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temporal variability of algal biomass and potential productivity in the Mauritanian upwelling zone, as estimated from CZCS data
0273—1177/87 $0.00 + .50 Copyright © COSPAR
Adv. Space Res. Vol. 7, No. 2, pp. (2)53—(2)62, 1987 Printed in Great Britain. All rights reserved.
SPATIAL/TEMPORAL VARIABILITY OF ALGAL BIOMASS AND POTENTIAL PRODUCTIVITY IN THE MAURITANIAN UPWELLING ZONE, AS ESTIMATED FROM CZCS DATA A. Bricaud, A. Morel and J. M. André *Laboratoire de Physique et Chimie Marines, Université Pierre et Marie Curie, BP 8, 06230 Villefranche-sur-mer, France ABSTRACT The mesoscale distribution of phytoplankton in the Mauritanian coastal upwelling area has been mapped using a series of CZCS scenes covering more than one year. The spatial and temporal evolution of the algal biomass has been analysed. The CZCS images have been combined to establish composite images (ca. 840 km x 1260 Kni) representing monthly averages of the surface pigment content. SatelTite-derived pigment concentrations have been tentatively transformed into potential primary production per unit area. For this purpose, data concerning in situ primary production, vertical profiles of algal biomass and water optical propertii~ iipreviously obtained, were combined with mean values of available radiant energy taken from climatic atlases. An estimate of the daily integrated primary production in the different zones influenced by the seasonally fluctuating upwelling is proposed. INTRODUCTION The hydrology of the Mauritanian upwelling area (Fig. 1) is mainly controlled by the combined influence of the Canary Current which carries cool surface waters southwards and of the coastal upwelling regime induced by the NE tradewinds, blowing along the NW African coast and driving surface waters offshore by Ekman transport. The hydrographic conditions in this area have been described by numerous authors (see reviews in /1,2/). Very briefly, the southern limit of the tradewind influence varies approximately between lO°Nin February March, and 2l°N in September - October. In winter (December to May), the whole coastal region (north of l5°N)experiences an upwelling regime, which is subject to short—term (and small-scale) fluctuations due to local changes in wind stress /3,4/. In summer (June to
25W
20W I
I
I
i5W I
I
/(~ape1 Bojado1~ çcape - 25 N Garnet
-
I
Cape
Bar’bas 2
•
3 4
I
i\~CapeBlanc ‘)Banc d’Argut~20 N ~Cape Ttmiris Nouakchott 1ape Verde I~
Fig. 1. Location of the study areas. (2)53
N
(2)54
A. Bricaud, A. Morel and J. M. André
November), variable westerly winds replace the tradewinds south of Cape Blanc, where warm waters of the equatorial countercurrent often appear. The upwelling remains active only north of Cape Blanc. These marked differences in the annual upwelling cycle have been described, at least partly, either using SST data /2,5,6/ or biornass and production data /l,7/.The zonal variations are also influenced by other factors, such as the cross—shelf morphology and the gradients in water mass properties within the upper layer (the warmer, saltier, low-nutrient North Atlantic Central Water and the cooler, fresher, high-nutrient South Atlantic Central Water ;see e.g. Ill). The main front between NACW and SACW oscillates between Cape Blanc and Cape Timiris /8/. Though very detailed, the previous studies, made on the basis of ship-bound measurements (except /2/), have unavoidably been restricted in both space and time. The first aim of this paper is, therefore, to take advantage of the synopticity and repetitivity of the satellite colour imagery to complete our knowledge of the annual upwelling cycle in the Mauritanian region. This will be done by delineating, at different periods of the year, the extent of eutrophic waters, as well as that of mesotrophic waters. Because of their large coverage, the mesotrophic waters presumably contribute to the global productivity of this region in a significant manner. In the same way as phytoplankton distribution, the spatial/temporal variations of the primary production are not well documented in this area. The second aim of this paper is therefore to study this evolution, by using a relationship between satellite-derived pigments and primary production inferred from historical data in this region. An estimate of the global amount of carbon which is photosynthetically fixed in the whole area, on a per-year basis, can then be derived. METHODOLOGY Estimation of the pigment content of the surface waters The CZCS data have been processed using a procedure described previously /9/. Briefly, the signals received by the sensor have been calibrated (taking into account the time decay of Channel 1 according to Mueller’s algorithms /10/) and then corrected for atmospheric effects (Rayleigh scattering, aerosol scattering and atmospheric attenuation of the water-leaving signal; see e.g. /11/). The aerosol concentration and the spectral dependence of aerosol scattering, which vary in space and time, are therefore a priori unknowns. The aerosol and water-leaving radiances were therefore partitioned in channel 4 by an iterative procedure using a statistical relationship established from sea-truth measurements. This method is similar to that developed by Smith and Wilson /12/. The spectral dependence of aerosol scattering (in A where n is the Angstrom exponent’) was estimated with a method adapted from Gordon’s clear water radiance concept” /13,14/, and using a reflectance model for Case 1 waters /9/. The average of the computed exponents was then applied to the whole scene. Table 1 gives the average exponents for the selected scenes. No systematic trend is evident; the striking feature is the occurrence of low exponents, or even of negative exponents expressing an increase of aerosol scattering towards longer wavelengths. Such “reversed” selectivities (generally associated with a high aerosol load) have been occasionally observed in Africa /15/. Using the reflectance model mentioned before, a discrimination between phytoplankton-dominated Case 1 waters and sediment-dominated Case 2 waters was achieved. Case 2 waters, where the pigment and mineral particle content cannot be separately estimated, were excluded before applying the pigment algorithms. In the Mauritanian upwelling, this was the case for the near-shore waters, where sediment resuspension by wave action is very high, and also for the shallow waters of Banc dArguin, south of Cape Blanc. Finally, in the Case 1 waters the pigment content C (including pheopigments) was estimated by using the algorithm: 3) = 1.73 (L(443)/L(55O))2~04 C (mg.m where L (443) and L (550) are the water-leaving radiances in channels 1 and 3. This algorith’~ was used eVen for waters with high pigment content, where L (443) becomes very weak. The ratio 520/550, used by some authors, is not very sensitive to%ariations in C; it is believed to induce a greater uncertainty in pigment estimates. The temporally-averaged pigment content of surface waters Table 1 gives the list of the scenes selected for this study. The chlorophyll maps were resampled on a Mercator grid. Then composite images were established by averaging the images over a given period. In these composite images, the chl a content at a fixed location is the arithmetic average of the chl a contents of the corre~ondingpixels in the
Variability of Algal Biomass and Potential Productivity
Date
2 3 8 30 8 i8 23 24 8 15 21 3 27 10 11 11
November November November November February February February February ilarch April April September September November Novemoer November
TABLE 1
1982 1982 1982 1982 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1933
Orbit
Latitude (°li) of southern limit
20320 20320 20403 20707 21675 21813 21882 21896 22052 22587 22670 24536 24863 25476 25490 25490
12.40 16.30 17.79 14.98 14.67 15.10 14.53 12.31 13.16 12.89 18.34 16.30 18.18 17.59 12.31 13.11
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Angstrom exponent
0.246 0.203 0.277 -0.214 0.001 0.322 0.193 -0.059 -0.214 -0.101 -0.156 -0.322 -0.070 0.168 -0.151 0.004
CZCS overpasses selected for this study.
