II. Nutrients and primary production

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 180: 148-183. 1982.

Nutrients and primary production in the upwelling region off Northwest Africa

H. J. Minas1, L. A. Codispoti2, and R. C. Dugdale3

By relying heavily on the information that has resulted from the CINECA pro­ gramme, we have been able to compare the different nutrient and primary produc­ tion regimes that exist within the Northwest African (Canary Current) upwelling system. Our study indicates that the differences are considerable, and they can be summarized as follows: 1. The Northwest African upwelling region is divided into two major zones by the existence of a front between North Atlantic Central Water (NACW) and South Atlantic Central Water (SACW) near Cape Blanc. These waters are the primary components of the upwelling off Northwest Africa, and because SACW is richer in nutrients than NACW, there is a meridional nutrient gradient in the ascending waters. 2. The situation described suggests that the highest production rates should be in the south, where, indeed, the maximum instantaneous rates may be found. How­ ever, the highest annual rates probably occur in the Cape Barbas-Cape Blanc region. Upwelling is a year-round process here, and the region is far enough south to benefit from the northerly transport of SACW by an undercurrent that “hugs” the continental slope. 3. South of Cape Blanc, upwelling takes place mostly in winter and spring. During active upwelling, very high nutrient and primary productivity rates may be obser­ ved close to the coast. Offshore of Nouakchott, there is a widespread area with relatively high nutrient concentrations in the surface layer that is probably rela­ ted to a mesoscale “dome”. This region is anomalous because the productivity and chlorophyll concentrations are low in relation to the nutrient concentrations. 4. To the north of Cape Bojador, upwelling is strongest in summer. Despite the predominance of NACW in the upwelling source waters, the few available obser­ vations indicate that maximum productivity rates may be similar to those found to the south. In certain regions, nutrient regeneration seems to be particularly efficient. For example, off Cape Dra, there is a “cold cushion” over the continen­ tal shelf that is enriched considerably by nutrient regeneration.

Additional results of our study include the following: 1. Silica limitation may be more a function of geomorphology and dynamics than of the initial Si/N ratios in the ascending waters. 2. The average annual productivity in the nearshore region between Cape Bojador and Cape Blanc may be more than 2 gC/m 2/d, four times higher than one might infer from some previous studies.

Sur la base de l'ensemble du matériel d’information résultant du programme CINE­ CA, il nous a été possible de comparer les caractéristiques du régime des sels nutritifs et les rapports avec la production primaire dans le système des résurgences côtières NW africaines. Des différences considérables peuvent exister entre les di­ verses régions; elles peuvent être résumées de la façon suivante: 1. L’existence de deux types d’eau, l’Eau Centrale Nord Atlantique (ECNA) et l’Eau Centrale Sud Atlantique (ECSA), partage la zone en deux secteurs dont la limite se situe aux environs du Cap Blanc. La plus grande richesse en sels nutritifs de l’ECSA conditionne un gradient nord-sud dans les eaux des flux ascendants.

1 Station Marine d’Endoume, Laboratoire d’Océanographie, Centre Universitaire de Luminy, 13288 Marseille Cedex 9, France. 2 Bigelow Laboratory for Ocean Sciences, McKown Point, West Boothbay Harbor, Maine 04575, USA. 3 Department of Biological Sciences, Allen Hancock Foun­ dation, University of Southern California, Los Angeles, Ca­ lifornia 90007, USA.

148 2. Compte tenu de la situation précédente, les productions devraient être les plus fortes dans le sud, ce qui est effectivement le cas. Toutefois, les taux annuels de production les plus élevés existent probablement dans la région Cap Barbas-Cap Blanc. En effet, les remontées d’eau y sont observées tout au long de l’année et la région est suffisamment au sud pour pouvoir bénéficier de l'apport de l'ECSA, assuré par le sous-courant orienté vers le nord et longeant le talus continental. 3. Dans toute la région méridionale au sud du Cap Blanc, les résurgences ont lieu surtout en hiver et au printemps. A proximité immédiate de la côte, les concen­ trations en sels nutritifs et les taux de production sont très élevés. Au large de Nouakchott existe une vaste zone avec des teneurs superficielles relativement élevées de sels nutritifs, dues probablement à une divergence provoquée par une circulation cyclonique. Cette zone présente une anomalie car les biomasses et la productivité sont basses par rapport à l’abondance relative de sels nutritifs. 4. Dans toutes les régions au nord du Cap Bojador, les résurgences présentent leur plus grande intensité en été. Malgré une prédominance de l’ECNA dans les eaux- sources, les quelques observations dont on dispose semblent indiquer une activité photosynthétique aussi importante que dans le sud. A certains endroits, une régénération de sels nutritifs semble particulièrement efficace; par exemple près du Cap Dra, un «bourrelet» d’eau froide sur le plateau continental est le siège d'un recyclage important d’éléments nutritifs. Par ailleurs, notre étude montre que le caractère limitant ou non du silicium dépend plus de la géomorphologie et de processus de dynamique biochimique que du rapport Si/N initial dans les eaux-sources de l’upwelling. Elle suggère aussi que la production moyenne annuelle dans la région littorale entre le Cap Bojador et le Cap Blanc est supérieure à 2 gC/m 2/j, quatre fois plus élevée que celle que l’on pouvait déduire d’études préliminaires.

