Mesoscale processes affecting phytoplankton abundance in the southern Caribbean Sea

ContinentalShelfResearch,Vol. 14, No. 2/3,pp. 199-221,1994.

0278--4343/94$6.00+ 0.00 (~ 1993PergamonPressLtd

Printedin GreatBritain.

Mesoscale processes affecting phytoplankton abundance in the southern Caribbean Sea FRANK EDGAR MULLER-KARGER* a n d RUBEN APARICIO CASTROt

(Received 6 December 1991; accepted 12 February 1992) Abstract--The variability of phytoplankton biomass in the Caribbean Sea south of 14°N and east of 80°W is examined with a time series of high spatial resolution Coastal Zone Color Scanner (CZCS) satellite images. These synoptic pigment fields are compared with SST, wind, sea level, and Orinoco River discharge series (1979-1982). The information helped outline the spatial extent of upweUing centers and their seasonal variability, and the influence of the Orinoco's discharge on the margin of the southern Caribbean Sea. A seasonal SST cycle was observed throughout the Caribbean, with peaks between August and November (ca 29°C), and minima between January and June (ca 25.5°C). Waters within 100 km of the continent were consistently -0.5°C cooler. Along-shore wind stress intensified from November (ca -0.05 N m -2) to maxima in April-May (ca -0.10 N m-2), then weakened through June and July, and remained low (-0.04 to -0.07 N m -2) until November. Low-frequency (>26 h) filtered coastal sea level was usually below the mean ( < - 0 . 0 4 m) between January and April, near the mean between May and August, and then above the mean (>0.04 m) from September to November. The Orinoco discharged a mean 3.9 x 104 m 3 s-1, ranging from 1 x 104 m 3 s-1 in March to about 7 x 104 m 3 s -1 in August. The data show an inverse relationship between pigment concentration and coastal sea level, and a direct relationship with zonal wind stress, both at seasonal time scales and at shorter event scales. The frequency and area of blooms was larger during periods of strong wind (January-April), and smaller during June-October. Mean pigment concentration was higher around Margarita (4 year mean of 1.2 mg m -3 ) than off central Venezuela (4 year mean of 0.45 mg m-3), which indicated that more intense upwelling occurs near the former broad continental shelf, where the horizontal, cross-wind scale of coastal upwelling may be smaller than over a narrow shelf. Pigment concentrations were particularly high near capes and headlands. Concentrations off eastern/central Venezuela and the Orinoco's flow were inversely related. In contrast, pigments near Tobago varied directly with discharge. During the first half of the year, the Orinoco plume was located between Trinidad and Tobago, and pigments around Tobago were low (<0.5 mg m-3). During July-November, the plume engulfed Tobago, Grenada and St Vincent, with values >1 mg m -3. Interannual variation was evident in the series: sea level was unusually high (daily mean sea level up to 0.25 m) during the second half of 1979, and unusually low (daily mean as low as -0.20 m) in early 1980. The latter corresponded with extended periods of unusually strong trade winds and coincided with periods of extremely large patches of high pigments along the coast.

INTRODUCTION

ANNUAL photosynthetic rates along the southern Caribbean Sea can reach levels of 200-400 gC m -2 (MARGALEF, 1965; MARGALEFet al., 1960; BALLESTERand MAROALEF, *Department of Marine Science, University of South Florida, St Petersburg, FL 33701, U.S.A. *Instituto Oceanografico, Universidad de Oriente, Cumana, Estado Sucre, Venezuela. 199

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F E. MULLER-KARGERand R. A. CASTRO

1965; MORRISet al., 1981; CURL, 1960; RICHARDS.1960; CORREDOR.1979), i.e. comparable to rates observed in mid- and high latitude shelf systems (WALSH. 1983; WALSHet al.. 1987). Such rates occur within phytoplankton patches of 10-1000 km scale which are easily detected with space-based ocean color s e n s o r s (MULLER-KARGERet al.. 1989: CORREDOR, 1979). Fishermen harvest nearly 0.3 x 106 metric tons of marine fish, crustaceans, and molluscs each year from these areas (FAo, 1986). However, in spite of the active fisheries and natural beauty of the region, the oceanographic processes leading to the high productivity of the southern Caribbean have remained unquantified and poorly understood. This paper focuses on the variability of phytoplankton biomass in the Caribbean south of 14°N and east of 80°W [Fig. l(a)]. We use a time series of high-resolution {1 km × 1 km pixels) Coastal Zone Color Scanner (CZCS: HovIs et al.. 1980) satellite images to examine near-surface phytoplankton pigment concentration, and to visualize the circulation traced by the dispersal of colored material. We complement these data with concurrent shipbased sea surface temperature (SST) series obtained from the National Marine Fisheries Service and SST series obtained with the satellite-borne Advanced Very High Resolution Radiometer (AVHRR). Finally, we attempt to determine the relationship between the pigment, SST, wind, sea level and river discharge data. The information helped outline the spatial extent of upwelling centers, the nearshore dispersal pattern of the Orinoco plume. and their seasonal variability.

METHODS P i g m e n t concentration

We estimated pigment concentration using data from 159 orbital passes of the CZCS realized between November 1978 and December 1982. This resulted in 444 full resolution subscenes, each covering approximately 400 x 400 km 2 (MoLLER-KAR6~R. 1988). The subscenes spanned the southern margin of the Caribbean Sea from the mouth of the Orinoco River to the delta of the Magdalena River. Six regions were defined to obtain series of spatially-congruent scenes (Table 1). The images were corrected for atmospheric radiance, and pigment concentrations were derived using the algorithm of Go~Dor~ et al. (1983a: see also GoRoo~ et al.. 1983b; BARALEet at.. 1986). Concentrations represent average levels of chlorophyll a plus phaeopigments and dissolved organic matter in the first optical depth ( - l / k z , where kz is the diffuse attenuation coefficient: see also MULLERKAR~ERet al.. 1989). Pigment scenes were individually remapped to a standard projection (Universal Transverse Mercator), and geographically registered to a coastline for accurate navigation. Finally, the individual subscenes were pasted into a mosaic showing the area of interest. To examine time variation in pigment concentration around known upwelling loci we further extracted regional pigment concentration means for the areas outlined in Fig. l(b). A time series for the Orinoco region was not obtained because the effect of riverine suspended and dissolved material on CZCS-derived pigment concentrations is unknown and could not be corrected, and because time coverage for the delta region was poor. Specifically, time series for the following five subregions were examined: (a) Tobago region [Fig. l(b)]. The sampling area covered 11.7 × t03 km2 around the island of Tobago (1 l°-12°N, 61°-60°W). This region is seasonally influenced by the plumes

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resolution CZCS images mapped for the subregions defined in Table 1. The map also shows the location for sea level (triangles), wind (solid circles) and monthly mean sea-surface temperature (4.1 x 1° squares) estimates, The stippled line indicates the approximate position of the shelf break (200 m isobath).

of the Amazon and Orinoco Rivers. Tobago also has an island-mass effect, and a meandering wake extending to the northwest of the island, occasionally shedding eddies >100 km diameter, can be seen in CZCS images during low Orinoco discharge. At this time, Orinoco waters remain between Trinidad and Tobago. The higher pigments seen in the island wake are probably associated with material washed from the island or with plant growth due to upwelling.