individual scenes. A given pixel is identified as belonging to Case 2 waters or to clouds only if this occurs for the corresponding pixels in all the individual images. This allows one to discard most of the artifacts due to improper atmospheric corrections and to extract “permanent” turbidity features (at the scale of about a month). Estimation of the potential production The primary production, P, in a given water column is dependent on the amount of the photosynthetically available radiation PAR (i.e. the amount of radiant energy available within the spectral range 350-700 nm, see /16/), and on its pigment content, C. At sufficiently low energy levels, P is proportional (empirically) both to C and PAR, with the slope controlled by phytoplankton physiology /17/. In the ocean, the possibility of establishing a relationship between production and (surface or integrated) pigment concentration has been investigated by several authors /18-23/. In spite of limitations due to physiological and environmental variability, they demonstrated that approximate estimations of the primary productivity could be obtained by using statistical site-specific algorithms. A similar approach has been made here, by using data acquired during the Cineca V-Charcot cruise in the Mauritanian upwelling area in March-April 1974 /24-26/. The in situ production in the water column can be written as: P
=
(1/39) 8 x PAR x C
(1
where C is the chlorophyll a content in the euphotic 1a~yer, ~l/39)is the factor converting the phot~synth~tical1ystorid radiation (PSR, in J.m .day ) into primary production (P, in gC.m .day ). 8 represents the ratio of e , the efficiency of utilization of radiant energy by phytoplankton (in reference to photosynthetic irradiation), to C, the water column chlorophyll a content; c defined in /16/ is dimensionless, thus 2)~) 8 = r~/ C , with 8 expressed as (% (mg.m In practice, the p_~-ameter derived from CZCS imagery is not C, but Csat, the pigment concentration (mg.m ) weighted over the upper attenuation length. A relationship similar to (1) can be written: P
=
(1/39) 8’ x PAR x Csat
(1’)
The “detectable” pigment concentrations, Csat, have been estimated from the pigment concentration vertical profiles, C(z), determined at each station of the cruise, and from the diffuse attenuation coefficient profiles for PAR, K, 1(z), using the relationship given in /27/. The energy in the spectral range 350-700 nm “is approximately 43% of the total energy /28/, and thus PAR was inferred from the daily irradiation, E, measured at each
A. Bricaud, A. Morel and J. M. André
(2)56
station. Using the values of the water column production, P, and of the daily irradiation in terms of PAR at each station, a mean value of ~‘ can be determined: 3)~) (2) 8= 0.184 ±0.059 (N = 14) , where 8’ is expressed as (% (mg.m When computing 8’ , turbid coastal stations have been excluded, since the corresponding e values are anomalously lower than for offshore stations (it has been recognized that the turbidity-induced light limitation depresses the efficiency /29/). Note that, compared to most of the previous studies linking P to the pigment content, the influence of environmental variability has been reduced by taking into account the actual photosynthetic irradiation. The standard deviation however remains relatively high (32%), likely due to variations in other factors such as species composition and physiological state of algae, nutrient availability or water temperature. When converting CZCS-derived pigment concentrations into primary production rates, the values of PAR corresponding to the different scenes are needed; they could be obtained using e.g. an analysis of Meteosat data. Here, as a first approximation, monthly-averaged values for the study areas have been derived from climatic atlases (see legend Fig. 2). These mean values for solar irradiance /30/ and for nebulosity N /31,32/ have thu, been combined in Matsuike et al’s formula /33/ to obtain mean daily irradiations:
1=
?~(l - 0.52 ~l.3)
As an example of this calculation, Fig. 2 shows the annual variations of ~ (and ~) at 25°N, and_evidences their importance when evaluating P. Then PAR has been computed as above = 0.43 E) and introduced in equation (1’).
(J.m2~d4
— h--~A
~ J
N
i
FMAMJJASOND
r,
Fig. 2. Monthly values of the maximal solar irradiance, at 25°N (from /30/), of the nebulosity off Cape Bojador (from /319), and of the resulting solar irradiance at 25°N, computed from Matsuike et al. /33/. These values have been applied to the zone 1. Similar curves have been obtained for the zone 2 (using the average of at 25 and 20°N), for the zone 3 (using En at 20°N)and for the zone 4 (using the average of E 0 at 15 and 20°N, ~nd the mean nebulosity off Dakar /32/).
RESULTS AND DISCUSSION Spatial-temporal variability of the surface pigment content. Fig. 3 shows five composite images corresponding to typical situations in the annual cycle of the upwelling: a) and e) in November, at the end of the warm season, when the tradewinds are progressing southwards (these images are shown as an example of interannual variations), b) in February, in the middle of the cold season, when the tradewinds are blowing everywhere including the zone south of Cape Verde, c) in March-April, when the winds regress northwards, and d) in September, when the southern limit of the tradewinds is at its northern position, near Cape Blanc. An individual image (November 3, 1982) is also shown as an example (Fig. 4). On all images the extent of the highly reflective Case
Variability of Algal Biomass and Potential Productivity
~
-
_____
u•,,.•
(2)57
:~ :~ ~
I P~ CM1
____
CM1 Fig. 3a.
t
Fig. 3b.
MAR ______
15-21 ~pR
Fig. 3c.
331
Fig. 3d.
Fig. 3. Composite images of surface pigment concentration in the NW African upwelling. The scenes selected for each image are indicated in Table 1. The dimension of the images is approximately 840 km x 1260 km, represented with a 2.5 km resolution. The western limit of the images is 24°W.Land and clouds are flagged in white. Turbid-Case 2 waters are masked in black. In Case 1 waters, the pigment con’cer~rations increase according to a_~ogarithmicscale, from dar~ blue (< 0.2 mg.m ) to the 3mean blue (0.2 - 1 mg.m ),_~ightblue (1 - 3 mg.m light green (3 - 5 mg.m ) and dark green (>5 mg.m ).
(2)58
A. Bricaud. A. Morel and J. M. André
I”
_
Fig. 3e.
Fig. 4. Image of surface pigment concentration for the scene of November 3, 1982, represented with a 1.6 km resolution. The colour scale is the same as in Fig. 3.
Fig. 5. Thermal data (CZCS channel 6, without atmospheric correction) for the scenes of February 18, 1983 (Fig. 5a) and September 3, 1983 (Fig. 5b). The temperature is increasing according to a linear scale from black to dark blue, light blue, green, yellow, orange and red.