Introduction will be concerned with the portion of the Atlantic adja­ cent to the region between Gibraltar and Cape Verde As expected from classical studies, generalized global (Fig. 119). This area corresponds to the CINECA and large-scale charts of marine productivity (e.g. study region. Sournia, 1969; Koblentz-Mishke et al., 1970) support the view that primary productivity in temperate and tropical regions is high when the nutrient supply is high. The coastal upwelling areas that are found in the eastern boundary regions of the temperate and tropical ocean are prime examples of such locales, and recently much effort has been expended on unraveling the rela­ tionships between nutrients and productivity in these areas. While the application of recently developed methods for the routine estimation of carbon, nitrogen, and sili­ con uptake (e.g. Steemann Nielsen, 1952; Dugdale, 1967; Nelson and Goering, 1977; Slawyk et al., 1977) supports the general view, many questions have arisen because the new techniques permit a better apprecia­ tion of the complexity of the interactions between phy­ CABO PENA GRANDEfc toplankton and the nutrient fields in which they are immersed. Recent studies also show that coastal up­ welling dynamics are primarily a mesoscale phenome­ non (e.g. Barton et al., 1977) and that the response of CAP SLAVIC the phytoplankton and higher trophic levels strongly depends on local and mesoscale factors (e.g. Ryther et al., 1971; Beers et al., 1971; Parsons, 1976; Huntsman and Barber, 1977). The purpose of this report is to demonstrate the existence and, insofar as possible, the significance of important mesoscale and local differences in nutrients and primary production within the Northwest African upwelling system. Since the definition of this system Figure 119. Location chart of the main CINECA area off can vary, we wish to make it clear that our discussion Northwest Africa.

149 25 2 5 ° - + 4 + + garnet

-+ B d GORREI

C BLANC

20 20 '

NOUAKCHOTT

VERT

C.ROXO +-

10°

SPACE-TEMPORAL dislocation o f t h e NORTH-W EST AFRICAN UPWELLING REGION

I II III IV V 1 VI 1 VII ' VIII ' IX 1 X ' XI ' XII 2 0 ' 16° W

Figure 120. Seasonal variation of upwelling along the Northwest African coast (from Schemainda and Nehring, 1975). The plus signs indicate the presence of upwelling, and the minus signs indicate the absence of upwelling.

Nutrients and hydrology while nutrient concentrations tend to increase (e.g. The Canary Current carries cool surface water south­ Weichart, 1970 and 1974; Codispoti and Friederich, ward along the Northwest African coast. In the near­ 1978). However, this situation is not necessarily indica­ shore region, this large-scale flow is enhanced by the tive of more intense upwelling in the south (e.g. Mittel- action of the winds, and these winds also cause a staedt, 1972). Instead it arises largely from differences further reduction in sea surface temperatures because in the ambient water masses. In the north, the upper they are generally favorable for upwelling. This latter layer of the ocean is dominated by relatively warm, influence of the trade winds (blowing mainly from salty, and low-nutrient North Atlantic Central Water north-northeast) has been recognized for many years. (NACW) while in the south, relatively cool, fresh, and Thermal anomaly charts from Böhnecke (1936) nutrient-rich South Atlantic Central Water (SACW) clearly show the cooling of the coastal waters. With dominates (Fig. 121). Low salinity, tropical surface these and other data, Wooster and Reid (1963) and waters also influence the southern region (e.g. Tom- Wooster et al. (1976) have described the seasonal czak, 1978a,b), but not to any great extent during upwelling cycles off Northwest Africa. They show that upwelling periods. there are considerable differences in the annual upwell­ Mittelstaedt (1972) presents a T-S diagram (Fig. ing cycle from place to place. In the Moroccan region 122) that suggests that salinity is a good indicator of the upwelling takes place predominantly during the sum­ relative proportions of NACW and SACW. Various mer. Upwelling is continuous off Cape Blanc and it authors (e.g. Anderson, 1973; Fraga, 1973 and 1974; generally takes place during winter and spring between Fraga and Manriquez, 1975; Codispoti and Friederich, Cape Blanc and Cape Verde (Fig. 120). 1978) have discussed how mixing between NACW and Analysis of the upwelling off Northwest Africa is SACW can produce marked nutrient gradients (Fig. further complicated by the existence of significant 123). While the nutrient-concentration changes be­ north-south gradients in water properties. Tempera­ tween NACW and SACW are considerable, nutrient: ture and salinities tend to decrease towards the south nutrient ratios (nutrient = nitrate, dissolved silicon,

150 20° 16° W 20' 16* W

Oberfläche 27. 8 - 0 .9.1970 /Bd. "Garnet ta rn e t

d.Gorrei d.Gorrei

0.2 \C. Blanc .C. Blanc 0.8

20° 0.5.

Nouak­ Nouak­ chott chott åg m

i0.5 C. Vert

< 0.2

Roxo r \ Roxo

1 2 ° \>0.3

Figure 121. Surface reactive phosphorus distribution off Northwest Africa during spring (right) and summer (left) (from Schemainda et al., 1975).

and reactive phosphorus) are not so variable (e.g. series of T-S diagrams to trace this poleward flow be­ Gardner, 1977), a fact that can prove useful in the tween 16° and 24°N (Fig. 124). They show that the interpretation of the nutrient regime in zones where poleward undercurrent occurs between depths of ~ 200 the upwelling source waters are composed of a varying to 400 m over the continental slope and that an SACW- mixture of NACW and SACW (e.g. Friederich and rich mixture is indicated by a salinity minimum and a Codispoti, 1979). small temperature inversion at a o, of 26-80. Gardner The major front separating NACW and SACW is (1977) has shown how the undercurrent influences the located off Cape Blanc at a latitude of ~ 21°N (e.g. reactive phosphorus distribution at 250 m (Fig. 125). Fraga, 1973; Tomczak, 1977). This boundary divides Tomczak (1977) has discussed the influence of SACW the CINECA study area into a southern part with a rich between 20° and 28°N, and Voituriez and Chuchla nutrient reservoir and a less rich northern part. If fac­ (1978) have suggested that the salinity minimum may tors such as upwelling duration and cross-shelf mor­ not be an exclusive characteristic of the undercurrent phology were the same everywhere, the southern re­ because they found salinity minima at some offshore gion would undoubtedly be the most fertile, but, as we stations. shall show, these factors differ considerably from place Recently, Tomczak (1978a,b) applied T-S diagram to place. analysis to the study of the ratios between the principal Beyond the front at ~ 21°N, the northward propaga­ water bodies. He also investigated the distribution of tion of SACW depends mainly upon a poleward under­ these water bodies and their dependence on seasonal current that is itself associated with the coastal upwell­ cycles. Figures 126 and 127 show the situations and ing system. Hughes and Barton (1974) have used the schematic T-S diagrams for the two seasons: winter water-mass definitions of Sverdrup et al. (1942) and a with a generalized upwelling situation and summer