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Table 1. Geographical boundaries of areas for which full-resolution CZCS subscenes were extracted and remapped

Region Orinoco River Tinidad Margarita Is. Codera Cape Goajira Pen. Barranquilla

Abbreviation

Latitude (rain)

Latitude (max)

(OR) (TR) (MA~ (CO) (GO) (BA)

06.50 N 10.00 N 10.00 N 10.00 N 10.83 N 10.83 N

10.25 N 13.70 N 13.70 N 13.70 N 14.53 N 14.53 N

L o n g i t u d e Longitude (rain) (max) 57.83 W 57.83 W 61.50 W 64.67 W 68.33 W 72.33 W

61.67 W 61.67 W 65.17 W 68.50 W 72.50 W 76.17 W

(b) Margarita region [Fig. l(b)]. This region (12.6 x 103 km 2, excluding island surface) was located on the broad (>70 kin) shelf around the Island of Margarita. The sampling box extended over the easternmost portion of the Cariaco basin but excluded waters east of the town of Carupano [Fig. l(a)], to minimize the influence of Orinoco discharge. (c) Central Venezuela region [Fig. l(b)]. This region covered 12.5 × 103 km 2. The width of the continental shelf varies from - 5 0 km off Cape Codera (Fig. la) to <5 km west of La Guaira. (d) Gulf of Venezuela region [Fig. t(b)]. This region covered 17.1 x 103 km 2 between 11.5°N and 13°N. 70°W and 71.5°W. The sampling box followed approximately the 30 m isobath within the Gulf of Venezuela to avoid turbid, shallow waters: to the north, the area extended to the shelf break. (e) Northeastern Colombia region [Fig. l(b)]. This region covered 39.3 × 103 km 2. The area was defined between (11.36°N, 74°W), (12.92°N. 74°W), (13.97°N. 72.2°W) and (12°N, 72.2°W) and extended approximately 150 km beyond the shelf break. The plume of the Magdalena River usually flowed west- or northwestward of its delta; therefore its effect on this series of means is negligible. Temporal resolution of the CZCS data over the 4 year study period was poor. but sampling frequency in the central Caribbean was higher than in the southern and eastern Caribbean. Very few images were available for the southeastern Caribbean after March 1980 because the instrument was frequently turned on just north of about 1 I°N. Since the instrument scanned from SW to NE. many orbits contained data from waters off the Goajira Peninsula but not off NE Venezuela. Furthermore. clouds, glint patterns or partial scanning of a region reduced coverage. Due to patchiness in phytoptankton distribution, significant correlation can develop between concentration and sample size when sample size is small. Therefore, only those means based on images containing ~200 pixels per sample box (or >1800 pixels for Margarita and Central Venezuela regions) were retained. This criterion minimized product-moment correlation coefficients (SOKALand ROnLF. 1981) between concentration and sample size. Sea surface temperature ( S S T )

To complement the pigment time series, we examined concurrent ship-based sea surface temperatures (SST) produced by the National Marine Fisheries Service (courtesy of D r Douglas McLain. NMFS, Pacific Fisheries Environmental Group. N O A A , Monterey,

Mesoscaleprocessesaffectingphytoplankton abundance

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CA). These were 1 x 1 degree-square monthly composites (1978-1982) of unclassified merchant and naval vessel SST reports to Fleet Numerical Weather Center (Monterey, CA). Since the number of SST observations within any area was small, monthly mean estimates for four adjacent 1 degree squares were pooled, weighting each by number of observations. Three SST regions were examined (Fig. lb): (a) Central Caribbean (center: 14°N, 67°W). (b) Central Venezuela (center: ll°N, 67°W). (c) Eastern Venezuela (center: ll°N, 64°W). To examine the time evolution of spatial patterns of SST, we also examined temperature fields based on the N O A A operational Multi-channel Sea Surface Temperature (MCSST) product for the period October 1981-December 1989. These satellite-derived data have a nominal pixel resolution of about 20 km (OLSONet al., 1988; MtlLLER-KARGERet al., 1991). Since the MCSST series started in October 1981, overlap with CZCS data is minimal. Daily estimates of wind stress were obtained for Orchila Island (11.78°N, 66.18°W) and Margarita Island (Punta de Piedras; 10.92°N, 64.16°W; Fig. lb). The 1979-1981 series were derived by averaging bi-hourly wind stress estimated with the relationship of BLANTOI~et al. (1984). We have no information on possible effects of land on anemometers used to collect these data. We assume that the data are representative of conditions over the adjacent ocean. Daily sea level estimates at La Guaira (10.60°N, 66.94°W) and Carupano [10.67°N, 63.26°W; Fig. l(b)] were obtained from bi-hourly tide gauge data for 1979-1981. The tide records were first detrended (3 year trend subtracted) and lowpass-filtered with a 26 h running mean to remove most tidal fluctuations.

RESULTS

Temperature, wind and sea level The Caribbean Sea undergoes a clear seasonal cycle in SST (Fig. 2). In the central Caribbean, peak SST occurred between August and November (ca 29°C), and minima between January and June (ca 25.5°C). Even though SST varied in a similar fashion throughout the Caribbean, waters within 100 km of the coast were consistently ~0.5°C cooler than waters offshore. The colder temperatures confirm the presence of upwelling near the coast. The weekly MCSST series shows the seasonal progression from warm regional SST fields typical of August-November [Fig. 3(a)], to cooler temperatures with stronger gradients in January-April [Fig. 3(b)]. During January-April, patches of very cold water (SST < 25.5°C) develop along the continental margin. Their size varied with time and location, with distinct 1-10 x 103 km 2 patches appearing off Venezuela (62-67°W) and northeastern Colombia (71.5-75°W) in February-March. On occasion, the entire continental margin (>10-20 x 103 km 2) experiences cold temperatures. The strongest wind stress and Ekman transport in the Caribbean Sea occur along the South American continental mar#n, with extreme values off the Goajira Peninsula (up to 0.16 N m -2 and 5.8 x 103 kg m -1 s -1, respectively, during February-April). The lowest values occur in the northern central Caribbean (0.01 N m -2, 0.4 x 103 kg m -1 s-l). This leads to a divergence along the southern Caribbean and convergence north of about 14°N.

