 0.1 Nm -2
-62
-60
-58
Ekman t r a n s p o r t 5.0x10~ Kg m-ls-1 <
Flo. 7. Monthly mean wind stress and Ekman ~'ansport in the Caribbean Sea based on 1000 mb FGGE IIIB ( gridded ) wind speed (First Global GARP Experiment (FC.,-GE) data). (a) April 1979, (b) September 1979.
48
MOLLER-KARGERet al.
produced by seasonal changes in temperature through steric effects. However, variability in coastal sea level is accentuated by fluctuations in wind stress and Ekman transport. We expand on the significance of these changes in the Discussion. CZCS data covering the western tropical Atlantic shows that the Amazon discharge flows along the continent toward the northwest during February-May periods (MOLLER-KARGER, McCLAIN and RaCH.AgDSON,1988; DEUS~, MOLLER-KARcERand HEMLEBEr~,1988). We inferred that a large fraction of this water entered the Caribbean, while some is entrained eastward off Trinidad tracing an ephemeral anticyclonic eddy (lifespan of 1-3 months, cf. Plates 2 and 4). Part of the Orinoco discharge may also become entrained in this eddy, while the remainder is carried into the Caribbean. But the Orinoco's discharge at this time is 6 to 7 times less than during AugustOctober (Fig. 2). In contrast, between July and November, the bulk of the Amazon water is carried eastward by the NECC. This water may be entrained northward into the North Equatorial Current (NEC), and reach the Caribbean after extensive mixing with Atlantic water. The images clearly showed a plume of high pigment concentrations that occupied the eastern Caribbean during this time. However, this plume originated at the mouth of the Orinoco River (Plates 5-9), rather than at the mouth of tbe Amazon. 4.3. Nutrient dynamics in the Orinoco River plume The CZCS data clearly show that the origin of the large pigment patch present in the eastern Caribbean during the second half of the year is the Orinoco River. Since at this time winds are weak (Fig. 7), sea surface temperature is high (Fig. 6), and the seasonal thermocline is well developed, it is unlikely that this seasonal increase in chlorophyll concentration is due to upward mixing of nutrient-rich deep water. It is likely that a large fraction of the pigments seen originating in the Orinoco represent viable phytoplankton, similar to what is observed in the Amazon's discharge (cf. WOOD, 1966; RYrrmR, ME~ZELand CORW~, 1967; HtJLatmT and CORWI~, 1969; DEMASTER,KNAPPand Nrrr~otmR, 1983; DUST:~, unpublished data). High concentrations of chlorophyll have been observed in surface "bucket samples" taken in April 1986 in the Gulf of Paria, directly north of the Orinoco delta (Table 4). Also, as pointed out above, the surface salinity minima seen during October-November in waters south of Puerto Rico coincide with maxima in surface chlorophyll (YosrnoKA, OWEr~and PESArcrE, 1985). Since the Orinoco is a point source of nutrients, concentrations of phytoplankton should decrease with distance from the mouth as nutrients are consumed and exported into adjacent water. To gain some insight into how nutrients are lost from the Orinoco's plume, we examined transects along the plume's axis and built a simple trophic model based on histograms of the distribution of CZCS-derived pigments. Individual images showed that the plume can have several branches, and that these may maintain coherence for periods of weeks to months. Our transects started south of Trinidad, continued east of Trinidad, and roughly followed the plume's axis to the end of the longest branch. Figure 8 shows pigments along transects obtained on 27 September 1979 and on 9 October 1979, when the Orinoco plume was not excessively obstructed from view by clouds or missing data. Due to movement of the plume, these transects are not spatially congruent. A filter was applied to each transect to replace both cloud pixels and pixels exceeding 7 mg m -3 with a value of 0. Based on historical data (Table 1), we did not expect values as high as 7 mg m -3 to occur offshore, even in the river plumes. We removed such values because we considered them noise associated with
Pigmentdistributionin the CaribbeanSea
• 6
=---4
~
o
1.1
5
E
49
T
1111111
.......
[
•
27 Sep '79 o
9 Oct '79
o...... ,,vo
~
Exponential
4
I"-. 1
o
qll I ~ .
E
II 2
i
i
" .
~'I
t I
~ " / i l l l ...... ?JlJ Ii
it
I i~ ILl i I 1- I
""
I ill rl F'-. ~I I~ I "~'-..
Ill
1
0
o'2
oi,
01s
C[housonds) Distance [kin]
;
,2
,,
FxG.8. Transectsof the Orinocoplumein the Caribbeanon 27 September 1979and 9 October 1979.Thestraightlinerepresentsdilutionby mixingof a conservativeproperty.The exponential decay modelwas obtainedusingan exponentialcurvefit to the transectdata of October 1979: Pigments [mg m"3]= 5.34 × e('°'°'5~×'~=°tn,y,lL cloud margins. The pixels excluded represented < 5% of the number of pixels sampled per transect. Note, however, that we did not exclude these extremes from the series of regional means shown in Fig. 5 or Table 5. The transects were approximately 1000-1400 km long, which is the typical length of the plume between September and November (cf. Plate 5). Plume waters frequently traced some eddy-like features, which appeared in the transects as depressions in the pigments concentrations (e.g. see transect for 27 September 1979 in Fig. 8). Since the eddies were not stationary, their position in the monthly CZCS composites was blurred. Within each transect, pigments decreased along the axis of the plume with distance from the river mouth. To obtain an estimate of the rate at which pigments decreased, we fit a curve to the transect data. Distance from the river mouth was converted to time by assuming that particles in the plume moved at an average speed of 0.3 m s -t (ca. 26 km d -1) in the direction of the plume axis. We tracked plume features in September and October 1979, and obtained velocities of 0.2- 0.4 m s -1, which agreed with previous estimates of the mean west-northwestward motion in the region (KINDER, HEBLrRn and G ~ N , 1985; Kn,~ER, 1983; WLLL,~vtS, 1986; MOLINneJ, SPILLANE,BROOKS,ATWOODand DucKm'r, 1981). The least squares curve that best fit the 9 October 1979 data was an exponential decay model: Pigment = 5.34 x e C-°.°61× t~), n =163, r 2= 0.51,where pigments are in [mg m "a] and time is in [days]. This suggested that pigments in the plume had a half life of approximately 11 days. On 27 September 1979, eddies with lower pigment concentration (plume water surrounding clearer oceanic water) were observed about 200-300 km and 600-700 km downstream of the plume's origin (Fig. 8). Assuming a mean plume velocity of 0.3 m s 1, the exponential curve fit for this date produced a flatter curve than for 9 October (i.e. a lower intercept and a smaller decay constant; curve not shown):
50
MOLLER-KARGERet al.
Pigment = 2.5 x e (*'°34 × "~'), n = 214, r2= 0.29, or a half-life of 20 days. This decay rate is probably an underestimate due to the effect of the eddies. Pigments disappeared from the O r i n o c o ' s plume faster than by conservative dilution in a onedimensional model. Exponential decay models fit the data much better than a straight line (Fig. 8), and the decay rate obtained from the transects included losses due to 3 dimensional dispersion in open waters, sinking, and grazing of phytoplankton cells. This implied that the plume is a significant source of organic matter which contributes to surface food chains and offshore sedimentation in the eastern Caribbean. We then used the C Z C S images to develop atrophic model of the plume. The m o d e l was based on a series of nominal conversions between pigment and nitrogen concentrations, and between pigment concentration and productivity (see Table 6a). This enabled us to compute estimates of average primary productivity and nitrogen uptake in the plume, DO~DAUS and GOERINO'S (1967) " f ' ratio, turnover times of freshwater, particulate carbon and nitrogen, and the average number of times that a nitrogen atom is recycled before export (Table 6c). We present 3 case studies based on histograms obtained during high discharge (27 September 1979, 9 October 1979, and 8 N o v e m b e r 1981 ). As in the case of the transects, we selected scenes in which the Orinoco plume was not excessively obstructed from view by large clouds or missing data. We excluded pixels in upwelling regions and defined the river plume as all C Z C S pixels with concentrations between 0.25 and 10 mg pigment m 3 (Fig. 9; Table 6b). In order to obtain the best estimate of the area of the plume and of the total biomass contained in it, non-valid pixels (clouds) were artificially assigned a pigment concentration. The value given to a particular cloud pixel was randomly selected from valid pixels outside the cloud's perimeter in a window centered at the pixel to be filled. This window expanded dynamically until an arbitrary threshold of 25 valid pixels was reached. Typically, this represented an area of about 425 km 2 around the periphery of the cloud. About 25% of the pixels in the plume had to be filled in this fashion for the cases examined. All calculations in the model requiring an input of chlorophyll used a value o f 50% o f the
TAe LE6. Model of the nitrogen and carbon turnover in the Orinoco River plume based on the pigment distribution observed with the CZCS (Fig. 9). The model was run as an electronic spreadsheet on a personal computer. The table contains three parts, as follows: (a) Variables: definition of parameters, thresholds, conversion factors, and model formulas, (b) Model input parameters and (c) Model results TASLE6a. Variables: Salinity end members [psu]: S(river) = 0 psu, S(ocean) = 36 psu, which is North Atlantic Surface water, S(plume) = 34.5 psu, salinity observed in the fiver plume. z(S(plume)) = 20 m. Is the depth of the 34.5 psu layer, which was visually estimated from transects presented by FROELICH,A'rwooo and GmSE,(1978). z qu~..... (0 psu) = z(S(plume)) * (1 - (S(plume)/S(ocean) ), this is the equivalent depth of flesh water needed to yield z(S(,plume))when mixed with S(ocean). This was estimated assuming linear (conservative) mixing of the salinity end members given above. N(min NO3-N + NI-I4-N) and N(max NOj-N + NH+-N) are the minimum and maximum Orinoco nitrogen concentration in [ I.J,M]: These are discharge-weighted average concentrations of nitrogen in Orinoco river water. Low and high turnover rates were estimated based on dissolved inorganic nitrogen (nitrate + anunonium) and on total nitrogen concentration in the fiver (dissolved inorganic + dissolved organic + particulate nitrogen), respectively (from LEwisand SAUr~DEES,1988; see also HAMILTONand LEWIS, 1987).