151 Figure 122. T-S diagram delineating the nutrient and density characteristics of upwelling water (U), Banc d'Arguin water (B) (slightly modified from Mittelstaedt. 1976; the figure also contains the straight line T-S curves defined by Sverdrup et al., 1942).

with the southern portion flooded by warm waters that 1973). Figure 121 (from Schemainda et al., 1975) re­ indicate little or no upwelling. From details of the study presents the large-scale phosphate distributions in sum­ it can be seen that the surface waters north of Cape mer and winter. These charts confirm the general con­ Blanc are the result of mixing between NACW coming ditions mentioned above. Nutrient concentrations are to the surface near the coast and the so-called “water of high during upwelling and highest in the upwelling the Atlantic Trade Winds Regime”. The surface waters center in the south. south of Cape Blanc are the result of mixing between SACW and tropical waters and, farther south, of tropi­ cal waters and waters from the Gulf of Guinea. Tom- The different upwelling regions off czak’s analysis makes it clear that the upwelling waters (composed predominantly of NACW and/or SACW) Northwest Africa are extensively modified by ambient surface waters as The area between Cape Blanc and Cape Corveiro they leave their place of origin. His analysis also indi­ ( -2 1 ° to 22°N) cates that interactions with the atmosphere do not sig­ nificantly affect the surface T-S mixing lines in his During the JOINT-I experiment, winds, currents, study regions. We lay stress on Tomczak’s description temperature, salinity, nutrients, and many biological of horizontal mixing, because Minas et al. (1978) have parameters were measured on a ~ 100 km long cross successfully applied a model based on this type of mix­ section extending seaward from Cape Corveiro. Mittel­ ing to the Cape Blanc region in order to calculate nut­ staedt et al. (1975) have described some of the current rient consumption and the production of photosynthe­ observations, and Barton et al. (1977) have described tic oxygen. (Fig. 128) the evolution of the surface temperature dis­ Many charts of the horizontal and vertical nutrient tributions observed from 24 February to 9 April 1974. distributions off Northwest Africa have been published During periods of high winds, the upwelling center (e.g. Jones and Folkard, 1970; Jones, 1972; Fraga, Contd on p. 157

152 23

24

23

2 5

? 4

20 27

15 - - 26

20

10 -- 4 0

29

PAN 29

9 6 30

Figure 123. T-S diagram showing the variation of nitrate concentration between NACW and SACW (modified from Fraga, 1973 and 1974).

S ta. numbers 6903 6906 6916 6920 6927 6933 6945 6946 6952

a D

Latitude °N Figure 124. North-south section indicating the salinity anomaly (shaded) associated with the undercurrent found over the Northwest African slope (from Hughes and Barton, 1974).

153 <û

■CABO BOJADOR

20

30

40 CAP BLANC

50

52-5

CAP VERT

Figure 125. Reactive phosphorus in ug/l at 250 m (from Gardner, 1977).

T ° C E A U GUINÉENNE - E A U E A U GUINÉENNE 'ATLANTIQUE- TROPICALE E A U 2 5 E A U - ATLANTIQUE ATLANTIQUE TROPICALE D E S A L IZ É S E A U _

'ATLANTIQUE I D E S ALIZÉS 20

N O V E M B R E MAI J U I N —

OCTOBRE ‘ECNA ECNA

1 5 3 4 S % o 3 5 3 6 3 7 3 4 S % o 3 5

Figure 126. Theoretical mixing diagrams for the waters off Northwest Africa (from Tomczak, 1977). ECNA = NACW; ECSA = SACW.

154 T ° C

2 5

20 20

ECNA ECNA ECSA« 'ZM

3 4 S % o 3 5 3 6 3 4 S % o 3 5 3 6

Figure 127. Mixing diagrams and water-mass composition at the surface for summer (27 August to 8 September 1970) and spring (13-26 April 1971). Left, summer; right, spring. The shaded areas represent frontal zones. The isolines on the figure at the left represent the percentage of NACW (top), temperature (middle), and Eau Guinéenne (bottom). On the right, the isolines represent the percentage of NACW (top) and SACW (middle). These figures are taken from Tomczak (1978a,b).

155 m/sec 30 ' <16.0 24 >175 16.5 16 5-17.5 <16.5 S 28 17.0

17.0 >18 17.0 16.5

\17.0 20

24 >18.

>18.0 28 \/7 .5 \I6.5

<17.5 < 16

16.5

► 17.0 ■17.5

100 50 0 DISTANCE FROM COAST (km)

Figure 128. Left: Variations in the zonal distribution of surface temperature along ~21o40‘N from hydrographic stations (solid circles) and moored thermographs (dashed lines) between 26 February and 9 April 1974. Contour interval is 0-5°C. Right: Components of low-passed wind velocity measured in midshelf over the same period. U is positive when motion is eastwards; V is positive when motion is northwards (from Barton et al., 1977).

o

<36.3 i oo 2 6 .6 > 36.5

TEMPERATURE ( C SALIN ITY(%.) SIGMA-T 2 I° 4 0 'N 16-17 March 1974197* 2I°4 0'N 2 l“40'N 16-17 March 1974 16-17 March 1974

4 0 0

500

DISTANCE FROM SHORE(km) DISTANCE FROM SHORE(km) DISTANCE FROM SHORE (km)

Figure 129. Vertical sections taken off Cape Corveiro during JOINT-I (from Barton et al., 1977).