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The zonal wind stress (rx) in the southern Caribbean was close to an order of magnitude stronger than the meridional stress (Fig. 4). Monthly mean meridional wind stress (ry) was usually between - 0 . 0 2 and +0.01 N m -2 [Fig. 4(a)]. Mean daily meridional winds of the order of - 0 . 0 6 - + 0 . 0 6 N m -2 occurred on fewer than 10 days between 1979 and 1981. However, even though ry was small, it showed seasonal variation. Values were about 0.01 N m -2 lower during J a n u a r y - M a y than during A u g u s t - N o v e m b e r . consistent with previous reports (HEI~,ERA and FEBRF.s-ORTE6A, 1975). Stress along the coast (rx) intensified from November (ca - 0 . 0 5 N m -2) to maxima in April-May [ca -0.10 N m-2; Fig. 4(b)]. Winds generally weakened through June and July and remained low ( - 0 . 0 4 to - 0 . 0 7 N m -2) until about November (see also ISEMERand HASSE, 1985; HERRERAand FEBRES-ORTEGA.1975). The seasonal cycle at Punta de Piedras was muted, which may have been due to orographic effects of the larger mass of Margarita Island relative to Orchila Island. Anomalously intense winds occurred in April 1980 (monthly mean of - 0 . 1 5 N m-2), and weak winds occurred throughout 1981 (ca 0.04 N m-2). Spectral analysis of the 3 year series o f bi-hourly wind estimates did not reveal significant periodicities shorter than a year. Cross-correlation analyses indicated that rx at Punta de Piedras and Isla Orchila were coherent at + 1 and +2 days (coefficients of 0.51 and 0.49.

Fig. 3. (Top) Weekly mean satellite-derived sea-surface tempe~'ature (SST) fields showing the seasonal development and extent of upwellingplumes along the southern Caribbean margin. Land was masked white, the coastline red and cloudsand missingdata, grey. SST fields are shownfor: (a)/he week of 22 August 1984(typical of the wet, low-windseason); (b) the week of 25 January 1984 (typical of the dry, high-windseason).

Mesoscale processes affecting phytoplankton abundance

Fig. 7. (Bottom) CZCS images showing pigment concentrations in the southern Caribbean Sea. Top panel is a mosaic of images collected between 15 and 24 October 1979. Middle panel shows a mosaic of images collected between 15 and 17 January 1980. The bottom panel is a mosaic of images collected between 6 and 13 March 1980.

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Fig. 6. CZCS images showing pigment concentrations in the southern Caribbean Sea. Top panel is a mosaic of images from 16 December 1978. The middle panel shows pigment concentrations on 12 March 1979. The bottom panel is a mosaic of images collected between 26 and 27 September 1979. Concentrations are color-coded in (mg pigment m-3). Blue indicates low concentrations, Yellow and red indicates high values. Land is dark grey and clouds or missing data are a lighter shade of grey or white.

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Fig. 4. 3 year time series (1979-1981) of monthly mean wind stress (N m-2) at Isla Orehila (11.78°N, 66.18°W) and Punta de Piedras (PPied; 10.92°N, 64.16°W; see Fig. l(b) for station locations). Lines are 3 month running means. (a) Meridional wind component (negative values are southward winds). (b) Zonal wind component (negative values are westward winds).

respectively). Based on distance between stations (-200 km), this suggested that westward propagation of wind events occurred at rates of 1-3 m s -1 Low-frequency (>26 h) sea level fluctuations at two tide gauge stations in the southern Caribbean [Fig. l(b)] were similar in amplitude and phase during the period of our CZCS observations (Fig. 5). Even though these stations were 400 km apart [Fig. l(b)], a crosscorrelation of 0.82 was found between same-day sea level at these locations. Sea level was usually below the mean ( < - 0 . 0 4 m) between January and April, near the mean between May and August, and then above the mean (>0.04 m) from September to November (Fig. 5). Sea level was very high (daily mean sea level up to 0.25 m) during the second half of 1979, and unusually low (daily mean as low as -0.20 m) in early 1980. A weak cross-correlation (+ 0.3 to + 0.4) between daily wind and sea level off Venezuela indicated that wind events preceded sea level changes by one day. In March-April 1980, and in February-March 1981, zonal wind stress lagged behind sea level by - 1 month (compare Figs 4 and 5). We could not adequately explain this lag.

Pigment concentration The CZCS imagery was helpful for tracing the circulation of surface waters and the dispersal of river discharge (cf. MULLER-KARGERet al., 1988, 1989) and examining the effect of upwelling along the coast. Table 2 gives the mean pigment concentration for the areas examined. Below we describe key features found in each of the subregions defined in Table 1. North Atlantic water with pigment concentrations <0.15 mg m -3 typically occupies the

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Table 2. Mean pigment concentrations and coefficients of variation [rag m -3 ] along the southern Caribbean for the 4-year (1979-1982) series of CZCS scenes (Table 1). For comparison, the larger areas examined by MraLLI~s-K~6EX et al. (I989) are included (southern Sargasso and southern and northern Caribbean)

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northeastern corner of the Caribbean in December-June, during the windy period. During this period, the CZCS also shows a mixture of tropical Atlantic and Amazon River water flowing into the Caribbeanin a broad ( - 2 0 0 kin)band of 0 . 1 5 4 . 3 mg pigment m -3 (MULLEB-KARGE~ et al., 1989). The northern boundary of this band emends from SW of Barbados to Hispaniola ( > t 0 0 0 km), at approximately 45°to the right og the wind, These concentrations show the minorinfluence of the Amazon on the Caribbean interior, but the satellite data allowed the definition of the meandering front. The series of high-resolution CZCS subscenes show that the shape and size of pigment

Mesoscaleprocessesaffectingphytoplankton abundance

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patches along the southern Caribbean Margin varied considerably from year to year. Concentrations very close to the coast (within a few pixels), particularly off the Orinoco River delta and the southern Gulf of Venezuela, are suspect. Here, suspended sediments or bottom reflection confound the signal detected by the CZCS. Furthermore, the Orinoco plume contains high levels of colored dissolved organic carbon (CDOC > 5 mg C 1-1 at the mouth to - 0 . 5 mg C 1-1 >200-300 km from the coast; cf. BLOU~H and ZA~mON, in press). This CDOC may lead to > 100% overestimation of biomass within the plume's boundaries (MULLER-KARGERet al., 1989). The bio-optical characteristics of the Orinoco plume remain unquantified, and therefore concentrations from this feature are suspect. Orinoco delta and Trinidad and Tobago waters