Pigment distribution in the Caribbean Sea
TABLE 6.
cont.
D = Orinoco discharge rate [m' s"]: Approximate discharge rate for the month previous to the date for which each plume histo/vam was obtained. Th(rain) and Th(max) are minimum and maximum threshold values [rag m3] for pixels defining the fiver plume. Ch = chlorophyll concentration [rag chlorophyll ra3]. Ch/Pig = 0.5, assumed ratio of chlorophyll a to total pigment. This ratio was applied to all CZCS pigment values for calculating parameters relating to viable phytoplankton. C/Ch = 50, typical carbon to chlorophyll ratio (wt:wt) in a bloom after 18-20 days (see ANTIA, McALLISTER, PARSONS, SamPrtENs and S T R I C K L A N O , 1 9 6 3 ; P A R S O N S , T A K A H A S H I a l a d HARORAVE, 1977). C/N = 6, assumed carbon to nitrogen ratio (atom:atom; REDFIELD,KETCHUMarid RICHARDS, 1963; BANSZ, 1974; PARSONS, TAKArtASmand HARGRAVE,1977). Tp~z = Arithmetic sum of all the pixels in the river plume falling within the defined range of concentrations. At, [kin 2] = T.,. * 17.5 lma2/pixel (based on 1/16 full CZCS resolution, reraapped to UTM projection). "I]fis'is the total area covered by the plume. V tw [m3]= A [w * z ¢quivldent (0 psu) ~ or the area of plume times the equivalent fresh water depth is the equivalent fresh water volume. This was estimated assuming that the plume had a constant depth. Sch [mg Ch]= Sum(Ch * frequency), is the sum of chlorophyll over all pixels encompassing the river plume. The frequency is the number of occurrences of particular concentrations (Ch) in the population of plume pixels. Avech [mg ra'~]. = S Ch/F.x,., is the mean chlorophyll concentration, estimated, from. the plume histogram (Figure 9) and a constant chlorophyll to C-7_~S pigment raao (Ch/Pig). Tcn [rag] = 17.5x106 m s * Sch * z(S(phlme)), is the total amount of chlorophyll in the plume, obtained by integrating pigment concentrations over the area and depth of the plume' s low-salinity layer (20 m in this case). Pigments were assumed to be uniformly distributed in this layer. 17.5x 10 ~ m 2 is the area represented by one pixd. Av%s ling-at N m -2] = Avech * (C/Ch) * z(S(plume)) / (C:N * 12), is the mean particulate nitrogen concentration in the plume. Tpn [rag-at] = Tch * (C:Ch) / (C:N * 12), is total particulate nitrogen concentration in the plume. Ave c [mg C m "2] = Avecb * z(S(plume)) * (C/Ch), is the mean particulate carbon concentration in the plume.
T c [metric tons] = Tch * (C:Ch) x 10"9[ton/mg], is the total particulate carbon concentration in the plume. PP [g C In 2 d-J] = SQRT(Ch), is an estimate of primary productivity based on the phytoplank-ton chlorophyll according to EPPLEY,S'rEWART,ABBO'rTand I-I~YMAN,(1985). SQRT(x) stands for square root ofx. This formulation was obtained by EVPLEY based on data from both MOREL Case I and Case II waters (see Fig. 1 in EPPLEY,STEWART,ABBOtt and HEYMAN,1985). Ave.. r e [g C rn "2d'~] = SumfPP * frequency)/T plx 1 where the sum is taken over all pixels encompassing the plume. This is a rough estimate of the average primary productivity in the plume. Ave h° [rag-at N m -2 d "t] = Av%p / (12 * C/N), is the average absolute nitrogen uptake rate by phytoplankton. f = D • N * 60 * 60 * 2 4 / ( T .Inx * 17.5x10 6 ra2 * A v erlao . ) "~ whichistheOrinocoNfluxrelativetonitrogen uptake by phytoplankton. This quantity is equivalent to the 'f-ratio' (of. EPPLEYand PETERSON, 1979). t c [d] = Tch * (C/Ch) / (Tpj, * 17.5x10 * m z * Av%p), which is an estimate of the turnover time for particulate carbon, based on the formulation: (Bioraass / Primary productivity). t v [d] = VI,' / (D * 60 * 60 * 24), is the time needed to fill the equivalent volume of fresh water in the area occupied by the river plume. t~ [d] = Tn~ / (N * D * 60 * 60 * 24),is the time needed to balance the PN of the standing crop with nitrogen supplied by the Orinoco. r = (1 - f) / f, is the average number of times that a nitrogen atom derived from the Orinoco is recycled in the river plume, assuming that there are no other new nitrogen sources (cf. EPPLEY and PErERSON, 1979).
51
52
MOLt.e~-KAxom~ et al.
TAat~ 6b. Model input. Variables and constants are defined and explained in Table 6a. Salinity end members: S(river) = S(ocean) = S(plume) = z(S(plume)) = N(min NO3-N + NH4-N) = N(max NO3-N + NH4-N) =
0 psu, 36.0 psu, 34.5 psu 20 m 8.2 p,M-N 19.7 IxM-N + 13,2 ItM-N 32.9 ttM-N
(NO3 + N H ) (Total dissolved N, organic plusinorganic) (Particulate N)
(Total N)
D (Orinoco discharge rate): 60000 [m3 s-'] ( for 27 September 1979) 50000 [m~s -t] (for 9 October 1979) 40000 [m~ s "~] (for 8 November 1981) Pixel values defining the plume : Th(min) = 0.25 [rag pigment m "3] Th(max) = 10.0 [rag pigment m'31 Ch/Pig = 0.5 C/Ch = 50(weight:weight) C/N = 6(atom:atom)
TABLE6C. Model results. Bracketed quantities are based on low and high nitrogen input by the Orinoco (see Table 6b). Results were based on histograms of the pigment distribution (Fig 10). Each date was run using a different discharge rate (Table 6b). Some quantities were rounded for presentation in the table Dme Or~t
1979 27 Sep 4677
1979 9 Oct 4843
1981 8 Nov 15360
T~ A~ [kin 2] z r~,~(O psu) V~, [m j
13046 228305 0.83 1.9E+l 1
16573 290028 0.83 2.4E+11
17475 305813 0.83 2.5E+11
So [mg Ch] AV%h [nag m "3] Tc~ [mg]
7560 0.58 2.6E+12
9500 0.57 3.3E+12
5560 0.32 1.9E+12
Ave~ [mg-at N m -2] T~ [rag-at] Avec [rag C m"2] T c [metric tons]
8.04 1.8E+12 579 1.3E+05
7.96 2.3E+12 573 1.7E+05
4.42 1.4E+12 318 9.7E+04
0.60 8.32
0.58 8.08
0.23 3.14
[0.02 - 0.091
[0.02 - 0.06]
[0.03 - 0.12]
0.97
0.98
1.41
Aveve [g C m"2d"l] Ave~ [rag-at N m -~d"] f
t c (B/PP) [d]
t,,
[d]
37
56
74
t~
[d]
[11 -43]
[16 -65]
[12 -48]
[10 -49]
[16 -65]
[7 -331
Pigmentdistributionin the CaribbeanSea
¢00i i
Oceanic concentrations
.........