156 17.0 362 16.0 15.5

15.0

,14.5"

14.5,

3.5^

O2 ml/l 02 sat. %

300

200- NH4 pg-at/l

Figure 130. Vertical sections taken from RV “Jean Charcot” on an NE-SW line with the inshore end near Cape Corveiro and the offshore end at the approximate latitude of Cape Blanc. This section was taken during JOINT-I, 14-17 March (from Minas et al., 1978)

would migrate towards the shelf break and remain with the temperature and salinity data explained by there until the winds relaxed. Such a situation was Barton et al. (1977). Figure 131 gives some idea of the observed on 15-16 March, with water at temperatures conditions that existed during a period of moderate lower than 15-8°C ascending to the surface near the upwelling. shelf break (Fig. 129). A number of investigators have The JOINT-I observations clearly reveal a strong presented the nutrient and oxygen distributions that correlation between local winds and the nutrient fields were observed during this period (e.g. Coste et al., off Cape Corveiro (Codispoti and Friederich, 1978), 1975; Minas et al., 1978; Friederich and Codispoti, but it is obvious that other factors are also involved. 1979), and they are in general accord (see Fig. 130) For example, fluctuations in the undercurrent have

157 17.5.

17.0 '

36.2 16.5

15.5 36.1

zoo MO-

288 - 02 ml/1 2 0 0- O2 sat. %

300- Figure 131. Vertical sections taken off Cape Corveiro during JOINT-I, 12-15 April (from Minas et al., 1978).

been shown to affect the nutrient concentrations at the isolation of the nearshore waters during periods of surface and changes in biological rates may cause sig­ shelf-break upwelling (Minas et al., 1978). nificant variations (Codispoti and Friederich, 1978). It Because Cape Blanc is the site of the frontal region is also possible that variations could be introduced by between NACW and SACW and because these water the distinct water mass that cascades off the Banc d’Ar- masses have very different nutrient concentrations, one guin, which lies to the south of Cape Corveiro. must expect considerable variability in the nutrient Although two-celled upwelling such as that de­ content of the upwelling source waters. Studies by Voi- scribed by Hart and Currie (1960) did not seem to turiez (1974) and by Le Corre and Tréguer (1976) shed occur during JOINT-I (R. L. Smith, personal com­ some light on these conditions (see Figs. 132-135). munication), there is some evidence to suggest a partial Some preliminary studies by Steemann Nielsen and

158 oO ECSA

CAPRICORNE 74 03

stations □ 2 4 9 • 03 o 54

to s X / X 47 * 36 □ 22 • ECNA A 38

S% o 35,5 56 p 36,5

Figure 132. T-S diagrai diagram from several stations taken during JOINT-I. Station 54 somewhat to the south of Cape Blanc and station 03 located offshore show a relatively high content of SACW (ECSA) in the 250 m upper layer. The rest of the stations were taken between Cape Corveiro and Cape Blanc (from Le Corre and Tréguer, 1976).

Jensen (1957) have indicated primary productivity in a nearshore region that has high turbidity (see Fig. rates of 0-67 and 0-56 gC/m 2/d near Cape Corveiro, but 136). During 1970 and 1974 an extensive series of prim­ these stations were located in relatively warm water (~ ary productivity measurements were taken from RV 19°C). A large-scale survey off Cape Blanc (Estrada, “Alexander von Humboldt” near Cape Blanc. Kaiser 1974) has revealed the highest primary productivity (1976) has used these data to make up an annual pro­ rates (0-71 gC/m2/d) at one station (station 12) about ductivity budget, but the reader should note that the 60 miles offshore. But this station is too far from the “von Humboldt” observations seem low in comparison coast to be considered representative of the coastal with the other available data. When the upwelling high productivity area. Productivity values published center is at the continental shelf break, one might by Lloyd (1971) range from 1T2 to 3-35 gC/m2/d. expect a production minimum at that location owing to Some values taken from RV “Jean Charcot” in 1971 in the recent origin of the upwelled water, and the thick­ water with a relatively high SACW content range from ness of the mixed layer. But results show that stations 3-7 to 7-6 gC/m 2/d, but the average value in 1974 was near the upwelling center are sometimes among the 2-4 gC/m2/d. An extensive series of observations taken most productive. Such is the case at three “in situ” along the JOINT-I transect (Barber and Huntsman, stations (17, 20, and 27) carried out by the “Jean Char­ 1975) indicate an average productivity of 2 gC/m 2/d. cot” (Fig. 137). It is possible that production is stimu­ Huntsman and Barber (1977) were also able to demon­ lated early by mixing with biologically conditioned strate that primary productivity is considerably reduced water masses from the continental shelf. Support for

159 N mine MQ tg /l

CAPRICORNE 74 03 stations

______i______,______,______*______I------3?------1------.------1-^2? 35,5 36,0 36,0

Figure 133. An inorganic nitrogen (jxgat/1) vs salinity diagram from the same stations given in Figure 132 (from Le Corre and Tréguer, 1976). The higher nutrient content of the SACW-rich waters becomes evident.