CZCS imagery covering the Orinoco delta show the effect of high suspended sediment concentration near shore: high reflectance in CZCS bands I and 5 resulted in a land/cloud mask (cf. Figs 6 and 7). Seaward of the delta, a sharp pigment concentration gradient (>>1.0 mg m -3 to <0.5 mg m -3 across <10 kin) dearly separates oceanic waters from plume waters. On 16 December 1978 (Top Panel, Fig. 6), two wave crests separated by - 7 5 km were seen off the delta along this front. Much larger disturbances can be seen in this region (Fig. 7). These features are probably the result of eddies generated in the area of the retroflection of the North Brazil Current (MULLER-KARGERet al., 1988; JOHNSet al., 1990), and which migrate northward along the coast. Unfortunately, the coarse time resolution of the CZCS series was inadequate to examine the motion of such frontal features. The influence of the Orinoco extended over 100 km further offshore, as indicated by the 0.3 mg m -3 isopleth. The bulk of the Orinoco discharge moved northward. Both during low discharge and during high discharge, part of the Orinoco's water moved directly into the Gulf of Paria, while a fraction moved around eastern Trinidad. The margins of the Gulf of Paria showed high concentrations, but patches of low-pigment water were observed frequently in the central part of the Gulf. We could not adequately explain this condition. The pigment distribution patterns around Tobago [Fig. l(b)] were strongly dependent on the river discharge rate: during the first half of the year the plume was located between Trinidad and Tobago, and pigments near Tobago were low (<0.5 mg m -3, Figs 6--8). In contrast, during July-November, when discharge is maximum, the Orinoco plume engulfed Tobago, Grenada and St Vincent, raising concentrations to >1 mg m -3 near these islands. Other patterns, notably ephemeral peaks in November 1978, late JanuaryFebruary 1980 (Middle Panel, Fig. 7), and in January 1982, were due to anticyclonic eddies traveling north from the region of the retroflection of the North Brazil Current. These eddies transport pigment-rich water from the continental margin offshore. Island wakes can enhance the productivity of tropical waters (FELDMAN,1985; FELDMAN et al., 1984; DOTV and OGtrRI, 1956). In the Caribbean Sea, wake patterns were seen downstream of some of the southernmost Antilles and islands off the continent during December-May periods. The island of Margarita showed a very persistent wake of higher concentrations. Concentrations in the wake of Tobago during the low-discharge periods were higher than oceanic concentrations, but much lower than the DOC-contaminated values in the river plume. Presumably pigments in the wake of Tobago were related to surface blooming of phytoplankton, but could also have been advected coastal material or

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Mesoscale processes affectingphytoplankton abundance

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resuspended near-bottom material. The northern Antilles showed no noticeable enhancement of surface concentrations in their wake. Orinoco waters flowing east and west of Trinidad entered the Caribbean and merged again in G r e n a d a Passage near l l ° N , 62°W. The typical northwestward elongation of pigment patches in this region suggested that the general flow was to the NW. During low discharge, the pigment front is typically located about 50 km south of Grenada, and follows the shelf edge for almost 200 km downstream of Tobago. It then separates from the continental margin (Top and Middle Panels, Fig. 6; Middle and Bottom Panels, Fig. 7). The southern front of the plume, which initially flowed parallel to the 75 m isobath, also separated from the shelf in this region. The entire plume typically veers to the northwest and extends into the Caribbean over 350 km even during low discharge (Figs 6 and 7), with pigment concentrations decreasing along the plume axis. During low discharge, concentrations at distances >300 km rarely exceeded 1 mg m -3. During high discharge (cf. MULLER-KARGERet al., 1989), pigment concentrations were higher and more viable far offshore, but a trend toward lower pigments was still dear.

Southern margin o f the Caribbean Flow through Grenada Passage (4-8 Sv) can carry 25--60% of the total flux of water flowing through the Caribbean (MAzEIr,A et al., 1983; KINDER et al., 1985). It is unclear, however, how the inflowing Atlantic water behaves in the southeastern Caribbean. Patterns on 16 D e c e m b e r 1978 (Top Panel, Fig. 6) and on 6-13 March 1980 (Bottom Panel, Fig. 7), suggest that a jet of Atlantic water, flowing through Grenada Passage, restricted the Orinoco plume to the continental shelf. Since wind has a strong influence on the shallow Orinoco plume, the plume may be decoupled from the flow beneath it. Based on the westward elongation of pigment patches north and west of Margarita Island, it may be inferred that this jet continued westward. Further evidence of a strong westward jet are a series of meanders that extended up to 500 km downstream of Margarita along -12°N.

Fig. 8(a). Tobago region. Time series of regional pigment concentration means (solid lines with squares), and mean + one standard deviation (broken lines), around Tobago [Fig. l(b)]. Standard deviationsare provided for reference, but pigment frequency distribution varied from scene to scene and was not normal. Fig. 8(b). Margaritaregion. Time series of regional pigment concentration means (solid lines with squares), and mean _+one standard deviation (broken lines), based on individualscenes of the Margarita region [see Fig. l(b)]. Fig. 8(c). Central Venezuela region. Time series of regional pigment concentration means (solid lines with squares), and mean _+ one standard deviation (broken lines), based on individual scenes of the Cape Codera region [see Fig. l(b)]. Fig. 8(d). Gulf of Venezuela region. Time series of regional pigment concentration means (solid lines with squares), and mean + one standard deviation(broken lines), based on individualscenesof the Goajira Peninsula ('GO') region [see Fig. l(b)]. Fig. 8(e). Northeast Colombia region. Time series of regional pigment concentration means (solid lines with squares), and mean + one standard deviation (broken lines), based on individual scenes of the Barranquilla ('BA') region off NE Colombia [see Fig. l(b)].