Variability of Algal Biomass and Potential Productivity
(2)59
2-turbid waters (masked in blac~k) coincides approximately with the 50 meter isobath. Beyond this turbid zone, the spreading of eutrophic and mesotrophic waters reveals important fluctuations. Four main areas are distinguished (see Fig.l): i) north of Cape Barbas, ii) from Cape Barbas to Cape Blanc, iii) from Cape Blanc to Cape Timiris, and iv) from Cape Timiris to south of Cape Verde. North of Cape Barbas (zone 1). In this area the seasonal variation in the upwelling activity appears weak. A slight minimum in algal biomass occurs in the cold season (Fig.3c), in agree_m.~nt with previous studies /1,5/. On all the images, the area of high biomass (> 3 mg.m ) is restricted to a coastal band, approximately 50 km wide, which narrows near Cape Garnet. Thermal data sh~wthat the cold waters are confined to the same belt. 3Mesotrophic waters ( = 0.2 - 3 mg.m ) are separated from oligotrophic waters (< 0.2 mg.m ) by a sinuous, well-defined front. This front parallels the coast at a distance of about 120 km, until it becomes E- W oriented near Cape Barbas, where mesotrophic waters invade the offshore region. The persistency of the eddy-like features along the front is apparent on Figs. 3a and 4. The limited extent of the upwelling area in this zone is consistent with the weakening of the Ekman transport, compared to the zone south of Cape Barbas, as demonstrated by Wooster et al. /5/. From Cape Barbas to Cape Timiris (z~iTer2 and 3). In these areas, the upwelling is known to be permanent, as attested by the infra-red data in February and September (Fig. 5a,b). However, pronounced seasonal variations are evident in Fig. 3, with a maximal coverage of the high biomass area in February, a well-marked minimum in September, and intermediate situations in November and March-April. South of Cape Blanc (zone 3), the eutrophic waters show a larger offshore extension than in the north (zone 2). The March-April image reveals a peculiar situation where the upwelling is active near Cape Blanc whilst it appears considerably weakened near Cape Barbas. In both zones the mesotrophic waters extend far offshore, approximately 700 km from the coast at the latitude of Cape Blanc, even in September, when the biomass is at its~lowest level. In February, they extend far beyond this limit (concentrations of 0.8 mg.m are still observed about 1400 km offshore on the scene of February 8). The advection processes inducing offshore (regenerated?) production have been predicted and analysed by theoretical models based on a two cyclonic cell circulation /33,34/. From Cape Timiris to Cape Verde (zone 4). In this area the seasonal variations of the upwelling are the most pronounced. In February and March-April, the area of high algal content extends as far as 200 km all along the coast, and even 300 km at the latitude of Nouakchott (this latter area has been previously recognized as subject to enhanced production, due to the intensifying effect of a submarine canyon /4,36/). Clear water patches appear inside the area of high biomass, which could indicate recently upwelled, not yet productive waters. Fig. 5a shows that the whole nearshore region is covered by cold waters. In September, the algal concentration remains very low, as corroborated by thermal data which show that equatorial low-nutrient waters have invaded this area (Fig. 5b). The images of November show a weakened, though distinct, upwelling activity. Note the large phytoplankton patches offshore, north-west of Cape Verde, in November 1983 (non-existent during the same period in 1982). Annual variations of the potential production Table 2 gives, for the different periods of the year, the values of Csat averaged over each of the four areas indicated on Fig. 1. These areas are limited on the west by the 24°W meridian. This includes all the eutrophic waters, and most of the mesotrophic waters (> 1 mg.m , in green and light blue on Fig.3). From these C~tvalues, average values of the primary production P over the same areas have been estimated, using the method described before *~ By integrating these values of P, an approximate value, Pint, for the global amount of fixed carbon in the different areas, can be obtained **~ To the north of Cape Barbas, as previously observed, annual Csat variations are rather weak. Nevertheless F values are significantly higher in March-April than in September and November, because of the higher PAR values. For the same reason, and though the pigment content between Cape Barbas and Cape Blanc is at its minimum in September, the primary production in September exceeds that occurring in November 1982. In this area, both pigment content and primary production reach their maximum in March-April. Between Cape Blanc and Cape Timiris, Csat and P reach their maximum in February. Between Cape Timiris and Cape * This computation assumes that the average pigment content and production inside Case 2 waters are equal to the average values over the whole area. This likely leads to a slight overestimation, because of the light limitation in the mid-shelf zone (see above).
The column pigment content, and therefore the production capability of the oligotrophic waters inside the study areas may be underestimated, since likely their vertical distribution of algal biomass is that of a “typical tropical structure”, with a maximum occurring within the thermocline, i.e. below the upper attenuation length “seen” by CZCS. **
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A. Bncaud, A. Morel and J. M. André
Nov
1 1982
Zone 1 2 3
4
Feb, 1983
Area PAR ~ 9m2) (mg.m3)(1O6J.m2d~) (10 279.2 0.64 8.21 115.8 1.09 8.60 2.35 131.1 1.87 8.60 2.95 411.3 (2.01) 9.03 2.78
Zone
~
PAR
PAR
9.37 (1.02) 9.72 (2.80) 9.72 2.20 10.06
2.43
pint
~
~I1~
Mar-Apr, 1983 Sept, 1983
P
~
PAR
11.70 11.70 11.70
0.85
25.9
26.3 26.3
0.62 8.21 2.34 8.60 2.94 8.60
11.95
(0.59)
23.6
2.53
9.03
Pint
l’
P
Pint
0.96 0.80
PAR
pint
(gC.m2.d~)(KtC.d1) 0.248 69.2 (O.563)(157.2) 0.447 124.8 0.442 51.2 1.077 124.7 (1.545)(178.9~ 0.512 59.3 0.759 99.5 1.352 177.2 1.214 159.2 0.427 56.0 (0.856) (352.1) 1.319 542.5 1.370 563.5 — —
1 2 3 4
Nov, 1983
0.241 67.2 0.949 109.9 1.193 156.4 1.078 443.3
TABLE 2 Upper part: Values of the pigment concentration weighted over the upper attenuation length, Csat, averaged over given periods (see Table 1) and over the areas indicated in Fig. 1. The monthly values of the photosynthetically available radiation (PAR) used for estimating the average primary production are also given. The values in parentheses are average values computed on incomplete areas. Lower part: Values of the average primary production (P) for each zone and period; values of the primary production when integrated over the different areas (Pint).
Nov, 1982 Zone
~P
Pint
Feb, 1983 ~P
Pint
Mar-Apr, 1983 AP
P
Pint
AP
(gC.m2.d1)(KtC.d~) 1%)
1 2 3
0.42
60.8
0.86 1.34
43.5 81.3
4
0.94
179.0
Zone
2 3 4
2.26 2.81
—
2.44
-
Sept, 1983 ~
1
12.1 15.0 18.3
0.75 0.76
0.56
(0.28)
-
114.4 170.6 4660
-
8.3 3.7 14.1
(0.83) (2.32) 2.49 2.73
(121.2) (117.4) 151.1 521.4
(22.9) (34.4) 5.1 7.5
Nov. 1983
Pint
AP
P
Pint
AP
108.5 38.3 34.3 (115.4)
13.1 35.4 38.7
0.35 2.14 2.43 1.68
51.2 108.3 147.5 320.9
23.8 1.4 5.7 27.6
-
TABLE 3 Average values of ~ and Pint over the same periods and the same areas as in Table, 2, except that the western limit of these areas is 20°W(instead of 24°W). ~ P represents the ratio of the primary production occurring in the band between 24 and 2O°W (corresponding mainly to mesotrophic waters), to the global primary production between 24°Wand the coast.