27 September 1979 9 October 1979
Orinoco plume 300
--;ore of Orinoco
53
8 November 1981
plume
at maturity o
LL
200
1 O0
0
--
[
I 1
"1 2
. . . . . . . . . . . . . . . . . 3
Pigments [rag
m--3]
Fla. 9. Histogramsshowingthe concentrationofpigmentsin the Orinocoplume.The histogramswere obtainedfrom three CZCS scenesduringthe period of high riverdischarge.Indicatedare the ranges of CZCS-derivedpigmentconcen~ationsusedto separateoceanicwatersfromthoserepresentingthe river plume (range = 0.25 - 10 mg pigmentsm'3). Each frequencycountrepresents approximately 17.5 km z. Valueshigher than4 mg rn"3were onlya smallfractionof the totalarea and were omitted from this figure. Frequenciesrepresentingoceanic waters (concentrations< 0.25 mg pigmentm .3) were omittedbecausethe area usedto constructthe histogramswas tailoredto the individualplumes. Thus, the number of pixels with oceanic pigment concentrationswas arbitrary, making any comparisonbetweenhistograms at eoncentxations< 0.25 mg rn'3meaningless. CZCS-derived pigment level. As discussed above, this correction was necessary since phaeopigments are a byproduct of grazing and senescence of the phytoplankton (Jm~azY, 1980; MAROAU~F,1965). Also, river plumes are classified as Case II waters (MORELand PRmtm, 1977), in which there may be a large concentration of Gelbstoffe which does not covary with phytoplankton. Typically this leads to an overestimate of phytoplankton concentration (see BAKERand Srcn'rI-L1982; CARDER,STEWARD,PAULand VARGO, 1986b). Unfortunately there is no data on the concentration of Gelbstoffe in Caribbean waters nor in the Orinoco's plume, and we could not correct for this effect. The model presented in Table 6 suggested that the nitrogen discharged by the Orinoco was sufficient to support the observed concentrations o f pigments. During the period of high discharge (September-November; Fig. 2), the average concentration of surrogate chlorophyll in the Orinoco river plume was about 0.5 mg m -3. We estimated that the standing stock of phytoplankton in the plume was approximately l x l 0 ~ metric tons carbon during this mature stage. Using the productivity estimates based on EPPt~V, STEWARTand ABBOaW(1985), it appeared that the turnover time (biomass/primary productivity) was roughly one day. Assuming that the Orinoco is the only source of "new" nitrogen to the bloom observed within the perimeter of the plume, we obtained " f ' ratios (DU6DALEand GO~.tNG, 1967) and recycling factors Cr"; EvPt.m: and P~rv.zSON, 1979) close to those typically observed in the deep, oligotrophic sea. This implied that nitrogen was recycled many times before being exported (Table 6c). Assuming that the turnover rate for carbon (t c, Table 6) also reflects the amount of time that
54
MOLLBR-KARoERet al.
it took to recycle ni~ogen, it appears that the time needed to fill the estimated volume occupied by fresh water in the plume at a given discharge rate (tv) was roughly equivalent to the average number of cycles undergone by nitrogen (i.e. t v ~ t c ×r; Table 6c). Also, the time needed to fill this equivalent volume of fresh water was approximately equal to the time needed to balance the surrogate particulate nitrogen with N discharged by the river (Table 6). Furthermore, if we assume that nitrogen disappeared from the plume at the decay rate obtained earlier from our transects (cf. Fig. 8), nitrogen input by the Orinoco decreased to trace levels (i.e. I% of the average input) in roughly the time needed for the plume to cover the area observed in Plate 5, 7, or 8, namely a • little over 1 month. The above estimates are independent except for the rate of discharge of the Orinoco. Assuming that the estimates of primary productivity based on the empirical formulation of E~LEV, STEWARTand ABBcrrr (I 985; Table 6c) are representative of each of the months for which we had a plume sample, we estimated that the Orinoco contributed about 11 x 106 ton C to the Caribbean during the approximately 90 days in which the plume was well developed. Assuming that all nitrogen discharged by the Orinoco is taken up by algae to fix carbon, however, only 29 x 105 ton C may be considered "new" productivity over 90 days (at an approximate discharge rate of 5 x 104 m 3 s'l). Only the new production is available for export, and this amounts to an annual export of between 7 x l0 s and 29 x l0 s ton C due to the Orinoco alone. We believe that our estimates of productivity and turnover rates in the river plume are not adversely affected by the high light levels typical of tropical oceanic environments. Even though the plume is maintained in the upper part of the water column, vertical mixing within the plume would provide a mechanism by which few cells are photoinhibited. In fact, photoinhihition seems to be an artifact observed only during extended incubation of phytoplankton in clear bottles at surface light intensities. Results of productivity experiments in which realistic in situ fight histories are simulated do not show indications of photoinhibition (e.g. ~ , 1980; YOD~ and BJsHoP, 1985). However, to our knowledge, in situ productivity measurements have never been carried out in the Orinoco River plume. Clearly, these questions need to be addressed in the future. Finally, we realize that the relationship between chlorophyll and productivity presented by E~LEY, STEWARTand ABBo'rr (1985) is simplistic and has a wide margin of error (E~LEY, personal communication). Our earlier comparisons with historical data from the Caribbean region suggest that this approach leads to overestimates of regional productivity. However, this is a convenient formulation for its simplicity, and it allows exploration of the lower trophic dynamics of large patches of phytoplankton based on a limited number of parameters. More sophisticated methods are now being developed (e.g. PLATr and SATnVF~RANATn, 1988). 5. DISCUSSION
5.1. Spatial structure of pigments and environmental forcing The large-scale (> 100 km) distribution of pigments in the Caribbean Sea seems to be controlled by wind stress, flux of water through the basin, and river discharge. Wind plays an important role by forcing the surface circulation in the open tropical Atlantic (BusAt,ACCm and PIcAtrr, 1983) and regulating Ekman drift within the Caribbean (cf. Fig. 7). The wind field clearly is upwelling-favorable for most of the year in the southern Caribbean, but an area of Ekman convergence occurs in the northern Caribbean. The changes in SST observed in the Caribbean are the result of several processes acting in concert. During the first half of the year, cool surface water from the subtropical Atlantic invades
Pigment distribution in the Caribbean Sea
55
the upper layers of the Caribbean and the stronger winds erode the seasonal thermocline formed during the previous summer. Along the coast, the seasonal intensification of the trade winds leads to upwelling (I~Drnm.J~, 1955; MARGntJW,Cm~VIOONand Ym,Ez, 1960; MARGAUW,1965; BALLESTm~ and MAgGAJ.~F,1965; Ricnam~s, 1960; W0ST, 1964; GORDON,1967; ~ and Fm3gES-ORTEOA, 1975a, 1975b; COmU~DOR,1976, 1979). Isotherms may be raised as much as 90-175 m during February-June periods, enough to bring Subtropical Underwater and Antarctic Intermediate Water to the euphotic zone (I~RRm~ and FEBRES-ORTEGA,1975a; KINDER,HEBtnU~and GREEN, 1985). Thus, SST along the continental margin is always lower than SST in the central Caribbean (cf. Fig. 6). The higher nutrient concentration characteristic of this deeper water (MoRR~SONand NowLtn, 1982) leads to the blooms observed in these regions. In contrast, little or no coastal upwelling may take place during July-November periods (MARGALEF,1965; HEggEgAand FEBRES-ORTE6A,1975a). Blooms near the coast become increasingly rare, in particular in September and October. This appears to be the combined result of reduced wind stress, reduced turbulence along the coast, formation or strengthening of a seasonal thermocline, and deepening of the permanent thermocline. Monthly mean wind stress between July and November is < 0.07 N m "z, and sea level and SST are higher than their means, or ca. +0.1 m and 29 ° C, respectively (MOLLER-KARGER,1988). In spite of the strong westward winds early in the year, under which we expected evidence of a strong northward Ekman transport throughout the region, pigments remained confined to the southern Caribbean. As mentioned above, it has been inferred that the flux of water through the Caribbean is larger during the first half of the year than during the second half (MoR~JSONand NOWLIN,1982; I-~Btnunand RHODES,1987). We propose that the jet of Atlantic water that moves westward through the Caribbean during the first part of the yearpresses the plumes and upwelled water against the coast, despite local Ekman forcing driven by the trade winds. In contrast, when the trade wind and the inflow of Atlantic water relax during the second half of the year, the circulation switches to a mode dominated by local Ekman transport, and a large pigment patch spreads across the Caribbean toward Puerto Rico. The change in the circulation is traced by the discharge of the Orinoco, which can cross the Caribbean and reach the vicinity of Puerto Rico in approximately 1 month. River plumes in the northern hemisphere typically turn to the right of their mouths (BEARDSLEYand HART,1978; CHAO and BOXCOLrRT,1986). But the Orinoco's plume turns to the left as it leaves the delta. This indicates that accelerations other than those due to the rotation of the earth are strong. Because of the decrease in the Coriolis parameter (f) toward the equator, the Ekman depth of frictional resistance in the tropics increases to depths which encompass the plumes of rivers (typically 20 m or greater). Thus, river plume dispersal is tightly coupled to variation in the wind. The movement of Orinoco water is probably controlled by the large-scale (> 1000 km) drift of the North Equatorial Current against the coast and island chain, as modified by local Ekman drift. Orinoco waters that reach the northern Caribbean are rapidly diluted by mixing or removed when Atlantic water moves westward early in the year. This westward flow may show interannual variation, as suggested by the different dispersal patterns traced by the plume in different years. For example, the absence of pigment north o f 14°N in March 1979 (Plate 1), as opposed to January and March 1980 (Plates 2 and 3), implied that there was stronger westward flow in early 1979 compared with early 1980. Flow during the second half of the year also varies. For example, pigment patches during the second half of 1979 (Plate 5) were more elongated to the northnorthwest than during the second half of 1980 and 1981 (Plates 7 and 8), which indicated stronger westward flow in the second half of the latter years. As mentioned in the introduction, measurements of dynamic height in the Caribbean Sea have OPO ~3:l-O
56
MOLLER-KARGERet al.