212

F.E. MULLER-KARGERand R. A. CASTRO

Several patches of high pigment concentration were observed along the coast of Venezuela west of the Orinoco plume. These patches were always enhanced near capes. indicating surface divergence of the flow downstream of headlands, similar to features seen off Spain (MCCLAINet al., 1986), California (PELAEZand McGOWAN, 1986) and other locations (see PINGREEet al., 1978). Ephemeral patches occurred around Carupano, Araya Peninsula, Margarita Island, and off central Venezuela at various times of the year (Figs 6 and 7). High pigments around Margarita occurred year-round ,although the spatial extent and frequency of blooms was smaller during June-October [Fig. 8(b)]. No large rivers enter the ocean at these locations, so a mechanism such as upwelling must supply the nutrients needed for blooming of phytoplankton. Waters around Margarita support a large fishery and represent a source of carbon for the Caribbean interior as phytoplankton growing in the upweUing plume are dispersed offshore. The blooms in December 1978, and in January and March 1980, extended > 100 km over the Cariaco Basin (Top Panel, Fig. 6, and Middle and Bottom Panels, Fig. 7), showing the effect of upwelling on this basin. These blooms have an important influence on the chemistry of the Cariaco Basin. where sinking of phytoplankton leads to anoxia in waters below sill depth (MORRISet al.. t985; IOCHARDS. 1975). Patches off central Venezuela were less frequent [Fig. 8(c)], but material from coastal lagoons and small rivers south of Cariaco was frequently observed flowing westward past Cape Codera (cf. Figs 6 and 7). The highest concentrations consistently occurred near the cape, and decreased to the west. Concentrations off central Venezuela were lower than those in other subregions, but visual inspection of the series suggests that some coherence exists between concentration peaks off central Venezuela and off Margarita Island. Further to the west, wind stress and offshore Ekman transport intensify. This, combined with the presence of a channel between the continental shelf and Curazao. leads to an acceleration in the flow (see also MORRISON and NOWLIr~. 1982). East of Paraguana Peninsula [Fig. l(a)], high pigment concentrations within 5 km of the coast probably reflect output of organic material and nutrients from abundant mangrove forests and small rivers. However, offshore, a front could frequently be observed extending along the shelf break for distances >350 km (Top Panel. Fig. 6: Bottom Panel, Fig. 7). The shape of the front suggested both offshore advection of coastal waters and strong shear between water flowing west of Aruba, Bonaire and Curacao, and waters on the shelf. In the Gulf of Venezuela [Fig. l(a)], the highest pigment concentrations (>5 mg m -3) were always observed in the southwest, shoreward of the - 5 0 m isobath. However. these values are unreliable because of the unknown optical properties of the outflow from eutrophic Lake Maracaibo, and because of bottom reflection in shallow waters. A narrow band (15-30 km) of high values (2-5 mg m -3) along the eastern margin of the Gulf was common, which agrees with high values repeatedly observed in situ (Ramon Varela. EDIMAR. Fundacion La Salle. personal communication). Upwelling of Subtropical Underwater has previously been detected here. producing the lowest surface temperatures observed in the Caribbean (REDFIELD,1955: EDIMAR. unpublished data for 1982. 1983 and 1984). This presumably is the base of the large fishery established in the Gulf of Venezeula (GINES, 1982). The series of mean pigment values for this region [Fig. 8(d)] excluded waters in the southeastern portion of the Gulf. Still. frequent events of high pigment concentration occurred [Fig. 8(d)], and values were on the average as high as those observed near Margarita (Table 2). It was unclear, however, if the peaks were due to blooming

Mesoscale processes affecting phytoplankton abundance

213

phytoplankton or redistribution of resuspended sediments and discolored outflow of brackish water from Lake Maracaibo. The series of images showed that such patches may flow out of the Gulf of Venezuela, and move around the Goajira Peninsula into the Caribbean interior or southwestward along the Colombia coast. Waters along the western side of the Goajira Peninsula experience strong upwelling (Cor,REDOR, 1976, 1979), and pigment plumes were frequently observed with the CZCS here. The combination of upwelling and advection of material from the Gulf of Venezuela can lead to very large patches of high concentrations, as seen for example in late 1979 and in early 1980 [Bottom Panel, Fig 7; Fig. 8(e)]. The series of pigment means [Fig. 8(e)], and a series of overview scenes showing pigment concentrations in the Caribbean basin (MULLER-KARGER,1988) suggested that the bloom observed in March-April 1980 developed gradually and lasted over 3 months. DISCUSSION Land runoff and coastal upwelling are important supply mechanisms of "new" nutrients (DUGDALE and GOERING, 1967) which result in high productivity of surface waters near continents (KOBLENZ-MlSrlKE et al., 1970). Several studies identify upwelling as an important phenomenon in the southern Caribbean (REDFIELD,1955; BALLESTER and MARGALEF, 1965; RICrIAROS, 1960; WUST, 1964; GORDON 1967; COmtEDOR, 1976, 1979). The Amazon and Orinoco Rivers also affect phytoplankton concentration in the region (MULLER-KARGERet al., 1988, 1989). However, few biological oceanographic measurements are available to evaluate the importance of these sources on regional productivity. Here we focus on upwelling-related patterns as seen from satellite. Upwelling is brought about by the divergence of surface waters. Over a large portion of the southern Caribbean, this divergence is the result of both wind-driven and geostrophic transport (e.g. GILL, 1982). The result is an upwarping of the thermocline, with movement of deep waters toward the surface. In stratified tropical waters, the upper mixed layer is usually depleted of nutrients due to active uptake by phytoplankton. Deeper waters contain relatively high concentrations of nutrients, and upwelling of these nutrient-laden waters into the euphotic zone can lead to enhanced productivity. Capes, headlands, and irregular topography enhance this process (PINGREE et al., 1978; JANOWITZand PIETRAFESA, 1982; BLANTONet al., 1981; PEFFLEYand O'BmEN, 1976; GILL and SCHUMANN,1979). Previous studies have inferred seasonal variation in upwelling in the Caribbean: intense upwarping of salinity and silicate isopleths was observed near the continent during the first half of 1973 but not in October 1972 (FORELICHet al., 1978; see also HERREnAand FEBRESORTEGA,1975; KINDERet al., 1985). Figure 2 indicates that the difference in SST between the coast and the interior Caribbean is maintained year-round, superimposed on the seasonal signal, which may be due to topographically- or wind-induced mixing (GILL, 1982). Inspection of series of parameters off Margarita and central Venezuela (Figs 9 and 10) reveals an inverse relationship between daily sea level and westward wind stress, both at annual and shorter time scales. While the series of monthly mean sea level and SST also tracked each other, the relationship between SST and wind stress seemed weaker. Changes in sea level reflect the combined effects of wind-driven upwelling and steric height changes due to advection of colder water into the Caribbean. These processes have similar seasonal cycles, which accentuates changes in coastal sea level. The magnitude of