this notion is provided by similar stations situated defined by Dugdale (1967) on the basis of assimilation farther offshore (stations 4 and 7) in warmer and more of different nitrogen compounds. saline waters. These show lower production values, Huntsman and Barber (1977) present a nitrate con­ near 1 gC/m2/d. The mean production value (2-4 centration vs assimilation number diagram (Fig. 138) gC/m2/d) measured on the “Jean Charcot” is similar to which shows that, in Peru, the higher nitrate concentra­ that from the “Atlantis II” (Huntsman and Barber, tions are related to higher assimilation ratios even at 1977: 2 gC/m2/d), but some values are very high during nitrate concentrations higher than 10 (j.gat/1. However, the period following the 9 and 10 April low-wind it should be noted that these latter values are defined as regime. These high values from the “Jean Charcot” the ratio of integrated production and chlorophyll val­ (station 92: 4-7 gC/m2/d) are confirmed by oxygen pro­ ues for an entire water column (euphotic zone). The duction measurements (Minas et al., 1978), which yield diagram in Figure 139 shows the relation between even higher rates. Chlorophyll a values during the “integrated AN" and the AN at saturating light inten­ JOINT-I study are generally lower than 10 (j.g/1. The sity, as usually found in the literature, and a new ordi­ mean of integrated values given by Huntsman and nate scale based on .this relation is drawn to the right of Barber (1977) is 68 mg/m2. Huntsman and Barber’s diagram. This makes it easier The expression mgC/mg Chi a/h (AN or NA), called to compare with the usual AN values. The new AN “a reasonably good index of comparative plant physiol­ values are very high (usually over 10). A diagram (Fig. ogy” (Strickland, 1965) or “assimilation ratio” (Curl 140) has been set up by Vedernikov (personal com­ and Small, 1965) or again, “assimilation number” munication) according to his procedure (Vedernikov, (Vedernikov, 1976b; Huntsman and Barber, 1977) is a 1976a). This figure combines values from the Peruvian way of comparing efficiency of production systems in and Northwest African upwellings. References for different oceanic areas. This value can give an idea of these values are given in Table 20. the specific growth rate, similar to the growth rate as These results confirm the values of Huntsman and

160 CAPRICORNE 74 03 stations

• 03 o 54 + Il X 47 * 36 A 38

- . . * * , 1 , , , , *_!%•_ 35,5 36p 36,5

Figure 134. An oxygen vs salinity diagram from the same stations given in Figures 132 and 133 (from Le Corre and Tréguer, 1976). The nutrient-poor NACW water below 250 m from station 03 is more highly oxygenated.

35.5 36 Barber (1977), most of the data for Peru being above T°c 10. The data points collected aboard the “Jean Char­ cot” and the “Atlantis II” in the JOINT-I region are all in the same area in the diagram. Values from the ves­ 15 sels of the German Democratic Republic are lower. This important shift, due mainly to lower production values (see above) becomes clearly visible in the medium range corresponding to a data set from a time­ series station in the spring of 1973 taken off Cape Blanc 10 (20°55'N 17°27-5'W; see Hagen and Kaiser, 1976). The mean for ANi is (0-757 g C/m2/d)/(0-1464 g Chi aim2) = 0-52, which gives, after transformation as before, /liVma, = 0-84 mg C/mg Chla/h. This value is very low, although conditions are similar, if not identical, to those taking place during the same period in 1974 (nut­ rients in sufficient amounts and temperatures between 35.5 36 16° and 18°C). Some abnormally high values found at Figure 135. T-S diagram including the envelope that covers all low nitrate concentrations may be due to the presence the RV “Jean Charcot” observations taken during the JOINT- of significant quantities of ammonia (Vedernikov, per­ I experiment (March/April 1974). This figure, taken from Go- stan and Nival (1976), shows the high content of NACW sonal communication). Table 21 gives a summary for (ECAN) during this period in the Cape Blanc and Cape Cor­ the data in the diagram. These are classified according veiro area. to three ranges of chlorophyll and nitrate concentra-

11 Rapports et Procès-Verbaux 161 100 veiro. They show that about 8-7 % of the particulate Chlorophyll a carbon production is excreted by autotrophic produc­ 80 - ♦ . m g n r2 ers. According to these authors, the uptake of these 60 excreted products by bacterioplankton considerably reduces their concentration. Measurements of organic 4 0 nitrogen and phosphorus carried out by Le Corre and 20 Tréguer (1976) on a shipboard culture confirm this, because the increase in dissolved organic matter re­ mains insignificant. Carbon fixed Upwelling areas are zones of massive nitrate trans­ g m2 day-1 port to the euphotic zone, where "new production” takes place, according to Dugdale and Goering (1967). However, ammonia-nitrogen provided by regenerative processes is also significant. Table 24 (Codispoti et al., 1982) indicates that about 30 % of the phytoplankton nitrogen uptake near Cape Corveiro was in the form of Assimilation ammonia during JOINT-I. Smith and Whitledge (1977) number have shown that much of this requirement could be met mg C mg Chi <7 by zooplankton regeneration, and Rowe et al. (1977) have shown that the sediments also make an important contribution. These findings indicate that when the effects of nekton are included it should be easy to show that local ammonia regeneration could meet the phyto­ plankton demand. A mixing model (after Broenkow, 1965) has allowed Minas et al. (1978) to evaluate the total N uptake dur­ • Nitrate ing a moderate wind period when a subsurface am­ ° Silicate monia maximum is present (see Fig. 141). They have /xg atoms also estimated the increase in dissolved oxygen (AOz). This parameter, which corresponds to the net increase in biological oxygen for the entire animal and plant community, is proportional to the chlorophyll a con­ centration (Figs. 142 and 143). The correlation be­ tween the quantities (Atotal N uptake) and A 0 2 (Fig. 141) is consistent with the classical relationship of Red- 20 30 40 50 60 field et al. (1963) and also suggests that the atmos­ Distance offshore,hore km pheric oxygen fraction is negligible in this system. The Figure 136. Cross-shelf variation in the average values of some same conclusion was reached by Maske (1976) after biological and chemical parameters measured off Cape Cor­ evaluating nutrient uptake in the Banc d’Arguin up­ veiro during JOINT-I (from Huntsman and Barber, 1977). welling. However, when the upwelling takes place at the shelf break the atmospheric contribution is greater due to a melange of processes. The relatively great depth of the mixed layers means that the phytoplank­ ton oxygen production is spread out over a large vol­ tions. In general, this grouping is similar to the one ume. This helps to keep the surface waters undersatu­ given by Curl and Small (1965) for data taken in the rated, a situation which in combination with the vigor­ Oregon upwelling system. ous mixing leads to a relatively high input of oxygen The values for A N t from Morocco, when added to from the atmosphere. It should be pointed out that Huntsman and Barber’s diagram by Le Corre and Tré­ offshore upwelling is always accompanied by mixing: guer (1976), are high and associated with relatively low the lowest temperatures recorded near the shelf break concentrations of nitrate. As a main reason for their (15-70° to 15-66°C from 0 to 50 m, 16 March 1974, low AN values, Huntsman and Barber (1977) put for­ station 12, “Jean Charcot”, CINECA V) being higher ward the inhibitory effect of the relatively deep mixed than nearshore upwelling temperatures with lower layer. They show that plankton sampled in this layer winds (15-57° to 15-29°C from 0 to 20 m, station 77, can rapidly adapt (24 h) to high light intensities and the 14 April 1974, “Jean Charcot”, CINECA V; see data assimilation number is then increased. report, Groupe Médiprod, 1976). Smith et al. (1977) have provided some information on the dissolved organic carbon regime near Cape Cor-