led to inferences of one or two currents flowing eastward near the surface (FtrloJOr,~, 1965; ATWOOD, 1976; F ~ e . s - O R ~ o A and ~ R A , 1976; Arml~,s, FEm~JES and H.mu~lu~,, 1979; MOm~ISON and NOWLIN, 1982; DUNCAN, Scrm.~x~ow and Wmt.tAMS, 1982). However, the 4-year series of CZCS images did not provide any evidence for near-surface, basin-scale countercurrents. Southward and eastward motions at scales of about 250 km occurred in association with eddies such as those observed in the model of I-IEBtrl~, Kn,rOF_X,A ~ and HtntLmrRT (1982). ThUS, as pointed out by Ka,mER, Hm3trRN and GREEN (I 985), the evidence for eastward flows may have been an artifact caused by non-synoptic mapping of the dynamic topography. Dramatic examples of the importance of the wind in the dispersal of tropical river water were observed in 1980 and 1984. Of the years for which we examined wind stress and coastal sea level ( 1979-1981 ), the strongest westward stress (daily mean stronger than - 0.18 N m2) and lowest sea level (daily mean < -0.2 m) occurred during March-May 1980 (MC.~£R-K~GER, 1988). Pigment levels in the southern Caribbean increased simultaneously (cf. Plate 3). High winds persisted until after June 1980, possibly causing a premature northward dispersal of pigments which had formed along the continental margin, e.g. in the plume of the Orinoco (cf. Plate 6, Fig. 5b). However, it was not possible to make clear inferences about the origin of the high pigments in the northern Caribbean in June 1980 since only a few CZCS images were available, and these did not cover the southern Caribbean nor Atlantic waters immediately east of the Antilles. There was no other comparable event during the time series. In November 1984, the Orinoco plume was observed drifting northeastward, extending over 1000 km into the northern Atlantic (Plate 9). This was the only instance in which this situation was observed. In fact, during the first half of November 1984, wind stress in the southern Caribbean (Isla Orchila, Venezuela) was about 0.020 N m -2 to the NNE (Cagigal Observatory records, Venezuela). Presumably, the Ekman drift induced by these winds, combined with residual flow in the western tropical Atlantic, explain the northeastward elongation of the plume. In mid-November, winds became westward again, but unfortunately there were no additional CZCS data available to examine the plume's behavior. This event supported inferences that a phenomenon analogous to that of El Nino occurred in the Atlantic in 1984 (see special edition of Nature, vol. 322, pp. 236-253, 1986).
5.2. Role of the Orinoco as a nutrient source The Caribbean is biologically productive due to an abundance o f nutrients derived from river discharge and upwelling. But the rate at which "new" nutrients (see DUGDALEand GOERIN6, 1967) are supplied to surface waters of the Caribbean via these processes has remained unquantified. With the CZCS, we had the capability to measure the area over which a nutrient source had an impact on phytoplankton biomass, and thus we could refine nutrient budgets for marginal seas such as the Caribbean. An essential variable in simple models is the rate at which nutrients are supplied. The Orinoco is a point source for which we have the daily discharge rates and discharge-weighted nutrient concentration (L~,as and SAtrNDERS, 1988; see also HAMILTONand L~wcIs, 1987). In contrast, upwelling is a source which is spread out over a larger area. Unfortunately, we did not have information on either the upward velocities at upwelling sites, nor of the depth from which upwelled waters are drawn in the Caribbean, at the time these CZCS data were collected. Therefore, we were unable to estimate the amount of nutrient supplied to the euphotic zone via upwelling. We also did not have good information on the amount of nitrogen discharged by the
Pigmentdistributionin the CaribbeanSea
57
Amazon River, and could not separate the effect of inorganic suspended matter and Gelbstoffe from phytoplankton in the Amazon plume as it drifted along the coast toward the Caribbean during the first half of the year (see MOLLER-KARGF_~,1988). Transects along the axis of the Orinoco's plume showed that pigments decreased exponentially with distance from the delta (Fig. 8). We estimated that concentrations along the plume decreased with a half life of 10-20 days. This suggested that nutrients introduced at the mouth of the river were slowly lost from the plume, and that additional input of new nutrients was small. Losses included spreading and mixing, as well as grazing and sinking of phytoplankton. This simple model did not explain local (pixel-size) maxima seen along the transects, which may argue for noise in the CZCS data or localized nutrient additions. Alternatively, the noise may have been due to losses of particles by localized grazing. From an atmospheric point of view, the noise may have been due to small or thin clouds that escaped detection by the land and cloud-flag algorithm. Since there are no estimates of grazing or sinking in the Caribbean Sea, and because transects do not represent the 3-dimensional nature of the plume, inferences about the dynamics of nutrients based on individual transects were limited. We examined the potential effect of the Orinoco by estimating the phytoplankton crop contained in the plume and comparing it to the river's output of nitrogen. The plume experiences growing, mature, and senescent stages, which are reflected in the shape of the histograms of pigment concentration (Fig.9). Given the young age of the plume on 27 September 1979, the model based on this date is probably the least erroneous. At maturity (October), the plume developed a distinct core of high pigment concentrations, which vanished again as the plume entered the senescent stage (November). At later stages of development, our simple model does not reflect the true history of river discharge nor the dynamics of nutrients within the plume. The model suggested that the Orinoco provided 2-12% of the daily nitrogen requirements of the phytoplankton in the plume (Table 6c). We assumed that upwelling was small during the season of maximum discharge, and in particular, that it was negligible in the areas affected by the plume. This assumption may not hold off the northern coast of Trinidad, but we had no data to explore the hydrography of this region in more detail. We also assumed that there were no additional sources of new nutrient to the plume other than the river. These assumptions may lead to an underestimate of 'f' (see DUGDALEand GOEP,~G, 1967), and imply that the balance of the nitrogen required for daily productivity was provided by recycled nitrogen. 'f' may be further underestimated because it is a surrogate of the EPPLEY,STEWARTand ABBcrrr (1985) formulation for primary productivity (see Table 6a). The result is that the recycling factor 'r" may be grossly overestimated. We examined the importance of recycling by ftrst estimating the time needed to supply the fresh water contained in the body of the plume (Table 6b). The volume discharged during September 1979 was comparable to the estimated volume of the plume at the end of September (Table 6c). Also, the nitrogen discharged during this period was roughly equivalent to the estimated total particulate nitrogen (PN) in the plume (see time needed to balance the PN in the plume with discharged N, tn, in Table 6c). This suggested that little nitrogen had been lost by 27 September 1979. But as the plume expanded and aged, the time needed to balance the standing PN crop with nutrients provided by the river remained relatively constant (Table 6c). This reflected a decrease in average pigment concentrations (Fig. 9) and a loss of nutrients. A loss may be expected because the discharge, and consequently the nutrient input, decreased after October. Also, the plume was progressively diluted offshore with nutrient-poor oceanic water. Assuming that the amount of nutrient supplied by the Orinoco relative to the rate of nutrient
58
MOLt:aR-KARanRet al.
uptake by phytoplankton is an estimate of 'f'. we estimated that nitrogen atoms were recycled 765 times (recycling factor = r = (1-0/f; Table 6c; see EI~LWeand l:'zrv_asoN, 1979; EPPLZY, 1981; I-IARP,ISON,DOUGLAS,FALKOWSga,ROWEand VID~a., 1983). The value of 65 is probably unrealistic, since even in the central oceanic gyres nitrogen is cycled only about 25 times (EPrLL'Y and Pm ~SON, 1979). The lower values, however, are comparable to values found in productive areas such as the Mid-Atlantic Bight (H.~asoN, Doubt.as, F~a~Kowsi,a, Rowe and V ~ , 1983). This suggested that the plume exported a large amount of nutrients while supporting large standing crops. However, the lower values of "r" are probably underestimates, since not all of the total nitrogen discharged by the river is likely to be available to or utilized by the plankton. Clearly, in order to refine nutrient budgets such as the one described above, we need to obtain better data on the concentration and productivity of viable plankton in these river plumes. To take full advantage of the remote sensing technology available in the future, we also have to define the bio-optical characteristics of these waters. To expand our nutrient budgets, we would need to examine the effect of the material in the plume on the submarine light field, on which the phytoplmtkton forming the deep chlorophyll maximum depend. We also need better data on upwelling near the margins and, in general, on biological processes affecting phytoplankton communities in the region (i.e. productivity, sinking, and grazing). A large amount of inorganic and biogenic material is exported to deep waters near the continental margin. However, the amount oftbe production stimulated by nutrient addition at the margins that sinks below the thermocline is unknown. These fluxes help maintain the meridional gradient in properties observed in deep waters of the Caribbean (Mov,v,asoN and Nowi.~, 1982; ATWOOD,FROELICH,Pa..so~, BARCELONAand VmEN, 1979; SV'EVa)Rt.rI',JOHNSONand FLEMING,1942). Our heuristic results suggest that efficient recycling within the river plume delays transport of some carbon and other nutrients to deeper waters of the basin. Nevertheless, materials that sink may be trapped in the basin for periods of at least 50 to 800 years (R.zDFmt~, KzrcrrLrM and Racr-t~s, 1963; K n , ~ , I-IEBt~,Yand G~',~, 1985), and much longer if they are incorporated into sediments. This is long compared to the time scale of increase of CO 2 in the ocean-atmosphere system by fossil fuel burning, deforestation, and cement manufacturing (see ~ A ~ , 1988), and could provide a regional sink for atmospheric CO 2. If we assume that half of the carbon fixed within the plume in a year is trapped within the basin below the thermocline, then approximately 3 to 15 x 105 ton C are removed from the atmosphere. This is a small amount relative to the carbon released annually from the harvest and clearing of the global tropical forests (i.e. less than about 0.01%; see WOODWELL,WHrlq'AKER,REINERS, LIx_~s, D~wlcrn~ and BOTr,m, 1978), but has an important effect on the local geochemistry and carbon cycling through the regional food web. Remote sensors like the CZCS are the only tool capable of providing synoptic estimates of plankton abundance which may help us further understand possible sinks of carbon. In anticipation of interdisciplinary earth observation studies planned for the decade of 1990, we need to develop a comprehensive data base of in situ optical information for the continental margins of the world. This information must be related to phytoplankton biomass or productivity, and to the fate of humic and fulvic compounds, or clay particles, which absorb blue light similar to phytoplankton pigments (cf. DAvxs-CoL~Y and V~rr, 1987; BAKERand S~wr,a, 1982; Y~'crsc~I, 1960). Particular attention should be given to rivers since, even though they are responsible for less than 1% of the new productivity of the world's oceans (HARRXSON, 1980), they are important local nutrient sources and help trace the circulation of nearby oceans.