Variability of Algal Biomass and Potential Productivity
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Verde, Vreaches comparable values in February and April, though Cut is at its maximum in February. From November to February, higher production values are found to the south of Cape Blanc compared to that to the north. This agrees with the study by Minas et al. /1/ who attributed this difference to the influence of nutrient-rich SCAW (which is reduced in summer). Interannual variations (November 1982 to November 1983) of Csat and P are the most important in the zone 2, where they exceed 100%. These production evaluations are not easily comparable with the previous ones made from measurements at sea, which have been restricted to the zone east of 18°Wor 20°W. The same computations as presented in Table 2 have therefore been repeated with the four areas limited to the meridian 20°W (T~le_~).The value obtained in March-April between Cape Blanc and Cape Barbas (2.32 gC.m .d ) falls in the range of the value~determined from ship at different stations within the same _~egt9n: 0.66 to 4.74 gC.m .d (mean valu~ 2.4~in March-April /26/, or 0.8 to 2.8 gC.m. .d in April-May /4/, and 1.1 to 3.4 gC.m .d in May—June /37/. 2.d~x 365 The annual production A value 0.59 gC.m = 215 gC.mme~nprirn~ry .year was proposedis not by well Schulzdocumented. and Kaiser /38/,of from time series of measurements along a section off Cape Blanc, as far west as 20°W.For the same zone (zone 2), a linear interpolation ~tween_jhe seasonal production values in Table 3 leads to a much higher value, 603 gC.m .year . The same inte.jpolati 1on for the whole area from Cape Verde to Cape Garnet provides a value of 510 gC.m .year . This estimate greatl~2exceed~ the value derived from the Koblentz-Mishke et al.’s map /39/ (0.5 x 365_f 182 ~j.m .year I and is closer to that proposed by MTWa~et al. /1/ (730 gC.m .year ) in their discussion of the production budget for the wholéThWATrican upwelling. From the seasonal areal fluxes in Tables 2 and 3, the ~l1obal mass of photosynthetically fixed carbon can be computed~ it amounts to 276 MtC.year for the entire zone (coast to 24°W), or to 228 MtC.year for the zone restricted to 20°W. This implies that the mesotrophic waters in the band between 20°and 24°Wwould be responsible for approximately 17% of the global production. This “offshore fraction” of the production (~P) varies from 1 to 39% according to the considered zones and periods (see Table 3). CONCLUSIONS The methodology used here for estimating the primary production in the Mauritanian upwelling area deserves some comments: i) the assumption of a unique relationship between the chlorophyll a content and the carbon fixation results in an uncertainty. By considering the hisforical data used for establishing this “site-specific” relationship, the uncertainty is estimated to be about
30%; ii) the question of the meaning of the in situ primary production experiments (gross production or something intermediate betwee~Tnétihd gross production?) exists as well for the present results; iii) in such a study, the distinction between “new” and “regenerated” production cannot be made on the sole basis of the colour information; iv) the deep chlorophyll maximum, if any, is not detected; therefore, a slight underestimate of the primary production in mesotrophic and overall in oligotrophic waters can be expected; v) the linear interpolation between selected seasonal values of primary production is obviously questionable; this drawback is easily circumvented if more scenes are available. Keeping in mind the above limitations, the temporal and spatial evolution of the surface chlorophyll content as revealed by CZCS data does not contradict what could be roughly expected from the general and well known meteorological events (i.e. the North-South oscillation of the tradewinds). The extent of eutrophic and overall mesotrophic waters appears however much larger than anticipated from historical ship-bound data. This is verified all over the year, particularly south of Cape Barbas and even when the upwelling activity is at its lowest level. The important consequence of this result lies in that the primary production capability of the entire zone affected by the upwelling has been most likely underestimated in the past. Within the nearshore eutrophic zone, the seasonal values of primary production as obtained agree with the previous in situ data. The offshore extent of mesotrophic waters, not investigated from ships but ei~TTy detected from space, causes the revision (towards a greater value) of the global carbon fixation by photosynthesis in the whole region influenced by the upwelling process. Acknowledgements This work was supported by Centre National de la Recherche Scientifique (UA 353 and GRECO 034), by Centre National d’Etudes Spatiales (under contract 84/1243) and by IFREMER (these institutions were associated in funding ATP Télédétection Spatiale 1984). The authors wish to thank R.G. Ingram for improving the English of the manuscript.
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A. Bricaud, A. Morel and J. M. André
REFERENCES H.J. Minas, L.A. Codispoti and R.C. Dugdale, Rapp. P-V. Réun. Cons. in Explor. Mer, 180, 148 (1982). 2 . F. Domain, Publications ORSTOM, Documentations techniques, n°42, 43 pp. (1979). 3 . E.D. Barton, A. Huyer and R.L. Smith, Deep-Sea Res., 24, 7 (1977). 4 . S.A. Huntsman and R.T. Barber, Deep-Sea Res., 24, 25 (1977) 5 . W.S. Wooster, A. Bakun and D.R. Mc Lain, J.Mar. Res., 34, 131 (1976). 6 . P. Speth and H. Detlefsen, Rapp. P.V. Rêun. Cons. i. Explor. Mer, 180, 29 (1982). 7 . L.A Codispoti, R.C. Dugdale and H.J. Minas, Rapp. P-V. R~un. Cons. in Explor. Mer, 180, 184 (1982). 8 . M. Manriquez and F. Fraga, Rapp. P-V. Rêun. Cons. in Explor. Mer, 180, 39 (1982). 9 . A. Bricaud and A. Morel, Oceanol. Acta, in press (1987). 10 . J.L. Mueller, Appl. Opt., 24, 1043 (1985). 11 . H.R. Gordon and A. Morel, Lect. Notes on Coastal and Estuarine Studies, edited by M. Bowman, Springer-Verlag, 114 pp. (1983). 12 . R.C. Smith and W.H. Wilson, In : Oceanography from Space, edited by J.F.R. Gower, Plenum Press, New-York, 281 (1981). 13 . H.R. Gordon and D.K. Clark, Appl. Opt., 19, 3428 (1981). 14 . H.R. Gordon, D.K. Clark, O.B. Brown and R.H. Evans, J. Mar. Res., 40, 491 (1982). 15 . A. Cerf, Contributions to Atmospheric Physics, 53, 414 (1980). 16 . A. Morel, Deep-Sea Res., 25, 673 (1978). 17 . T. Platt, ~~-Sea Res., 33, 149 (1986). 18 . C.J. Lorenzen, Limnol. Oceanogr., 15, 479 (1970). 19 - R.C. Smith and K.S. Baker, Limnol. Oceanogr., 23, 247 (1978). 20 . T.L. Hayward and E.L. Venrick, Mar. Bid., 69, 247 (1982). 21 . R.C. Smith, R.W. Eppley and K.S. Baker, Mar. Biol., 66, 281 (1982). 22 . T.Platt and A.W. Herman, Int. J. Remote Sensing, 4, 343 (1983). 23 . R.W. Eppley, E. Stewart, )1.P. Abbott and V. Heyman, J. Plankt. Res., 7, 57 (1985). 24. G. Jacques, M. Panouse and J. Gostan, In : Résultats de la campagne Cineca 5-J Charcot-Capricorne, Publ. CNEXO, n°lO, 1.1.4 (1976). 