214

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216

F.E. MULLER-KARGERand R. A. CASTRO

sea level change resulting from steric effects may be estimated using salinity (S) and temperature (7) values from meridional profiles by FaOELICnet al. (1978), MoR~soN and NOWLIN(1982) and Fig. 2. Assuming that conditions remain relatively constant at 200 m (S = 37 psu. T = 18°C), that S and Tat the surface are 36.5 psu and 26°C in April vs 36.0 psu and 29°C in October. and that the gradient in properties between the surface and 200 m is linear, the seasonal change amounts to 0.13 m, which accounts for - 5 0 % (or more) of the annual range in sea level observed along the continental margin (Figs 9 and 10). This effect would double if properties in the upper 200 m were homogeneous. Based on CZCS imagery, and on the similarity between sea level off central Venezuela and off Carupano, we believe that the Orinoco plume affects sea level primarily in the vicinity of Dragon's Mouth and Trinidad. Detailed vertical temperature and salinity time series would be of great value to understand sea level change in this region. Pigment concentrations and coastal sea level showed an inverse relationship (Fig. 11), as did daily pigment concentrations and zonal wind stress. Both central and northeastern Venezuela have long coasts a l i g n ~ ~ the wind, which represents a classical coastal upwelling regime. Therefore, we expected similar v a r i a ~ i n : ~ a n d pigment concentration series. However, by inspection it appears that a close ret~ioaship exists only off northeastern Venezuela. In general, pigment concentrations in-the southern Caribbean were lower during the second half of the year than during the first half, which supports previous inferences of seasonality in upwelling. Wind stress in the southern Caribbean is upwelling favorable year-round (usually -0.06 to -0.16 N m-2; Figs 9 and 10, and cf. Fig. 12). It is comparable in magnitude to stress off Peru during upwelling (0.079 N m-2 in March-May; BAaBERand St,fire, 1981). During the first half of the year, the trades are strongest, and the volume of Atlantic water advected through the Caribbean is largest (with high geostrophic flow). Because transport varies inversely with latitude, such forcing can lead to more pronounced upwelling in the region. The apparent mitigation of upwelting during the second half of the year may be due to a deepening of the thermocline by a decrease in the volume of Atlantic water advected through the Caribbean (with concomitant decrease in the geostrophic flow), and greater stability of the water by local heating due to insolation. Presumably, upwelling occurs during this period (Fig. 3), but it may only bring to the surface waters shallower than the depth of Subtropical Underwater (150-250 m; MomusoN and NOWLIN, 1982). Wind stress off central Venezuela is usually stronger (-0.08 N m -2) than stress off northeastern Venezuela [-0.06 N m-2; Fig. 4(b)]. Nevertheless, mean pigment concentrations are usually higher around Margarita (4 year mean of 1.2 mg m -3) than off central Venezuela (4 year mean of 0.45 mg m - ). Even though sea level variation at both locations was almost identical, and even though pigment concentrations in both regions seemed to be in phase, pigment concentrations seemed to undergo larger changes in response to changes in sea level around Margarita than off central Venezuela. For a given sea level. pigments off eastern Venezuela were generally about twice the concentration off central Venezuela (Fig. 11). Pigments near Margarita and central Venezuela varied inversely with Orinoco River discharge. This reflects the difference in timing between wind-driven upwelling during the first half of the year and the calmer wind conditons during the rainy season, when river discharge is maximum. In contrast, pigment series near Tobago varied directly with the Orinoco's flow. since this island is completely surrounded by Orinoco waters during high discharge.

Mesoscale processes affecting phytoplankton abundance

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218

F.E. MULLER-KARGERand R. A. CASTRO

Fig. 12. Average wind stress and Ekman transport in the Caribbean Sea based on 1000 mb FGGE winds for the period 12 December 1978 (0000 h~ to 16 December 1978 (1200 h). Sea surface temperature contours are based on monthly mean data from the Climate Analysis Centre (CAC).

Higher pigment concentrations around Margarita Island and over the shelf off N E Venezuela m a y b e in part due to blooming of resuspended cells, which has b e e n observed in other upwelling and shelf systems (S~ETACEK, 1985; BARaER and SMIxn. 1981; SAt,laROa'ro et al.. 1986). In contrast, because off central Venezuela the shelf is narrow, such a "seeding" mechanism may not be available there. Further west, resuspension may also be an important mechanism for surface blooming off the Paraguana and Goajira peninsulas and in t h e Gulf of Venezuela, where the shelf is broad and shallow. More intense blooms may occur near the broad portions of continental shelves also due to the influence of b o t t o m topography on wind-driven upwelling. The horizontal, crosswind scale of coastal upwelling is a radius of deformation (H*N/f. where H i s the depth of the water, N is the m e a n B n m t - V a i s a l a frequency, and f is the Coriolis parameter: CI~ARnEY, 1955; YOSnIOA, 1967). This suggests t h a t wide shelves have smaller radii of deformation than narrow shelves, since the latter are typically steep a n d d e e p Given that the coast of Venezuela has an east-west orientation (same f ) , and assuming similar mean Brunt-Vaisala frequency, the radius of deformation will vary in direct proportion to the depth of the water along the continental margin. Since the E k m a n flux is 0 at the coast, but reaches its oceanic value over a distance of about an internal Rossby radius of deformation, the transport generated by wind-driven coastal upwellitig is proportional to the change in E k m a n flux across the Rossby radius (GILL. 1982). Thus, intense upwelling over the shallow shelf off northeastern Venezuela will be focused in a narrow band along the coast. Over the steep and narrow shelf off central Venezuela. upwelling would occur

Mesoscale processes affecting phytoplankton abundance

219

across a b r o a d e r band. The C Z C S images supported this view. Pigment patches off central Venezuela were rarely as p r o n o u n c e d as they were off the N E Venezuelan coasts. Seasonal cycles in pigment concentration, wind stress, and sea level were not as well defined as those of sea surface t e m p e r a t u r e and river discharge (Figs 9 and 10). In each of the regions, variability in concentrations was probably a function of a n u m b e r of processes which may shift in dominance with time. For example, concentrations around T o b a g o were highest during peaks in Orinoco discharge [Fig. 8(a)]. O f f Margarita and central Venezuela, pigments seemed to follow sea level and wind intensity. The lack of clear seasonal trends in the Gulf of Venezuela [Fig. 8(d)] is presumably due in part to the confounding effects of sediment resuspension and upwelling. E v e n though each of the regions examined seemed to have its own variability, higher pigment concentrations were frequent between D e c e m b e r 1979 and March 1980 relative to other similar periods in all regions. T h e larger blooms a p p e a r e d to be related to m a x i m a in westward wind stress and minima in sea level (Figs 9 and 10), suggesting seasonal upwelling intensification. All regions showed higher than normal values also in January and D e c e m b e r 1982 [Fig. 8(a)-(e)]. Unfortunately, due to the low temporal resolution and short pigment series, it was not possible to determine the significance of interannual changes. Currently there are no models that permit quantification of the m e a n and fluctuating c o m p o n e n t s of pigment concentration in the southern Caribbean. With the C Z C S and other environmental data, it was possible to separate factors influencing the pigment fields such as river plumes from upwelling p h e n o m e n a . But we still know little about the effects of local physical and biological variables on pigment distribution. To develop the capability of predicting the fate of material introduced along the coast, we need detailed estimates of the growth and nutrient uptake rates of phytoplankton in the region, along with m o r e detailed hydrographic series, circulation measurements, and observations of zooplankton abundance, grazing, and nutrient cycling. These processes act synergistically, and therefore should f o r m the focus of an interdisciplinary program. Such information would be of great value towards understanding the role of continental margins in global carbon and nutrient cycles. Acknowledgements--The guidance offered by Charles McClain and Wayne Esaias (NASA Goddard Space