Pigment distribution in the Caribbean Sea
59
6. ACKNOWLEDGEMENTS The suggestions of TI~OMASMALONEand BILL BOICOURT(University of Maryland, UMCEES), WATSONGREOO (University of South Florida), and two anonymous reviewers for revising the manuscript are greatly appreciated. We thank JOHN SISSALA,MInE DOLINEand RICaARDSn,~ (Nimbus Operations office, General Electric) for their help in selecting the imagery. JUDY CnEN, MIKE DARZI and JIM FmBSZONE(General Sciences Co.) implemented the image analysis software (SEAPAK). OHER--OTEC project reports on chlorophyll observations off Puerto Rico were kindly provided by JOROECORREDOR and PAUL YOSmOKA (Department of Marine Sciences, University of Puerto Rico, Mayaguez). This work was supported by the Ocean Processes Branch at NASA Headquarters and by the NASA Graduate Student Researcher's Program at the Goddard Space Flight Center (Grant No. NGT 21-002-822). University of Maryland (UMCEES) publication No. 1983. 7. REFERENCES
ALBERrS,J. (1973) Reportof"DISCOVERER" cruise. Nutrient chemistry. Data Report S.C.O.R. Expedition, May 1970. Volume 1. University of California, SIC) Ref. 73-16, pp. L1-LI2. ANDRES, J. M., G. A. FEBRES and L. E. HERRERA(1979) Circulacion geostrofica fxente a Venezuela, Mar Caribe Suroriental. Boletin Instituto Oceanografico Universidad de Oriente, 18, 115-126. ANTXA,N. J., C. D. MCALLIS'rgR,T. IL PARSONS,K. STEPr~ENSand I. D. H. S'rRICKLAND(1963) Further measurements of primary production using a large-volume plastic sphere. Limnology and Oceanography, 8, 166-183. ATWOOD,D. K. (1976) La oceanografia regional con respecto a los problemas actuales y futuros de la contaminacion y de los recursos vivos - CARIBE. Intergovernmental Oceanographic Commission, FAO, Reunion internacional de Trabajo IOC/FAO/UNEP/IWMPCAR/8, Paris, 18 November 1976, pp. 1-36. AXWOOD,D. K., P. N. FROELXCH,M. E. PlrsoN, M. J. BARCELONAand J. L. VmE~ (1979) Deep silicate content as evidence of renewal processes in the Venezuela Basin, Caribbean Sea. Deep-Sea Research, 26A, 1179-1184. BAreD, I. E. (1973) Report of "DISCOVERER" cruise. Chlorophyll a and Phaeopigments. Data Report S.C.O.R. Expedition, May 1970. Volume 1. University of California, SIO Ref. 73-16, lap. C1-C25. BAKER, K. S. and IL C. SMTm (1982) Bio-optical classification and model of natttral waters. Limnology and Oceanography, 27,500-509. BALLESrER, A. and R. MAROALEF(1965) Produccion primaria. Memoria, Sociedad de Ciencias Naturales La Salle, 25, 209-221. BAtqSE, K. (1974) On the interpretation of data for the carbon-to-nitrogen ratio of phytoplank-ton. Limnology and Oceanography, 19, 695-699. BANSE, IC and C. R. McCLAxN(1986) Winter blooms of phytoplankton in the Arabian Sea as observed with the Coastal Zone Color Scanner. Marine Ecology - Progress Series. Inter-Research, 34, 201-211. BARALE, V., C. R. MCCLAtN and P. MALArqOrrE-RIzzoLI(1986) Space and time variability of the surface color field in the northern Adriatic Sea. Journal of GeophysicalResearch, 91:C11, 12957-12974. BEAROSLr~Y,R. C. and J. HART (1978) A simple theoretical model for the flow of an estuary onto a continental shelf. Journal of Geophysical Research, 83,873-883. BEERS,J. IL, D. M. STEVEr~and J. B. l.~wxs (1968) Primary productivity in the Caribbean Sea off Jamaica and the tropical North Atlantic off Barbados. Bulletin of Marine Science, 18, 86-104. BERMUDABIOLOOICALSTATION(1965) Primary productivity in the tropical North Atlantic offBarbados and the Caribbean Sea off Jamaica. Final Report submitted to the Biology Branch, Office of Navai Research, St. Georges's West, Bermuda, 130pp. B~'rz~R, P. 1L, D. W. EOOXMAr~,K. L. CARDER,D. R. IC~_SrERand S. B. BErZ~R (1977) Seasonal patterns in suspended calcium carbonate concentration during the dry and wet seasons in the eastern Caribbean. In: The Fate of Fossil Fuel CO: in the Oceans, N. R. ANDERSENand A. MALAHOPF,editors, Plenum Press, 63-79. B ORSTAD,G. A. (1982a) The influence of the meandering Guiana Current and Amazon River discharge on surface salinity near Barbados. Journal of Marine Research, 40,421-434. B ORSTAO,G. A. (1982b) The influence of the meandering Guiana Current on surface conditiom near Barbados - temporal variations of Trichodesmium (Cyanophyta) and other plankton. Journal of Marine Research, 40, 435-452. BRows, O. B., IL H. EVANS,J. W. BROWN,H. IL GORDON,R. C. SMIThand IC S. BAKSR(1985) Phytoplankton blooming off the U. S. east coast: a satellite description. Science, 229, 163-167. BURKHOLDER,P. R., L. M. BURKHOLDERand L. R. ALMODOVAR(1967) Carbon assimilation of marine flagellate blooms in nerilSc waters of southern Puerto Rico. Bulletin of Marine Science, 17, 1-15. BUSALACCMt, A. J. and J. PICAUT(1983) Seasonal variability from a model of the tropical Atlantic Ocean. Journal of Physical Oceanography, 13, 1564-1588.
MOLLER-KARQER
60
et al.
G. W. and G. D. GRICE (1967) Influence of the Amazon River outflow on the ecology of the western tropical Atlantic. Il. Zooplankton abundance, copepod distribution, with remarks on the fauna of low-sali& areas. Journal of Marine Research, 25,84-94. CAMPBELL, J. W. and W. E. E~AIAS (1985) Spatial patterns in temperature and chlorophyll on Nantucket Shoals from airborne remote sensing data, May 7-9, 198 1. Journal of Marine Research, 43, 139- 161. CARDER, K. L., R G. STEWARD. D. L. BETZER and J. M. PROSPERO (1986a) Dynamics and composition ofparticles from an aeolian input event to the Sargasso Sea. Journal of Geophysical Research, 91 Dl, 1055-66. CARDER, K. L., R. G. STEWARD, J. H. PAUL and G. A. VARGO (1986b)Relationships between chlorophyll and oceancolor constituentsastheyaffectremote-sensingretlectancemodels. LimnologyandOceanography,31(2).403413. CARLSON, T. N. and J. M. PROSPLIRO (1972) The large scale movement of Saharan air outbreaks over the north equatorial Atlantic. Journal of Applied Mereorology, 11.283-297. CARPENTER. E. J. (1983) Niuogen fixation by marine OsciUaroria (Trichodesmium) in the world’s oceans. In: Nitrogen in rhe Marine Environmenr, E. J. CARPENTER and D. G. CAPONE, editors, 65-103. CARPENTER, E. J. and C. C. PRICE (1977) Nitrogen fixation, distribution, and production of Oscillaroria (Trichadesmium) spp. in the western Sargasso and Caribbean Seas. Limnology and Oceanography, 22,60-72. CBRVIGON, F. and P. J. MARCANO (1965) Zaoplsnkton Memoria, Sociedad de Ciencias Naturales Lm Salle, 25,263-360. CHAO, S.-Y. and W.C. Bo1cou~~(1986)Gnsetofestuarineplumes. JournalofPhysicalOceanography, 16,2137-2149. CORREDOR, J. E. (1976) Aspects of phytoplardcton dynamics in the Caribbean Sea and adjacent regions. In: Cooperative Invesrigations of the Caribbean and Adjacent Regions (CIGAR) - II. Symposium on Progress in Marine Research in the Caribbean andAdjacent Regions, Caracas, 12-16 July 1976, FAO Fisheries Report NO. 200, 101-l 14. CORREDOR, J. E. (1979) Phytoplankton response to low level nutrient enrichment through upwelling in the Colmnbian Caribbean Basin. Deep-Sea Research, 26A. 731-741. CULLBN, J. J. (1982) The deep chlorophyll maximum: comparing vertical profiles of chlorophyll a. Canadian Journal of Fisheries and Aquatic Science, 39,79 1. CURL, H. (1960) Primary production measurements in the north coastal waters of South America. Deep-Sea Research,
CALEP,
7. 183-189.