25 . A. Morel and L. Prieur, In : Résultats de la campagne Cineca 5 - J.Charcot Capricorne.,Publication CNEXO, n° 10, 1.1.10. (1976). 26 . M. Ninas, In : Résultats de la campagne Cineca 5 - J. Charcot - Capricorne, Publication CNEXO, n° 10, 1.2.2. (1976). 27 . R.C. Smith, Proceedings of the Workshop on the Global Ocean Flux Study, National Academy Press, 103, 124 (1984). 28 . H.R. Jitts, A. Morel and V. Saijo, Aust. J. Mar. Freshwater Res., 27, 441 (1976). 29 . A. Morel, Rapp. P-V Réun. Cons. in Explor. Mer, 180, 94 (lg~2T. 30 . T.G. Berliande, Met. Gidrol., 6 (1960). 31 . Marine Climatic Atlas of the World, North Atlantic Ocean, NAVAER 5O-1C-528 (1955). 32 . M.F. Courel, 3rd cycle thesis, Univ. P. M. Curie, 407 pp (1984). 33 . K. Matsuike, T. Morinaga and T. Sasaki, J.Opt. Soc. of Japan, 6, 1 (1970). 34 . C.N.K. Mooers, C.A. Collins and R.L. Smith, J. Phys. Oceanogr., 6, 3 (1976). 35 . J.S. Wrobleski, J. Mar. Res., 35, 357 (1977). 36 . A. Herbland and B. Voituriez, Rapp. P.V. Réun. Cons. i. Explor. Mer, 180, 131 (1982). 37 . I.J. Lloyd, J. Cons. i. Explor. Mer, 33, 312 (1971). 38 . S. Schulz and W. Kaiser, Third (nt. Symp. lipwelling Ecosystems, Kiel, comm. n° 43 (1975). 39 . 0.J. Koblentz-Mishke, V.V. Volkovinsky and J.G. Kabanova, Scientific Exploration of the South Pacific, Standard book n° 309 - 01755-6, National Academy of Sciences, lJashington, D.C. (1970).
Adv. Space Res. Vol. 7, No. 2, pp. (2)53—(2)62, 1987 Printed in Great Britain. All rights reserved.
SPATIAL/TEMPORAL VARIABILITY OF ALGAL BIOMASS AND POTENTIAL PRODUCTIVITY IN THE MAURITANIAN UPWELLING ZONE, AS ESTIMATED FROM CZCS DATA A. Bricaud, A. Morel and J. M. André *Laboratoire de Physique et Chimie Marines, Université Pierre et Marie Curie, BP 8, 06230 Villefranche-sur-mer, France ABSTRACT The mesoscale distribution of phytoplankton in the Mauritanian coastal upwelling area has been mapped using a series of CZCS scenes covering more than one year. The spatial and temporal evolution of the algal biomass has been analysed. The CZCS images have been combined to establish composite images (ca. 840 km x 1260 Kni) representing monthly averages of the surface pigment content. SatelTite-derived pigment concentrations have been tentatively transformed into potential primary production per unit area. For this purpose, data concerning in situ primary production, vertical profiles of algal biomass and water optical propertii~ iipreviously obtained, were combined with mean values of available radiant energy taken from climatic atlases. An estimate of the daily integrated primary production in the different zones influenced by the seasonally fluctuating upwelling is proposed. INTRODUCTION The hydrology of the Mauritanian upwelling area (Fig. 1) is mainly controlled by the combined influence of the Canary Current which carries cool surface waters southwards and of the coastal upwelling regime induced by the NE tradewinds, blowing along the NW African coast and driving surface waters offshore by Ekman transport. The hydrographic conditions in this area have been described by numerous authors (see reviews in /1,2/). Very briefly, the southern limit of the tradewind influence varies approximately between lO°Nin February March, and 2l°N in September - October. In winter (December to May), the whole coastal region (north of l5°N)experiences an upwelling regime, which is subject to short—term (and small-scale) fluctuations due to local changes in wind stress /3,4/. In summer (June to
25W
20W I
I
I
i5W I
I
/(~ape1 Bojado1~ çcape - 25 N Garnet
-
I
Cape
Bar’bas 2
•
3 4
I
i\~CapeBlanc ‘)Banc d’Argut~20 N ~Cape Ttmiris Nouakchott 1ape Verde I~
Fig. 1. Location of the study areas. (2)53
N
(2)54
A. Bricaud, A. Morel and J. M. André
November), variable westerly winds replace the tradewinds south of Cape Blanc, where warm waters of the equatorial countercurrent often appear. The upwelling remains active only north of Cape Blanc. These marked differences in the annual upwelling cycle have been described, at least partly, either using SST data /2,5,6/ or biornass and production data /l,7/.The zonal variations are also influenced by other factors, such as the cross—shelf morphology and the gradients in water mass properties within the upper layer (the warmer, saltier, low-nutrient North Atlantic Central Water and the cooler, fresher, high-nutrient South Atlantic Central Water ;see e.g. Ill). The main front between NACW and SACW oscillates between Cape Blanc and Cape Timiris /8/. Though very detailed, the previous studies, made on the basis of ship-bound measurements (except /2/), have unavoidably been restricted in both space and time. The first aim of this paper is, therefore, to take advantage of the synopticity and repetitivity of the satellite colour imagery to complete our knowledge of the annual upwelling cycle in the Mauritanian region. This will be done by delineating, at different periods of the year, the extent of eutrophic waters, as well as that of mesotrophic waters. Because of their large coverage, the mesotrophic waters presumably contribute to the global productivity of this region in a significant manner. In the same way as phytoplankton distribution, the spatial/temporal variations of the primary production are not well documented in this area. The second aim of this paper is therefore to study this evolution, by using a relationship between satellite-derived pigments and primary production inferred from historical data in this region. An estimate of the global amount of carbon which is photosynthetically fixed in the whole area, on a per-year basis, can then be derived. METHODOLOGY Estimation of the pigment content of the surface waters The CZCS data have been processed using a procedure described previously /9/. Briefly, the signals received by the sensor have been calibrated (taking into account the time decay of Channel 1 according to Mueller’s algorithms /10/) and then corrected for atmospheric effects (Rayleigh scattering, aerosol scattering and atmospheric attenuation of the water-leaving signal; see e.g. /11/). The aerosol concentration and the spectral dependence of aerosol scattering, which vary in space and time, are therefore a priori unknowns. The aerosol and water-leaving radiances were therefore partitioned in channel 4 by an iterative procedure using a statistical relationship established from sea-truth measurements. This method is similar to that developed by Smith and Wilson /12/. The spectral dependence of aerosol scattering (in A where n is the Angstrom exponent’) was estimated with a method adapted from Gordon’s clear water radiance concept” /13,14/, and using a reflectance model for Case 1 waters /9/. The average of the computed exponents was then applied to the whole scene. Table 1 gives the average exponents for the selected scenes. No systematic trend is evident; the striking feature is the occurrence of low exponents, or even of negative exponents expressing an increase of aerosol scattering towards longer