Flight Center), and Thomas Fisher, Thomas Malone, and Bill Boicourt (University of Maryland, Horn Point Environmental Laboratories) in the preparation of this study is greatly appreciated. Specialthanks go to Ramon Varela (Fundacion La Salle/EDIMAR) for his help in the interpretation of imagery and sharing of ideas and data. Two anonymous reviewers provided useful comments on the manuscript. John Sissala,Mike Doline and Richard Sipes (Nimbus Operations office, General Electric) helped select the imagery and provided the raw CZCS data. CZCS images were processed using the "SEAPAK" software (NASA/GSFC), AVHRR data were processed using "dsp" (University of Miami/RSMAS), and post-processing was done using software developed at USF. Time series of wind measurements and sea level estimates were kindly provided by the Cagigal Observatory (Caracas, Venezuela), and by Paul Mazeika (Fundacion La Salle/EDIMAR), respectively. Orinoco stage height data and volume discharge formulae were shared by Dr William LewisJr (Universityof Colorado, Boulder). 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). REFERENCES BALLESTERA and R. MARGALEF(1965) Produccion primaria. Memoria, Sociedad de Ciencias Naturales La Salle, XXV, 209-221. BARALE,V., C. R. McCLAINand P. MALANOTTE-RIzzOLI(1986) Space and time variability of the surface color field in the northern Adriatic Sea. Journal of Geophysical Research, 91, 12957-12974.

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BARBERR. T. and R. L. SMITH(1981) Coastal upwelling ecosystems. In: Analysis of marine ecosystems. A. R. LONGrtURST, editor. Academic Press, New York, pp. 31-68. BLANTONJ. O., L. P. ArrdNSON, L. J. PIETRAFESAand T. N. LEE (1981) The intrusion of Gulf Stream water across the continental shelf due to topographically-induced upwelling. Deep-Sea Research. 28,393--405. BLANTONJ. O.. L. P. ATrdNSON, F. FERNANDEZDECASTILLEJOand A, LAVINMorrrERO(1984) Coastal upwelling off the Rias Bajas, Galicia, Northwest Spain. I. Hydrographic studies. Rapport et proct~s-verbaux des reunions Conseil permanent international pour l'Exptoration ele la MeT, 183, 79-90. BLOUGH N. V. and O. C. ZAFImON (in press) Optical absorption spectra of waters from the Orinoco River outflow: terrestrial input of colored organic matter to the Caribbean. Journal of GeophysicaIResearch. CHARNEYJ. G. (1955) The generation of oceanic currents by wind. Journal of Marine Research. 14.477-498. CORBEDOR J. E. (1976) Aspects of phytoplankton dynamics in the Caribbean Sea and adjacent regions, In: Cooperative investigations of the Caribbean and adjacent regions ( CICAR)-II, Symposium on Progress m Marine Researchin the Caribbean and Adjacent Regions, Caracas, 12-16 July 1976, FAO Fisheries Report No. 200, pp. 101-114. CORREDOR J. E. (1979) Phytoplankton response to low level nutrient enrichment through upwelhng in the Columbian Caribbean Basin. Deep-Sea Research, 26, 731-741. CURL H. (1960) Primary production measurements in the north coastal waters of South America. Deep-Sea Research. 7. 183-189. DOTY M. S. and N. OGUm (1956) The island mass effect. Journal du Conseil permanant international pour l'Exploration de la MeT, 22, 33-37. DUGDALE R. C. and J. J. GOERING (1967) Uptake of new and regenerated forms of mtrogen in primary productivity. Limnology and Oceanography, 12,685-695. FAO (1986) Yearbook of Fishery Statistics. Catches and Landings, 1984. Vol. 58. Food and Agriculture Organization of the United Nations. Rome. pp. 89-91 FELDMAN G, C. (1985) Satellite Observations of Phytoplankton Variability in the Eastern Equatorial Pacific. Ph.D. dissertation, Marine Sciences Research Center, State University of New York at Stony Brook. 217 PP. FELDMANG. C.. D. CLARKand D. HALPERN(1984) Satellite color observations of the phytoptankton distribution in the eastern equatorial Pacific during the 1982-1983 El Nino, Science, 226, 1069-1071 FROELICHP. N.. D. K. ATWOODand G. S. GtESE (1978) The influence of Amazon River water on surface salinity and dissolved silicate concentration in the Caribbean Sea. Deep-Sea Research. 25. 735-744. GILL A. E. (1982) Atmosphere-Ocean Dynamics. Academic Press, New York, 662 pp. GILL A, E. and E. H. SCHUMANN(1979) TopographicaUy induced changes in the structure of an inertial coastal jet: application to the Agulhas Current. Journal of Physical Oceanography. 9. 975-991. GINES H. (1982) Carla Pesquera de Venezuela (2): Areas Central y Occidental. HNO. H. GINES. editor. Monografia No. 27. Fundacion La Salle de Ciencias Naturales. Caracas. Venezuela. 227 pp. GORGON A L. (1967) Circulation of the Caribbean Sea. Journal of Geophysical Research, 72(24), 6207-6223. GORDON H. R., D. K. CLARK. J W. BROWN, O. B. BROWN. R. H. EVANS and W. W. BROENgOW (1983a) Pbytoplankton pigment concentrations in the Middle Atlantic Bight: Comparison of ship determinations and CZCS estimates, Applied Optics: 22, 20-35. GORDONH. R.. J. W. BROWN.O. B. BROWN. R. H. EVANSand D. K. CLARK(1983b) Nimbus 7 CZCS: reduction of its radiometric sensitivity with time. Applied Optics, 22(24), 3929-3931. HERRERAL. E. and G. FLaRES-ORTEGA(1975) Procesos de surgencia y de renovacion de aguas en la Fosa de Cariaco, Mar Caribe. Boletin del Instituto Oceanografia Universidad de Oriente, 14(1), 31-44. Hovls W. A.. D. K. CLARK.F. ANDERSON.R. W. AUSTIN.W. H. WILSON.E. T. BAKER,D. BALL. H. R. GORDON, J. L. MUELLER, S. Z. EL-SAYED,B. STURM. R. C. WRIGLEYand C. S. YENTSCH (1980) Nimbus-7 Coastal Zone Color Scanner: system description and initial imagery. Science, 210, 60-63. ISEMER H.-J. and L. HASSE (1985) The Bunker Climate Ath~ of the North Atlantic Ocean. Volume l: Observations, Springer-Verlag, Berlin, 218 pp. JANOWITZ G. S. and L. J. PIETRAFESA(1982) The effects of alongshore variation in bottom topography on a boundary current - (topographically induced upwelling). Continental Shelf Research, !(2). 123-141. JOHNS W. E.. T. N. LEE, F. A SCHOT!r. R. J. ZANTOPPand R. H. EVANS (1990) The North Brazil Current retroflection: Seasonal structure and eddy variability. JournalofGeophysical Research, 95, 22.103-22.120. KINDER T. H.. G. W. HEtlURN and A. W. GREEN (1985) Some aspects of the Caribbean circulation. Marine