162 2-553 gC/m2/ d S 1-277 2-966 1-284^ O yn 5 * /© X 10 15 / 20 i 30

4 0 © 5 0 ~'C 4-452 i

4-348

50 100

1-874 1-579 1-261 1-884

I i ® 1-141 V > 1-670 1-241 ./ / 1-065 ; \i ( i © ®

3-268 2 * 278

68

2-250

0-663

2-538 /® 4-735

/ ©

Figure 137. Primary production curves taken from RV “Jean Charcot” observations during JOINT-I (from Minas, 1976).

i r 163 200-1 N » j (j) « » i mu (h) 35

150-

• •

•10 50-v r% & NQl uala/l IZ 4 8 12 IE 20 2 I Figure 138. Assimilation numbers (NA) vs nitrate off Peru Figure 139. The relationship between assimilation numbers (solid circles), Cape Corveiro (triangles), and Morocco (NAmax) based on the maximum uptake values observed at (stars). See text and Figure 139 for a full explanation of the each station vs values (NA,) based on integrating conditions in two NA scales, but note that the scale on the left is based on the photic zone. Data from RV “Atlantis II” during JOINT-I uptake/d while the rest are based on uptake/h. These data are (Barber and Huntsman, 1975). taken from Le Corre and Tréguer (1976) and Huntsman and Barber (1977).

25

-C o 01"20 ! 03 E z < O 15 OO □ □

□ □ □ □ □ □ □ □□ nn 10 * D

* □ O

° A * ° 0 A. c $ 0 c o 0 1. A € € o o C «* ' o « * • W * «• I« • • • i i i 1 i ]_ 10 0 0 1 10 N .N O j |jg a t/l

Figure 140. Assimilation numbers (M 4max) vs nitrate concentrations off Peru (boxes and double circles) and Northwest Africa (all other symbols) for samples with primary production > 0-1 g C/m3/d. Open symbols, 0-5 ug/1 Chi a. Half-darkened symbols, 5-10 ng/1 Chi a. Closed symbols, > 10 ug/1 Chi a. See Table 20 for the data sources employed.

164 Table 20. Sources of the data used in Figure 140.

Peru Northwest Africa “T. G. "Akademik “Atlantis II" “J. Charcot” Thompson" Kurchatov” “Alexander von Humboldt”

Pisco □ © Joint-I A CINECA V ☆ O O O O

1969 1974 1974 1974 1970 1971 1972-73 1973 May-June February- March-May March-April August- April November- February- March November February April

Anonymous Vedernikov Barber and Groupe Schemainda Nehring Schulz Nehring (1970) et al. Huntsman Médiprod et al. et al. et al. et al. (1975) (1975) (1976) (1972) (1973) (1975) (1975)

water, and the current regime has been directly meas­ Area from south of Nouakchott to Cape Timiris ured (Gostan and Guibout, 1974). Drogue experiments Data from different cruises4 representing the winter (Herbland et al., 1973; Herbland and Voituriez, 1974), and spring upwelling situations along the shores of this which represent the main body of data concerning the region (Fig. 144) show an area characterized by cold plankton dynamics in this area, started at this point. surface waters that are nutrient-rich (P 0 4 > 1 -5 ngat/1 The buoy drift took place parallel to the coast down to and N 0 3 ~ 20 u.gat/1). Inshore, the northern bound­ a region south of Nouakchott, which represented the aries of this zone are formed by waters characteristic of southern limit of the system as indicated by a large the Banc d’Arguin fringe. These waters have already increase in the chlorophyll gradient. produced important quantities of chlorophyll. The For both drogue experiments (March-April 1972 southern boundaries are very variable. Numerous and March 1973), the characteristics of the upwelled underway charts (Groupe Médiprod 1974a,b) have water were the following: temperature, 14-5° to 15-5°C; shown that a canyon at 18°45'N channels the ascending salinity, 35-55 to 35-60 %c; oxygen, 50 to 60 % satura­ tion; nitrate, 19 to 20 ngat/1; phosphate, 1-5 to 1-6 jigat/1; silicate, 10 |xgat/l; and chlorophyll, 1-2 to 2-0 4 - Maps in the series of publications on RV ‘Alexander von Hgat/1. The water mass drifted southward, parallel to Humboldt" cruises. the coast at a speed of 0-5 knots, and the experiments - Weichart (1970, 1971) - CINECA-Charcot II cruise, 1971 (Groupe Médiprod, were terminated 110 km from the starting point 1974a,b). (17°45‘N), where the phytoplankton suddenly - Capricorne 7209 cruise, 1972 (Herbland et al., 1973). increased and the nutrients were exhausted. Total

Table 21. Assimilation numbers (AJVmax) for different ranges of chlorophyll and nitrate concentrations corresponding to the data used in Figure 140 (by Vedernikov, personal communication). M = mean value, n = number of observations.