wavelengths. Such “reversed” selectivities (generally associated with a high aerosol load) have been occasionally observed in Africa /15/. Using the reflectance model mentioned before, a discrimination between phytoplankton-dominated Case 1 waters and sediment-dominated Case 2 waters was achieved. Case 2 waters, where the pigment and mineral particle content cannot be separately estimated, were excluded before applying the pigment algorithms. In the Mauritanian upwelling, this was the case for the near-shore waters, where sediment resuspension by wave action is very high, and also for the shallow waters of Banc dArguin, south of Cape Blanc. Finally, in the Case 1 waters the pigment content C (including pheopigments) was estimated by using the algorithm: 3) = 1.73 (L(443)/L(55O))2~04 C (mg.m where L (443) and L (550) are the water-leaving radiances in channels 1 and 3. This algorith’~ was used eVen for waters with high pigment content, where L (443) becomes very weak. The ratio 520/550, used by some authors, is not very sensitive to%ariations in C; it is believed to induce a greater uncertainty in pigment estimates. The temporally-averaged pigment content of surface waters Table 1 gives the list of the scenes selected for this study. The chlorophyll maps were resampled on a Mercator grid. Then composite images were established by averaging the images over a given period. In these composite images, the chl a content at a fixed location is the arithmetic average of the chl a contents of the corre~ondingpixels in the
Variability of Algal Biomass and Potential Productivity
Date
2 3 8 30 8 i8 23 24 8 15 21 3 27 10 11 11
November November November November February February February February ilarch April April September September November Novemoer November
TABLE 1
1982 1982 1982 1982 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1983 1933
Orbit
Latitude (°li) of southern limit
20320 20320 20403 20707 21675 21813 21882 21896 22052 22587 22670 24536 24863 25476 25490 25490
12.40 16.30 17.79 14.98 14.67 15.10 14.53 12.31 13.16 12.89 18.34 16.30 18.18 17.59 12.31 13.11
(2)55
Angstrom exponent
0.246 0.203 0.277 -0.214 0.001 0.322 0.193 -0.059 -0.214 -0.101 -0.156 -0.322 -0.070 0.168 -0.151 0.004
CZCS overpasses selected for this study.
individual scenes. A given pixel is identified as belonging to Case 2 waters or to clouds only if this occurs for the corresponding pixels in all the individual images. This allows one to discard most of the artifacts due to improper atmospheric corrections and to extract “permanent” turbidity features (at the scale of about a month). Estimation of the potential production The primary production, P, in a given water column is dependent on the amount of the photosynthetically available radiation PAR (i.e. the amount of radiant energy available within the spectral range 350-700 nm, see /16/), and on its pigment content, C. At sufficiently low energy levels, P is proportional (empirically) both to C and PAR, with the slope controlled by phytoplankton physiology /17/. In the ocean, the possibility of establishing a relationship between production and (surface or integrated) pigment concentration has been investigated by several authors /18-23/. In spite of limitations due to physiological and environmental variability, they demonstrated that approximate estimations of the primary productivity could be obtained by using statistical site-specific algorithms. A similar approach has been made here, by using data acquired during the Cineca V-Charcot cruise in the Mauritanian upwelling area in March-April 1974 /24-26/. The in situ production in the water column can be written as: P
=
(1/39) 8 x PAR x C
(1
where C is the chlorophyll a content in the euphotic 1a~yer, ~l/39)is the factor converting the phot~synth~tical1ystorid radiation (PSR, in J.m .day ) into primary production (P, in gC.m .day ). 8 represents the ratio of e , the efficiency of utilization of radiant energy by phytoplankton (in reference to photosynthetic irradiation), to C, the water column chlorophyll a content; c defined in /16/ is dimensionless, thus 2)~) 8 = r~/ C , with 8 expressed as (% (mg.m In practice, the p_~-ameter derived from CZCS imagery is not C, but Csat, the pigment concentration (mg.m ) weighted over the upper attenuation length. A relationship similar to (1) can be written: P
=
(1/39) 8’ x PAR x Csat
(1’)
The “detectable” pigment concentrations, Csat, have been estimated from the pigment concentration vertical profiles, C(z), determined at each station of the cruise, and from the diffuse attenuation coefficient profiles for PAR, K, 1(z), using the relationship given in /27/. The energy in the spectral range 350-700 nm “is approximately 43% of the total energy /28/, and thus PAR was inferred from the daily irradiation, E, measured at each
A. Bricaud, A. Morel and J. M. André
(2)56
station. Using the values of the water column production, P, and of the daily irradiation in terms of PAR at each station, a mean value of ~‘ can be determined: 3)~) (2) 8= 0.184 ±0.059 (N = 14) , where 8’ is expressed as (% (mg.m When computing 8’ , turbid coastal stations have been excluded, since the corresponding e values are anomalously lower than for offshore stations (it has been recognized that the turbidity-induced light limitation depresses the efficiency /29/). Note that, compared to most of the previous studies linking P to the pigment content, the influence of environmental variability has been reduced by taking into account the actual photosynthetic irradiation. The standard deviation however remains relatively high (32%), likely due to variations in other factors such as species composition and physiological state of algae, nutrient availability or water temperature. When converting CZCS-derived pigment concentrations into primary production rates, the values of PAR corresponding to the different scenes are needed; they could be obtained using e.g. an analysis of Meteosat data. Here, as a first approximation, monthly-averaged values for the study areas have been derived from climatic atlases (see legend Fig. 2). These mean values for solar irradiance /30/ and for nebulosity N /31,32/ have thu, been combined in Matsuike et al’s formula /33/ to obtain mean daily irradiations:
1=
?~(l - 0.52 ~l.3)
As an example of this calculation, Fig. 2 shows the annual variations of ~ (and ~) at 25°N, and_evidences their importance when evaluating P. Then PAR has been computed as above = 0.43 E) and introduced in equation (1’).
(J.m2~d4
— h--~A
~ J
N
i
FMAMJJASOND
r,
Fig. 2. Monthly values of the maximal solar irradiance, at 25°N (from /30/), of the nebulosity off Cape Bojador (from /319), and of the resulting solar irradiance at 25°N, computed from Matsuike et al. /33/. These values have been applied to the zone 1. Similar curves have been obtained for the zone 2 (using the average of at 25 and 20°N), for the zone 3 (using En at 20°N)and for the zone 4 (using the average of E 0 at 15 and 20°N, ~nd the mean nebulosity off Dakar /32/).