DAVIS-C•
LL~Y,R. I. and W.N. V~~~(1987)Absorbtionoflight by yellow substanceintieshwaterlakes. Limnologyand Oceanography, 32,416425. DELPTHYDRAULICS LABORATORY (1962) Demerara coastal investigation: Report on siltation of Demerara Bar Channel and coastal erosion in British Guiana. The Nerherlands Del&, v. l-3,240 pp. DBMASTFZR, D. J., G. B. KNAPP and C. A. NITTROUHR (1983) Biological uptake and accumulation of silica on the Amazon continental shelf. Geochimica et Cosmochimica Acta, 47, 1713-1723. DENMAN, K. L. and M. R. ABBOTT (1988) Time evolution of surface chlorophyll patterns from cross-spectrum analysis of satellite color images. JOWMI of Geophysical Research, 93 C6,6789-6798. DEUSER. W. G.. F. E. M~~LLIX-LARGER and Ch. H~MLEBEN (1988) Temporal variations of psrticle fluxes in the deep subtropical and mopical North Atlantic: Eulerianversus Lagrangian effects. Journal of GeophysicalResearch, 93 C6.6857-6862.
DOTY, M. S. and N. OOURI (1956) The island mass effect. J. Conreilpenn. inl. E.xpIor. Mer., 22, 33-37. DUGDALE, R. C. and J. J. GOHUNG (1967) Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography. 12.685695. DUNCAN, C. P., S. G. SCHLADOW and W. G. WILLIAMS (1982) Surface currents near the Greater and Lesser Antilles, Int. Hydrog. Rev., Monaco., LIX. 2,67-78. EISMA, D., S. J. VAN DIZR GAAST, J. M. MARTIN and A. J. THOMAS (1978) Suspended matter and bottom deposits of the Grinoco delta: Turbidity, mineralogy, and elementary composition Netherlands Journal ofSea Research, 12, 224-25 1. EPPLEY,R W. (1981)Autotrophicproductionofparticulatematter. In:AnalysisofmarineEcosystems,A. R. LONGHURST, editor, 343-361. EPPLEY, R. W. and B. J. P~TIZRSON (1979) Particulate organic matter 5ux and planktonic new production in the deep ocean. Nature, 282, 677-680. EPPLEY. R. W.. E. STEWART, M. R. ABBOTT and U. HEYMAN, (1985) Estimating ocean primary production from satellite chlorophyll. Introduction to regional differences and statistics for the Southern California Bight. Journal of Plankton Research, 7.57-70. ESAIAS. W. E. (1980) Remote sensing of oceanic phytoplankton: present capabilities and future goals. In: Primary Productivity in rhe Sea, P. G. FALKOWSKI, editor, Plenum, New York, 321-337. Erra~,P. C., P. J. LAMB andD.H. Poar1s(l987)Heatand heshwaterbudgets ofthecaribbean Seawithrevisedestimates for the cenb-al american seas. Journal of Physical Oceanography, 17, 1232-1248. FAO (1986) Yearbook of Fishery Statistics. Catches and Landings. 1984. Vol. 58, Food and Agriculture Organization of the United Nations, Rome, 89-91. FASHAM, M. J. R. (1978) The application of some stochastic processes to the study of plankton patchiness. In: Spatial Pattern in Plankton Communiries. J. H. STEELE, editor, 131-156. FEBRES-GRTBGA, G. and L. E. HIZRREIRA (1976) Caribbean Sea circulation and water mass transports near the Lesser k’Uilles. Boledn Institute Oceanografico Universidadde Orienre, Cumana, Venezuela, lS,83-96.
Pigment disffibution in the Caribbean Sea
61
Fm.OMAN,G.C., D. CLARKand D. I-Ixu,m~ (1984) Satellite color observations of the phytoplankton distribution in the eastern equatorial Pacific during the 1982-83 El Nino. Science, 226, 1069-71. Ftsrn~, J., R. D o m u ~ and H. GR.~SSL(1986) Factor analysis of mdtispectral radiances over coastal and open ocean water based on radiative transfer calculations. Applied Optics, 25, 446-456. FRom.lc~, P. N., D. IC As'WOODand G. S. GIr~B(1978) The influence of Amazon River water on surface salinity and dissolved silicate concentration in the Caribbean Sea. Deep-Sea Research, 25, 735-744. Foruor,~, J. (1965) Condiciones meteorologicas e hidrograficas de los mares adyacentes a Venezuela. 1962-1963. Mernoria, Sociedad de Ciencias Naturales La Salle, XXV, 11-38. GraBs, IL J. (1980) Wind-controlled coastal upwelling in the western equatorial Atlantic. Deep-Sea Research, 27A, 857866. GmEs, H. (1972) Carta Pesquera de Venezuela (1): Areas del Nororiente y Guayana. I-ino. GmF.s,editor, Monografia No. 16, Fundacion La SaUe de Ciencias Naturales, Caracas, Venezuela. 328 pp. Gnx,v.s, H. (1982) Curta Pesquera de Venezuela (2): Areas Central y Occidental. Hno. Gl~.S, editor, Monografla No. 27, Ftmdacion La Salle de Ciencias Naturales, Caracas, Venezuela. 227 pp. GORDON,A. L. (1967) Circulation of the Caribbean Sea. Journal of Geophysical Research, 72, 6207-6223. G omaor~,H. R. and D. K. C ~ (1981) Clear water radiances for atmospheric correction of Coastal Zone Color Scanner Imagery. Applied Optics, 20, 4175--4180. GORDOr4,H.R., D. K. CLARK,J. L. M ~ and W. A. Hoxas (1980) Phyloplankton pigments derived from the Nimbus7 CZCS: comparisons with surface measurements. Science, 210, 63-66. GoRooN, H. R., D. K. CLARK, J. W. BROWN,O. B. BROWNand R. H. EVANS(1982) Satellite measurement of the phytoplankton pigment concentration in the surface waters of a warm core GttlfStream ring. Journal ofMarine Research, 40, 491-502. GORDON,H. R., D. K. CLARK,J. W. BROWN,O. B. BROWN,R. H. EVANSand W. W. Baom,n~ow (1983a) Phytoplankton pigment concentrations in the Middle Atlantic Bight: Comparison of ship determinations and CZCS estimates. Applied Optics, 22, 20-35. GoaooN, H. R., J. W. BROWN,O. B. BROW~, R. H. Ew~Nsand D. K. CLARr (1983b) Nimbus 7 CZCS: reduction of its radiomea-ic sensitivity with time. Applied Optics, 22, 3929-3931. HAMm'ros, S. K. and W. M. LEwis., Jr. (1987) Causes of seasonality in the chemistry of a lake on the Orinoco River floodplain, Venezuela. Limnology and Oceanography, 32, 1277-1290. I-[gacP.xv~, P. E., R. W. B~ODYand P. R. BtntKnOLD~ (1970) A study of phytoplankton in the Lesser Antilles region. Bulletin of Marine Science, 20, 331-349. t-IAmtlSON,W. G. (1980) Nutrient regeneration and primary production m the sea. In: Primary Productivity in the Sea, P. G. F~a.rowsKJ,editor, Plenum Press, 433-460. HARRISON,W. G., D. DOUGLAS,P. FAtXOWSKX,G. ROWEand J. VmAL(1983) Summer nutrient dynamics of the middle Atlantic bight: nitrogen uptake and regeneration. Journal of Plankton Research, 5, 539-556. I'-Ir:auva~,G. W. and R. C. RHODES(1987) Numerical simulation of wind forced seasonal and interannual variability of the transport through the Caribbean Sea. EOS, Vol. 68, No. 16, Abstract O31A-07, p. 337. I4a~tn~, G. W., T. H. KINDER,J. H. Ata.m'~m and H. E. Hta~LBta~a"(1982) A numerical model of eddy generation in the southeastern Caribbean Sea. In: Hydrodynamics ofSemi-EnclosedSeaa, J. C. J. Nmou-t.(ed.), Elsevier, 299328. ~ , L. E. and G. FmR.eS43RrEOA (1975a) Procesos de surgencia y de renovacion de aguas en la Fosa de Cariaco, Mar Caribe. Boletin Instituto Oceanografico de la Universidad de Oriente, 14, 31-44. ~ , L. E. and G. FmaVaES-ORaaZ~A(1975b) Kinematics of the wind-generated velocity field in the surface waters off eastern Venezuela in the Caribbean Sea. Boletin Instituto Oceanografico de la Universidad de Oriente, 19, 165-186. HOLLIES, P. M., M. VIOLLma,D. S. HAm3ou~,P. CAMVS and M. C~Ac~r~-Prm.t~p~ (1983) Satellite and ship studies of coccolithophore production along a continental shelf edge. Nature, 304, 339-342. HOI~L, J. D., V. E. KousKYand M. KAOAr~O(1986) Atmospheric conditions in the Atlantic sector during 1983 and 1984. Nature, 322, 248-251. HtJLat~T, E. M. and N. CoRwIr~(1969) Influence of the Amazon River outflow on the ecology of the western tropical Atlantic. HI. The planktonic flora between the Amazon River and the Windward Islands. Journal of Marine Research, 27, 55-72. I s E ~ , H. J. and L. HAssr~(1985) The Bunker Climate Atlas of the North Atlantic Ocean. Volume 1: Observations. Springer-Verlag. 218 lap. Jm~.Ev, S. W. (1980) Algal pigment systems. In: Primary Productivity in the Sea, P. G. FAt~owsra, editor, Plenum Press, 33-58. KATZ, E. J. (1987) Seasonal response of the sea surface to the wind in the equatorial Atlantic. Journal of Geophysical Research, 92 C2, 1885-1893. Kn~D,R. and F. S ~ (1979) Influence of Amazon River discharge on the marine production system offBarbados, West Indies. Journal of Marine Research, 37,669-681. Kn~SrV.AD, H. and L. B. SLOBODr.m(1953) The size of water masses containing plankton blooms. Journal of Marine Research, 12, 141-147. Kn,ro~, T. H. (1983) Shallow currents in the Caribbean Sea and Gulf of Mexico as observed with satellite-tracked drifters.