N -N O 3 ngat/1 All data for ail concen­ Chl a mg/m3 < 1-0 1-10-0 > 10-0 trations of nitrate

2-17-5 3-20-5 2-5-30-4 2-30-4 < 5 n = 9 n = 2 5 n = 41 n = 75 M = 7-4 M = 7-1 M = 11-8 M = 9-7

1-9 1-6 10-10-8 1-10-7 5-10 n = 8 n = 17 n = 2 n = 2 7 M = 3-8 M = 3-2 M = 10-3 M = 3-9 Few data

1-3-5 0-5-2 0-5—3-5 > 10 n = 3 n = 10 No data n = 13 M = 1-9 M = 1-3 M = 1-45 Few data

All data for all 1-17-5 0-5-20-5 2-5-30-4 concentrations of n = 20 n = 52 n = 43 chlorophyll a M = 5-1 M = 4-7 M = 11-8

165 — 10-

161.3.09 fl?

Figure 141. Ntota, ( N 0 3 + N 0 2 + N H 4+) consumption in Hgat/1 vs 0 2 production in ml/1 (from Minas et al., 1978).

Figure 143. Distribution of net oxygen produced by photo­ synthesis in 1/m2 (from Minas et al., 1978).

organic production (particulate production + excre­ tion) was in both cases 19-5 gC/m2 for 5 days, i.e. 3-9 gC/m2/d. The net amount (grazing taken into account) of produced phytoplankton corresponds to 350 to 450 mg/m2. The assimilation ratios of nutrients relative to carbon synthesized have been estimated by these authors to be AC/AN03/ASi03/AP0 4 = 43/11/7-4/1 in 1972 and 60/12/9/1 in 1973. The first compound to become limiting was silicate. During the first part of the experiment, ammonia uptake was low (4 %) com­ pared with the great amount of nitrate assimilated. At the end of the experiment, the ammonia produced by zooplankton was very high and its assimilation by the phytoplankton then became very important (40 % of the nitrate uptake). When compared with data for Peru, where zoo­ plankton produce 0-018 to 0-480 mgat NH4/m2/d, the authors show that in this area of Mauritania, zooplank­ ton are more abundant and excrete 0-35 to 24-5 mgat NH4/m2/d. Some of this may be compensated by fish excretion (anchovy) which should be more predomi-

Figure 142. Chi a in ng/1 vs 0 2 production in ml/1 (from Minas et al., 1978).

166 CAP TI MIR IS

,22

26 .27

29 NOU KCHO

32

Figure 144. Trajectory of the drogue in the 1972 experiment described by Herbland et al. (1973).

nant in Peru. According to more recent studies (Le considered, while in reality, during the drift of the dro­ Borgne, 1978) on the high ammonium formation in this gue, a vertical component of water movement remains, area “it is suggested that much of the ammonium is which leads to underestimates of production due to a produced from phytoplankton decay or nitrogen excre­ dilution effect by rich upwelling waters. Such an exper­ tion during the bloom”. It is worth noting how much iment is not possible under the conditions in the Cape the second experiment confirms the first results. This Blanc-Cape Corveiro area. Mixing takes place very emphasizes the relatively small annual variations off rapidly (Gostan et al., 1976) and this must be taken Northwest Africa when compared with the changing into account (Minas et al., 1978). annual patterns in Peru (see Codispoti et al., 1982). In the Cape Timiris area, Herbland and Voituriez Some variations in the ratios AC/AN/ASi/AP are due (1974) have found an important fraction of atmos­ to weaknesses in the estimates, as pointed out by the pheric oxygen in the undersaturated water mass which authors. The ideal case of one water mass has been remains untouched for a long time (constant salinity).

167 Table 22. Production rates and assimilation numbers in the Cape Timiris-Nouakchott area.

March-April 1971 March 1973 CINECA-Charcot II Capricorne Groupe Médiprod (1974) Herbland et Voituriez (1974)

mg C/m’/d (max) 1 mg C/nr/h mg Chi a/m3 12 mg Chi a/m2 A N = AN (mg C/mg Chi alh) AN (mg C/mg Chi a/h) 1-86 AN, -0-13 Station no. assimilation number Station no. integrated assimilation number

48 279/2-55 x 1/12 = 9.1 36 163-0/30-0 = 5-43 9-7 49 241/4-02 x 1/12 = 5-0 39 122-0/27-7 = 4-11 7-5 44 62-4/0-80 x 1/12 = 6-5 40 160-0/48-2 = 3-32 6-0 43 176-7/(0-52?) x 1/12 = (28-3)? 42 134-0/64-8 = 2-06 3-7 42 160-4/3-12 x 1/12 = 4-3 43 132-7/98-5 = 1-35 2-4 35 105-4/2-10 x 1/12 = 4-2 45 163-0/82-3 = 1-98 3-6 46 161 -2/183-5 - 0-88 1-5 48 312-7/251-2 = 1-25 2-2 49 200-3/316-3 = 0-63 1-0 51 245-7/214-7 = 1-14 2-0 52 376-4/261-4 = 1-44 2-5