RESULTS AND DISCUSSION Spatial-temporal variability of the surface pigment content. Fig. 3 shows five composite images corresponding to typical situations in the annual cycle of the upwelling: a) and e) in November, at the end of the warm season, when the tradewinds are progressing southwards (these images are shown as an example of interannual variations), b) in February, in the middle of the cold season, when the tradewinds are blowing everywhere including the zone south of Cape Verde, c) in March-April, when the winds regress northwards, and d) in September, when the southern limit of the tradewinds is at its northern position, near Cape Blanc. An individual image (November 3, 1982) is also shown as an example (Fig. 4). On all images the extent of the highly reflective Case
Variability of Algal Biomass and Potential Productivity
~
-
_____
u•,,.•
(2)57
:~ :~ ~
I P~ CM1
____
CM1 Fig. 3a.
t
Fig. 3b.
MAR ______
15-21 ~pR
Fig. 3c.
331
Fig. 3d.
Fig. 3. Composite images of surface pigment concentration in the NW African upwelling. The scenes selected for each image are indicated in Table 1. The dimension of the images is approximately 840 km x 1260 km, represented with a 2.5 km resolution. The western limit of the images is 24°W.Land and clouds are flagged in white. Turbid-Case 2 waters are masked in black. In Case 1 waters, the pigment con’cer~rations increase according to a_~ogarithmicscale, from dar~ blue (< 0.2 mg.m ) to the 3mean blue (0.2 - 1 mg.m ),_~ightblue (1 - 3 mg.m light green (3 - 5 mg.m ) and dark green (>5 mg.m ).
(2)58
A. Bricaud. A. Morel and J. M. André
I”
_
Fig. 3e.
Fig. 4. Image of surface pigment concentration for the scene of November 3, 1982, represented with a 1.6 km resolution. The colour scale is the same as in Fig. 3.
Fig. 5. Thermal data (CZCS channel 6, without atmospheric correction) for the scenes of February 18, 1983 (Fig. 5a) and September 3, 1983 (Fig. 5b). The temperature is increasing according to a linear scale from black to dark blue, light blue, green, yellow, orange and red.
Variability of Algal Biomass and Potential Productivity
(2)59
2-turbid waters (masked in blac~k) coincides approximately with the 50 meter isobath. Beyond this turbid zone, the spreading of eutrophic and mesotrophic waters reveals important fluctuations. Four main areas are distinguished (see Fig.l): i) north of Cape Barbas, ii) from Cape Barbas to Cape Blanc, iii) from Cape Blanc to Cape Timiris, and iv) from Cape Timiris to south of Cape Verde. North of Cape Barbas (zone 1). In this area the seasonal variation in the upwelling activity appears weak. A slight minimum in algal biomass occurs in the cold season (Fig.3c), in agree_m.~nt with previous studies /1,5/. On all the images, the area of high biomass (> 3 mg.m ) is restricted to a coastal band, approximately 50 km wide, which narrows near Cape Garnet. Thermal data sh~wthat the cold waters are confined to the same belt. 3Mesotrophic waters ( = 0.2 - 3 mg.m ) are separated from oligotrophic waters (< 0.2 mg.m ) by a sinuous, well-defined front. This front parallels the coast at a distance of about 120 km, until it becomes E- W oriented near Cape Barbas, where mesotrophic waters invade the offshore region. The persistency of the eddy-like features along the front is apparent on Figs. 3a and 4. The limited extent of the upwelling area in this zone is consistent with the weakening of the Ekman transport, compared to the zone south of Cape Barbas, as demonstrated by Wooster et al. /5/. From Cape Barbas to Cape Timiris (z~iTer2 and 3). In these areas, the upwelling is known to be permanent, as attested by the infra-red data in February and September (Fig. 5a,b). However, pronounced seasonal variations are evident in Fig. 3, with a maximal coverage of the high biomass area in February, a well-marked minimum in September, and intermediate situations in November and March-April. South of Cape Blanc (zone 3), the eutrophic waters show a larger offshore extension than in the north (zone 2). The March-April image reveals a peculiar situation where the upwelling is active near Cape Blanc whilst it appears considerably weakened near Cape Barbas. In both zones the mesotrophic waters extend far offshore, approximately 700 km from the coast at the latitude of Cape Blanc, even in September, when the biomass is at its~lowest level. In February, they extend far beyond this limit (concentrations of 0.8 mg.m are still observed about 1400 km offshore on the scene of February 8). The advection processes inducing offshore (regenerated?) production have been predicted and analysed by theoretical models based on a two cyclonic cell circulation /33,34/. From Cape Timiris to Cape Verde (zone 4). In this area the seasonal variations of the upwelling are the most pronounced. In February and March-April, the area of high algal content extends as far as 200 km all along the coast, and even 300 km at the latitude of Nouakchott (this latter area has been previously recognized as subject to enhanced production, due to the intensifying effect of a submarine canyon /4,36/). Clear water patches appear inside the area of high biomass, which could indicate recently upwelled, not yet productive waters. Fig. 5a shows that the whole nearshore region is covered by cold waters. In September, the algal concentration remains very low, as corroborated by thermal data which show that equatorial low-nutrient waters have invaded this area (Fig. 5b). The images of November show a weakened, though distinct, upwelling activity. Note the large phytoplankton patches offshore, north-west of Cape Verde, in November 1983 (non-existent during the same period in 1982). Annual variations of the potential production Table 2 gives, for the different periods of the year, the values of Csat averaged over each of the four areas indicated on Fig. 1. These areas are limited on the west by the 24°W meridian. This includes all the eutrophic waters, and most of the mesotrophic waters (> 1 mg.m , in green and light blue on Fig.3). From these C~tvalues, average values of the primary production P over the same areas have been estimated, using the method described before *~ By integrating these values of P, an approximate value, Pint, for the global amount of fixed carbon in the different areas, can be obtained **~ To the north of Cape Barbas, as previously observed, annual Csat variations are rather weak. Nevertheless F values are significantly higher in March-April than in September and November, because of the higher PAR values. For the same reason, and though the pigment content between Cape Barbas and Cape Blanc is at its minimum in September, the primary production in September exceeds that occurring in November 1982. In this area, both pigment content and primary production reach their maximum in March-April. Between Cape Blanc and Cape Timiris, Csat and P reach their maximum in February. Between Cape Timiris and Cape * This computation assumes that the average pigment content and production inside Case 2 waters are equal to the average values over the whole area. This likely leads to a slight overestimation, because of the light limitation in the mid-shelf zone (see above).