Geology, 68.25-52. KOBLENZ-MISHKEO. J.. V. V. VOLKOVINSKYand J. G. KABANOVA{19701 Plankton primary production of the

Mesoscale processes affecting phytoplankton abundance

221

world ocean. In: Scienufic Exploration of the Southern Pacific, W. S. WOOSTER,editor, National Academy of Science, Washington, D.C., pp. 183-193. IVIARGALEFR. (1965) Composicion y distribucion del fitoplancton. Memoria, Sociedad de Ciencias Naturales La SaUe, XXV, 141-205. MARGALEFR., F. CERWGONand G. YEPEZT. (1970) Exploracion preliminar de las caracteristicas hidrograficas y de la distribucion del fitoplancton en el area de la Isla Margarita (Venezuela), Memorias, Sociedad de Ciencias Naturales La SaUe, Caracas, Venezuela, 20,211-221. MAZEIKA P. A., T. H. KINDER and D. A. BURNS (1983) Measurements of subtidal flow in the Lesser Antilles passages. Journal of Geophysical Research, 88(C7), 4483-4488. McCLArN C. R., S.-Y. CHAO, L. P. ATKINSON, J. O. BLANTONand FEDERICODE CASTILLEJO(1986) Wind-driven upwelling in the vicinity of Cape Finisterre, Spain. Journal of Geophysical Research, 91(C7), 8470-8486. MORRIS I., A. E. SMrrn and H. E. GLOVER(1981) Products of photosynthesis in phytoplankton off the Orinoco River and in the Caribbean Sea. Limnology and Oceanography, 26(6), 1034-1044. MomUsoN J. M. and W. D. NOWLINJR (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. MULLER-KARGERF. E. (1988) Pigment variability in the Caribbean Sea: A study using the Coastal Zone Color Scanner. Ph.D. Dissertation, University of Maryland, College Park, Maryland, 218 pp. MULLER-KARGERF. E., C. R. McCLAIN, T. R. FISHER, W. E. ESAIASand R. VARELA(1989) Pigment distribution in the Caribbean Sea: Observations from Space. Progress in Oceanography, 23, 23--69. MULLER-KARGER F. E., C. R. McCLAIN and P. L. RICHARDSON (1988) The dispersal of the Amazon's water. Nature, 333, 56--59. MtJLLER-KARGER F, E., J. J. WALSH, R. H. EVANS and M. B. MEVERS (1991) On the Seasonal Phytoplankton Concentration and Sea Surface Temperature Cycles of the Gulf of Mexico as Determined by Satellites. Journal of Geophysical Research, 96(C7), 12,645-12,665. OLSON D. G., G. P. PODESTA, R. H. EVANS and O. B. BRows (1988) Temporal variations in the separation of Brazil and Malvinas currents. Journal of Geophysical Research, 35(12), 1971-1990. PEFFLEVM. B. and J. J. O'BmEN (1976) A three-dimensional simulation of coastal upwelling off Oregon. Journal of Physical Oceanography, 6, 164-180. 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. PINGREER. D., M. J. BOWMANand W. E. ESA1AS(1978) Headland fronts. In: Oceanicfronts in coastalprocesses, M. J. BOWMAr~and W. E. ESAIAS, editors, Springer-Verlag, Berlin, pp. 78-86. REDFIELD A. C. (1955) The hydrography of the Gulf of Venezuela. Deep-Sea Research, Supplement, 3. RlCnARDS F. A. (1960) Some chemical and hydrographic observations along the north coast of South America. I. Cabo Tres Puntas to Curacao, including the Cariaco Trench and the Gulf of Cariaco. Deep-Sea Research, 7, 163-182. RaCrlAnDS F. A. (1975) The Cariaco Trench. Oceanographic Marine Biology Annual Review, 13, 11-67. RaCHARDSON P. L. and T. K. McKEE (1984) Average seasonal variation of the Atlantic equatorial currents from historicial shipdrifts. Journal of Physical Oceanography. 14, 1226-1238. SAMBROTTOR. N., H. J. NIEBAUER, J. J. GOERING and R. L. IVERSON(1986) The relationship among vertical mixing, nitrate uptake, and growth during the spring bloom on the southeast Bering Sea middle shelf. Continental Shelf Research, 5, 161-198. SMETACEK V. (1985) Role of sinking in diatom life-history cycles: Ecological, evolutionary, and geological significance. Marine Biology, 84, 239-251. SOKALR. R. and F. J. ROHLF (1981) Biometry, 2nd edition, W. H. Freeman and Co., New York, 859 pp. WALSH J. J. (1983) Death in the sea: enigmatic phytoplankton losses. Progress in Oceanography, 12, 1-086. WALSH J. J., D. A. DIETERLEand W. E. ESAIAS(1987) Satellite detection of phytoplankton export from the midAtlantic Bight during the 1979 spring bloom. Deep-Sea Research, 34,675-703. WUST G. (1964) Stratification and circulation in the Antillean-Caribbean basins. Part 1. Spreading and mixing of the water types. Columbia University Press, NY, 201 pp. YOSHIDA K. (1967) Circulation in the eastern tropical oceans with special reference to upwelling and undercurrents. Japanese Journal of Geophysics, 4, 1-75.