62
MOLL~R-KAg~ et al.
Bulletin of Marine Science, 33,239-246.
KINDER,T. H., G. W. H~BURNand A. W. GREEN(1985) Some aspects of the Caribbean circulation. Marine Geology, 68, 25-52. KOBLEr~z-MIsnKe,O. J., V. V. VOLKOVICSKY and J. G. IG~BAr~OVA(1970) Plankton primary production of the world ocean. In: Scientific Exploration of the Southern Pacific, W. S. WOOSTER,editor, National Academy of Science, Washington, DC, 183-193. LAWS, E. A., G. R. DITULLIOand D. G. REDALJE(1987) High phytoplankton growth and production rates in the North Pacific subtropical gyre. Limnology and Oceanography, 32,905-918. LEwxs, JR., W. M. and J. F. SAUNDERS,III (1988, in press) Concentration and transport of dissolved and suspended substances in the Orinoco River. Biogeochemistry. MACKAS,D. L., K. L. DENStANand M. R. A~ aoT'r (1985 ) Plankton patchiness: biology in the physical vernacular. Bulletin of Marine Science, 37,652-674. IVlxLor~E,T. C. ( 197 I) The relative importance ofnannoplankton and netplankton as primary producers in tropical oceanic and neritic phytoplankton communities. Limnology and Oceanography, 16,633~539. MALONE, T. C. (1973) Report of "DISCOVERER" cruise. Nanoplankton (<22) and net plankton (>22) chlorophyll concentrations in mg m -~. Data Report S.C.O.1L Expedition, May 1970. Volume 1. University of California, SIO Ref. 73-16, p. El-E7. IVIARGALEF,R. F., CERVIGONand G. YEPEZT (1960) Exploracion preliminar de las caracteristicas hidrograficas y de la distribucion del fitoplancton en el area de la Isla Margarita (Venezuela). Meraorias Sociedad de Ciencias Naturales La Salle, Caracas, Venezuela, 20, 211-221. MAROALEF,R. (1965) Composicion y distribucion del fitoplancton. Memorias. Sociedad de Ciencias Naturales La Salle, XXV, 141-205. MARRA,J. (1980) Vertical mixing and primary production. In: Primary Productivity in the Sea, P. G. FALKOWSKt(editor), Plenum Publishing Co., New York, 121-137. MARRA,J. and K. R. I"I~tNEMAr~N(1987) Primary production in the North Pacific central gyre: Some new measurements based on x4C.Deep-Sea Research, 34, 18-21. MAZEIKA,P. A. (1973) Circulation of water masses east of the lesser Antilles. Deutsche Hydrographische Zeitschrifi, 2, 49 -73. MAZEIKA,P. A., T. H. KINDERand D. A. BURNS(1983) Measurements of subtidal flow in the Lesser Antilles passages. Journal of Geophysical Research, 88 C7, 4483-4488. MAZPARROTE, S. (1967) Observaciones preliminares sobre la composicion y aspectos del "Turbio" o marea roja en las costas orientales de Venezuela. Memorias Sociedad Ciencias Naturales La Salle, 76, 37-45. MXZPARROTE,S. (1970) Composicion del Turbio o Marea Roja en las costas orientales de Venezuela. Memorias Sociedad Ciencias Naturales La Salle, 86, 102-121. MCCLAIN, C. 1~, L. J. PIETRAF.ESA&/ld J. A. YODER(1984) Observations of Gulf Stream-induced and wind-driven upwelling in the Georgia Bight using ocean color and infrared imagery. JournalofGeophysicalResearch, 89 C3, 3705-3723. MEAD.~,R. H., C. F. NORDIN,Jr., D. PER~ZHZRNANDEZ,A. M~IA B. and J. M. PEREZGODOY(1983) Sediment and water discharge in Rio Orinoco, Venezuela and Colombia. In: Proceedings of the Second International Symposium on River Sedimentation, 11-16 October, 1983, Nanjing, China. Water Resources and Electric Power Press, Beijing, China, 1134-1144. MXLLIMAN,J. D. and R. H. M~ADE(1983) World-delivery of river sediment to the oceans. The Journal of Geology, 91, 1-21. MILLIMAN,J. D., J. BU'fENKO,J. P. BARBOTand J. I-I~DBERG(1982) Depositional patterns of modern Orinoco/Amazon muds on the northern Venezuelan shelf. Journal of Marine Research, 40,643-657. MIRO, M. (Unpublished, 1969) Productividad primaria y pigmentos fotosintetizadores del Caribe Sur, Venezuela. Manuscript archived at Em MAR,Fundacion La Salle, Ptmta de Piedras, Isla Margarita, Estado Nueva Esparta, Venezuela. MOLINARI,R. L., M. SPILLANE,I. BROOKS,D. ATWOODand C. DUCKE'fT(1981) Surface curlent in the Caribbean Sea as deduced form Lagrangian observations. Journal of Geophysical Research, 86 C7, 6537-6542. Mooa~, W. S., J. L. SARMmNrOand R. M. K~Y (1986) Tracing the Amazon component of surface Atlantic water using 228Ra, salinity and silica. Journal of GeophysicalResearch, 91 C2, 2574-2580. MOREL, A. and L. PRmUR(1977) Analysis of variations in ocean color. Limnology and Oceanography. 22, 709-722. MORRIs,I., A. E. SMTXHand H. E. GLOVER(1981) Products of photosynthesisin phytoplankton offthe Orinoco River and in the Caribbean Sea. Limnology and Oceanography, 26, 1034-1044. MORRISOr~,J. M. and W. D. NOWLIN,Jr. (1982) General distributions of water masses within the eastern Caribbean Sea during winter of 1972 and fall of 1983. Journal of Geophysical Research, 87 C6, 4207-4229. MOLLER-KAR~ER,F. E. (1988) Pigment variability in the Caribbean Sea: A study using the Coastal Zone Color Scanner. Ph.D. Dissertation, Horn Point Environmental Laboratories, University of Maryland, 218 pp. MOLLER-KARonR,F. E., C. R. McCLAINand P. L. RaCHARDSON(1988) The dispersal of the Amazon's water. Nature, 333, 56-59. NITTROUER,C. A., T. B. CURTINand D. J. DEMASTI~R( 1986) Concenlxation and flux of suspended sediment on the Amazon continental shelf. Continental Shelf Research, 6, 151-174.