The column pigment content, and therefore the production capability of the oligotrophic waters inside the study areas may be underestimated, since likely their vertical distribution of algal biomass is that of a “typical tropical structure”, with a maximum occurring within the thermocline, i.e. below the upper attenuation length “seen” by CZCS. **
(2)60
A. Bncaud, A. Morel and J. M. André
Nov
1 1982
Zone 1 2 3
4
Feb, 1983
Area PAR ~ 9m2) (mg.m3)(1O6J.m2d~) (10 279.2 0.64 8.21 115.8 1.09 8.60 2.35 131.1 1.87 8.60 2.95 411.3 (2.01) 9.03 2.78
Zone
~
PAR
PAR
9.37 (1.02) 9.72 (2.80) 9.72 2.20 10.06
2.43
pint
~
~I1~
Mar-Apr, 1983 Sept, 1983
P
~
PAR
11.70 11.70 11.70
0.85
25.9
26.3 26.3
0.62 8.21 2.34 8.60 2.94 8.60
11.95
(0.59)
23.6
2.53
9.03
Pint
l’
P
Pint
0.96 0.80
PAR
pint
(gC.m2.d~)(KtC.d1) 0.248 69.2 (O.563)(157.2) 0.447 124.8 0.442 51.2 1.077 124.7 (1.545)(178.9~ 0.512 59.3 0.759 99.5 1.352 177.2 1.214 159.2 0.427 56.0 (0.856) (352.1) 1.319 542.5 1.370 563.5 — —
1 2 3 4
Nov, 1983
0.241 67.2 0.949 109.9 1.193 156.4 1.078 443.3
TABLE 2 Upper part: Values of the pigment concentration weighted over the upper attenuation length, Csat, averaged over given periods (see Table 1) and over the areas indicated in Fig. 1. The monthly values of the photosynthetically available radiation (PAR) used for estimating the average primary production are also given. The values in parentheses are average values computed on incomplete areas. Lower part: Values of the average primary production (P) for each zone and period; values of the primary production when integrated over the different areas (Pint).
Nov, 1982 Zone
~P
Pint
Feb, 1983 ~P
Pint
Mar-Apr, 1983 AP
P
Pint
AP
(gC.m2.d1)(KtC.d~) 1%)
1 2 3
0.42
60.8
0.86 1.34
43.5 81.3
4
0.94
179.0
Zone
2 3 4
2.26 2.81
—
2.44
-
Sept, 1983 ~
1
12.1 15.0 18.3
0.75 0.76
0.56
(0.28)
-
114.4 170.6 4660
-
8.3 3.7 14.1
(0.83) (2.32) 2.49 2.73
(121.2) (117.4) 151.1 521.4
(22.9) (34.4) 5.1 7.5
Nov. 1983
Pint
AP
P
Pint
AP
108.5 38.3 34.3 (115.4)
13.1 35.4 38.7
0.35 2.14 2.43 1.68
51.2 108.3 147.5 320.9
23.8 1.4 5.7 27.6
-
TABLE 3 Average values of ~ and Pint over the same periods and the same areas as in Table, 2, except that the western limit of these areas is 20°W(instead of 24°W). ~ P represents the ratio of the primary production occurring in the band between 24 and 2O°W (corresponding mainly to mesotrophic waters), to the global primary production between 24°Wand the coast.
Variability of Algal Biomass and Potential Productivity
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Verde, Vreaches comparable values in February and April, though Cut is at its maximum in February. From November to February, higher production values are found to the south of Cape Blanc compared to that to the north. This agrees with the study by Minas et al. /1/ who attributed this difference to the influence of nutrient-rich SCAW (which is reduced in summer). Interannual variations (November 1982 to November 1983) of Csat and P are the most important in the zone 2, where they exceed 100%. These production evaluations are not easily comparable with the previous ones made from measurements at sea, which have been restricted to the zone east of 18°Wor 20°W. The same computations as presented in Table 2 have therefore been repeated with the four areas limited to the meridian 20°W (T~le_~).The value obtained in March-April between Cape Blanc and Cape Barbas (2.32 gC.m .d ) falls in the range of the value~determined from ship at different stations within the same _~egt9n: 0.66 to 4.74 gC.m .d (mean valu~ 2.4~in March-April /26/, or 0.8 to 2.8 gC.m. .d in April-May /4/, and 1.1 to 3.4 gC.m .d in May—June /37/. 2.d~x 365 The annual production A value 0.59 gC.m = 215 gC.mme~nprirn~ry .year was proposedis not by well Schulzdocumented. and Kaiser /38/,of from time series of measurements along a section off Cape Blanc, as far west as 20°W.For the same zone (zone 2), a linear interpolation ~tween_jhe seasonal production values in Table 3 leads to a much higher value, 603 gC.m .year . The same inte.jpolati 1on for the whole area from Cape Verde to Cape Garnet provides a value of 510 gC.m .year . This estimate greatl~2exceed~ the value derived from the Koblentz-Mishke et al.’s map /39/ (0.5 x 365_f 182 ~j.m .year I and is closer to that proposed by MTWa~et al. /1/ (730 gC.m .year ) in their discussion of the production budget for the wholéThWATrican upwelling. From the seasonal areal fluxes in Tables 2 and 3, the ~l1obal mass of photosynthetically fixed carbon can be computed~ it amounts to 276 MtC.year for the entire zone (coast to 24°W), or to 228 MtC.year for the zone restricted to 20°W. This implies that the mesotrophic waters in the band between 20°and 24°Wwould be responsible for approximately 17% of the global production. This “offshore fraction” of the production (~P) varies from 1 to 39% according to the considered zones and periods (see Table 3). CONCLUSIONS The methodology used here for estimating the primary production in the Mauritanian upwelling area deserves some comments: i) the assumption of a unique relationship between the chlorophyll a content and the carbon fixation results in an uncertainty. By considering the hisforical data used for establishing this “site-specific” relationship, the uncertainty is estimated to be about
30%; ii) the question of the meaning of the in situ primary production experiments (gross production or something intermediate betwee~Tnétihd gross production?) exists as well for the present results; iii) in such a study, the distinction between “new” and “regenerated” production cannot be made on the sole basis of the colour information; iv) the deep chlorophyll maximum, if any, is not detected; therefore, a slight underestimate of the primary production in mesotrophic and overall in oligotrophic waters can be expected; v) the linear interpolation between selected seasonal values of primary production is obviously questionable; this drawback is easily circumvented if more scenes are available. Keeping in mind the above limitations, the temporal and spatial evolution of the surface chlorophyll content as revealed by CZCS data does not contradict what could be roughly expected from the general and well known meteorological events (i.e. the North-South oscillation of the tradewinds). The extent of eutrophic and overall mesotrophic waters appears however much larger than anticipated from historical ship-bound data. This is verified all over the year, particularly south of Cape Barbas and even when the upwelling activity is at its lowest level. The important consequence of this result lies in that the primary production capability of the entire zone affected by the upwelling has been most likely underestimated in the past. Within the nearshore eutrophic zone, the seasonal values of primary production as obtained agree with the previous in situ data. The offshore extent of mesotrophic waters, not investigated from ships but ei~TTy detected from space, causes the revision (towards a greater value) of the global carbon fixation by photosynthesis in the whole region influenced by the upwelling process. Acknowledgements This work was supported by Centre National de la Recherche Scientifique (UA 353 and GRECO 034), by Centre National d’Etudes Spatiales (under contract 84/1243) and by IFREMER (these institutions were associated in funding ATP Télédétection Spatiale 1984). The authors wish to thank R.G. Ingram for improving the English of the manuscript.
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A. Bricaud, A. Morel and J. M. André
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