Pigment distribution in the Caribbean Sea
63
NOF, D. and D. B. OLsorq(1983) On the flow through broad gaps with application to the Windward Passage. Journal of Physical Oceanography, 13, 1940-1956. OHER-OTEC S.rUDY (1980) Data Report (compiled and edited by J. M. Lopez). Center for Energy and Environment Research. University of Puerto Rico. U.S. Department of Energy. Bimonthly cruise reports, 1980. 80 pp. OKOBO, A. (1980) Diffusion and ecologicalproblems: mathematical models. Springer-Verlag, New York, 254 pp. OLTMAN,R. E. (1967)Reconnaissance investigations of the discharge and water quality of the Amazon. Atas. Syrup. Biota Amazonica 3 (Limnologia), 163-185. PARSONS,T. P,., M. TAKAHASHIand B. HARGRAVE(1977) Biological Oceanographic Processes. 2nd Edition, Pergamon International Library, 332 pp. PELAEZ, J. and J. A. McGOWAN (1986) Phytoplankton pigment patterns in the California Current as determined by satellite. Limnology and Oceanography. 31(5), 927-950. PE.rERSON,B. J. (1980) Aquatic primary productivity and the )4C-CO2method: a history of the productivity problem. Ann. Rev. Ecol. Syst. 11,359-385. PHILANDER)S. G. H. and R. C. PACANOWSKI(1986) A model of the seasonal cycle in the ta'opical Atlantic. Journal of Geophysical Research, 91 C12, 14192-14206. PLA'r'r T. and A. W. HERMAN(1983) Remote sensing ofphytoplanktonin thesea: surface-layer chlorophyll as an estimate of water-column chlorophyll and priniary production. International Journal of Remote Sensing, 4, 343-351. PLA.rr, T. and S. SATHYENDRANA'rH(1988) Oceanic primary production: estimation by remote sensing at local and regional scales. Science, 241, 1613-1620. PROSWRO,J. M., R. T. NEEs and M. UEraATStJ(1987) Deposition rate of particulate and dissolved aluminum derived from Sahara dust in precipitation at Miami, Florida. EOS, 68, 46, p. 1602. PROSPnRO,J. M. E. BONA.r.rl,C. SHUI3ER.rand T. N. CARLSON(1970) Dust in the Caribbean atmosphere traced to African dust storm. Earth Planetary Science Letters, 9, 287 293. PROSPERO, J. M. and T. N. CARLSON(1972) Vertical and aerial distribution of Saha.ran dust over the western equatorial Adantic Ocean. Journal of Geophysical Research, 77, 5255-5265. RAMANATHAN,V. (1988) The greenhouse theory of climate change: a test by an inadvertent global experiment. Science, 240,257-364. REd,toLD, A. C. (1955) The hydrography of the Gulf of Venezuela. Deep Sea Research, Supplement, 3. REDFmLD,A. C., B. H. K~TCHUMand F. A. POCHAROS(1963) The influence of orgamsms on the composition of sea water. In: The Sea. M. N. HILL,editor, Chapter U, interscience, New York, 26-77. RICHARDS,F. A. (1960) Some chemical and hydrographic observations along the north coast of South America. I. Cabo Tres Plmtas to Curacao, including the Cariaco Trench and the Gulf of Cariaco. Deep-Sea Research, 7, 163182. RICHARDSON,P. L. and T. K~ MCKEE(1984) Average seasonal variation of the Atlantic equatorial currents from historical shipdrifts. Journal of Physical Oceanography, 14, 1226-1238. RINE, J. M. and R. N. GINSBURG(1985) Depositional facies of a mud shoreface in Suriname, South America - A mud analogue to sandy, shalJow-marine deposits. Journal of Sedimentary Petrology, 55, 633-652. ROEMMICH, D. (1981) Circulation of the Caribbean Sea: a well-resolved inverse problem. Journal of Geophysical Research, 86 C9, 7993-8005. RV.rnER, J. H. (1969) Photosynthesis and fish production in the sea. Science, 166, 72-76. Rv .rnER,J. H., D. W. MENZ~'Land N. CORWIN(1967) Influence of the Amazon River outflow on the ecology of the western tropical Atlantic. I. Hydrography and nutrient chemistry. Journal of Marine Research, 25, 69-83. SAIJO, Y. and N. NAKAMOTO(1973) Report of "DISCOVERER" cruise. In situ photosynthesis. Data Report S.C.O.R. Expedition, May 1970. Volume i. University of California, SIO Ref. 73-16, p. A1-AI5. SELLNER, K. G. (1981) Primary productivity and the flux of dissolved organic matter in several marine environments. Marine Biology, 65, 101-112. SIOLLH. (1975)Tropical river: the Amazon. In: River Ecology. B. A. WRE'r'rON,editor, BlackweU Scientific Publication, London, 18,461-488. SMITH, R. C. and K. S. BAKER (1978) The bid-optical state of ocean waters and remote sensing. Limnology and Oceanography, 23,247-259. SOKAL,R.R. and F. J. ROrtLE(1981) Biometry. Second edition, W. H. Freeman and Co., 859 pp. STALCUP,M. G. and W. G. MaeTCALF(1972) Current measurements in the passages of the Lesser Antilles. Journal of Geophysical Research, 77, 1032-1049. STEELE, J. H. (1978) Some comments on planktnn patches. In: SpatialPattern in Plankton Communities. J. H. STEELE, editor, Plenum, New York, 1-20. STEELE, J. H. and E. W. HENDERSON(1979) Spatial patterns in North Sea Plankton. Deep-Sea Research, 26, 955-963. STORM, B. (1986) Correction of the sensor degradation of the Coastal Zone Color Scanner on Nimbus-'/. ESA/EARSeL Symposium on Europe from Space, Lyngby, DK, 25-28 June 1986. ESA SP-258, December 1986. SVERJKOVSKY,J. (1988) Sea surface flow estimation from Advanced Very High Resolution Radiometer and Coastal Zone Color Scanner satellite imagery: a verification study. Journal of Geophysical Research, 93 C6, 6735-6743. SVERDRtrP, H. U., M. V. JOHNSONand R. H. FLEMING(1942) The oceans: their physics, chemistry andgeneral biology. Prentice-Hall, 1087 pp.
64
MOLLER-KARGERet al.
UNESCO (1979) Discharge of selected rivers of the world. Volume I11 (part rlI). Mean monthly and exta-eme dischaxges (1972-1975), p. 54-70. VAN AND~L,T. H. and H. POSTMA(1954) Recent sediments of the Gulf ofParia. Vehr. der K. Nederl. Adad. Van Wetensch., Afd. Natuurkunde, Eerste Reeks, 20, 1-244. WALSa, J. J. (1988) On the nature of continental shelves. Academic Press, 508 pp. WALSH,J. J. (1977) A biological sketchbook for an eastern boundary current. In: The Sea, Volume 6, E. D. Go-Dsm~, I. N. McCAvE, J. J. O'BamN and J. H. S~sLS, editors, Wiley, New York, 923-968. WALSH,J. J., D. A. DI~'rmu.E and W. E. ESAIAS(1987) Satellite detection ofphytoplankton export fi'om the mid-Atlantic Bight during the 1979 spring bloom. Deep-Sea Research, 34, 675-703. W~LLS,J. T. and J. M. COLEMAr~(1981) Physical processes and fine-grained sediment dynamics, coast of Surinam, South America. Journal of Sedimentary Petrology, 51, 1053-1063. WtLKINSON, C. R. (1987) Interocean differences in size and nutrition of coral reef sponge populations. Science, 236, 1654-1657. Wtt.LIAMS, W. G. (1986) Lagrangian measurements of surface circulation near Puerto Rico. Journal of Physical Oceanography, 16, 1715-1719. WILSON,R. E., A. OKUBOand W. E. ESAIAS(1979) A note on time-dependent speclra for chlorophyll variance. Journal of Marine Research, 37,485-491. WOOD, E. J. F. (1966) A phytoplankton study of the Amazon region. Bulletin of Marine Science, 16, 102-123. WOODWELL,G. M., R. H. WHITTAKER,W. A. R~lr4EaS, G. E. LIKBr~S,C. C. DELWICHEand D. B. BOTKIN.1978. The biota and the world carbon budget. Science, 199, 141-146. WORTmrqG'rON,L. V. (1976) On the NorthAtlantic circulation. Johns Hopkins University Press, Baltimore, Md., 120pp. Wr~OBLBWSKI,J. S. and J. O'BRmN (1976) A spatial model of phytoplankton patchiness. Marine Biology, 35, 161-175. WUST, G. (1964) Stratification and circulation in the Antillean-Caribbean basins. Part 1. Spreading and mixing of the water types. Columbia University Press, New York, 201 pp. YErqTSCr~,C. S. (1960) The influence of phytoplankton pigments on the colour of sea water. Deep-Sea Research, 7, 1-9. YODER, J. A. and S. S. BISHOP (1985) Effects of mixing-induced i~adiance fluctuations on photosynthesis of natural assemblages of coastal phytoplankton. Marine Biology, 90, 87-93. YODEg, J. A., C. R. McCLAIN, J. O. BLANTONand L.-Y. OEr (1987) Spatial scales in CZCS-chlorophyll imagery of the southeastern U.S. continental shelf. Limnology and Oceanography, 32,929-941. YOSHIOKA,P. M., G. P. Owr~N and D. PESAi'qTE(1985) Spatial and temporal variations in Caribbean zoopla.n_kton near Puerto Rico. Journal of Plankton Research, 7,733-751.