On the other hand, oxygen concentrations toward the ricorne” confirm each other. The “Charcot” has also end of the water-mass evolution in the Nouakchott recorded high values in the same area south of Nouak­ area become extremely high and can reach 190 % of chott (Chi a > 15 [xg/1 and production > 600 mg Cl saturation. It is not surprising that biomasses corres­ m3/d, underway mapping no. 15, Groupe Médiprod, ponding to such oversaturations could also reach high 1974b). It should be noted that the underway mapping values (maximum chlorophyll concentrations beyond no. 9 (Groupe Médiprod, 1974b) cited by Walsh (1976) 30 (j.g/1, according to Herbland et al., 1973). did not reach southward enough to record higher The issue of high values (two sporadic values reach values. 40 to 50 (xg/1 in 1972) has been discussed by Walsh Table 22 gives daily production rates and assimilation (1976). The results from the “Charcot” and the “Cap­ numbers obtained from “Capricorne” and “Charcot”.

40

19.0 17.0 16.0 17.0 -

15.0 .15.0 . 14.0 '

.11.0

.10.0

9.0 .

8.0

7.0 TEMPERATURE

Figure 145. Temperature (°C) section off Nouakchott taken in April 1971 (from Minas et al., 1974).

168 20* W 15* 'Canaries

Cabo Cabo Bojador Bojador 25*N 25*N

Cabo Barbas Cabo Barbas

Cap Blanc Cap Blanc

Cap Timiris Cap Timiris

S Nouakchott Nouakchott

-100 m — 300m I5*N 15* N Cap Ver' .Cap Vert, 20*W 15* 20* W 15* Figure 146. Currents at 100 and 300 m as given by Shaffer (1976).

The most striking characteristic, which differentiates ing et al., 1973; Gardner, 1977; Tomczak, 1978b; Figs. this upwelling from those off Cape Blanc or off 121 and 145) show an offshore doming structure which Morocco, is that nutrient levels remain high at the sur­ leads us to think about a divergence caused by a cyc­ face up to 120 miles offshore. Horizontal and vertical lonic current. This is confirmed by the current map distribution maps at the latitude of Nouakchott (Nehr- given by Herbland et al. (1973) and Shaffer (1976; Fig.

St 42 43 48 49 50 51 52 53 5 4 55 56

xxxx 1% de penetration de la lumière Limile de la couche à forte activité bactérienne (lygC/m ^/h)

Figure 147. Ammonia in (j.gat/1 in a section extending offshore from Nouakchott (from Herbland et al., 1973).

169 146). Chlorophyll levels are generally low and surface to the Atlantic Ocean, after extensive dispersal takes primary production can be extremely low in spite of the place. Thus, a fundamental difference exists between presence of nutrients (NO3 ~ 10 ugat/1). These obser­ both systems (north and south), and the consequences vations could suggest a lack of biological conditioning regarding higher trophic levels remain the major prob­ (Barber and Ryther, 1969; Barber et al., 1971; Groupe lem to be solved. Médiprod, 1974a). However, relatively high levels of ammonia in the subsurface (Fig. 147) suggest an active zooplankton metabolism that should be associated with relatively high concentrations of biological condition­ Banc d’Arguin area (~ 19-5° to 21° N) ing agents. Another explanation could be zooplankton grazing No direct study has been carried out on the production pressure that may maintain a low chlorophyll level and of the Banc d’Arguin area. But some physical also provide appreciable amounts of ammonia. The phenomena and chemical properties of its western studies of Packard and Blasco (1974) on the relation­ boundary are very well known. Mittelstaedt (1974) has ships between ammonia and nitrate reductase (inhibi­ shown a cyclonic circulation pattern affecting the Banc tion for NH4+ > 015 (xgat/1) which have been carried d’Arguin waters. It is driven by a strong surface current out in this region favor such an hypothesis. skirting the bank in the N-S direction (Mittelstaedt, The main question is the following: is the upwelling 1976). This type of circulation helps the upwelled in the Cape Timiris-Nouakchott area, including the waters to “climb” on the bank where they are subject offshore zone, more productive than the one in the to a strong evaporation leading to the formation of very Cape Corveiro-Cape Blanc area, for the same distance saline and warm water. The density of these “light in the offshore direction? Relative to the shore length green or brownish bank waters” is high enough (see unit, the southern region contains in its euphotic zone a “B ” water in Fig. 122; T > 16°C, S > 36-3 %c) for them nutrient supply going far offshore, which implies a to sink and interleaf with other waters, down to 100 to potential production considerably higher than its 150 m (Mittelstaedt, 1974). Figure 148, taken from northern counterpart. However, the actual production Shaffer (1976), clearly shows such cascading of the in portions of this area seems inhibited, and it is only saline bank water which, in spreading, leads to a well- still more offshore that this potential becomes a benefit marked thermal inversion. The description of these

146 MS 144 143 142

.3 5 7 0 -

100

200

■3 5 4 0 3 0 0 o Section 4

4 0 0 S %o 333 0 . March 5 -6 , 1972 5 0 0

30 25 20 (5 10 5 kmo

Figure 148. Salinity distribution indicating the cascading of waters off the Banc d’Arguin (from Shaffer, 1976).

170 Figure 149. Block diagram giving a general idea of the movement of the waters that cascade off the Banc d’Arguin (Peters, 1976).

diving waters going deeper in the southern region is and skirting the continental slope has been detected. presented by Peters (1976), Figure 149. Being incorpo­ The area extending from Dunford Point to Cape rated at several levels in the undercurrent, the influ­ Bojador has the widest (90 miles off Punta Leven) and ence of these waters can remain visible even north of the shallowest continental shelf of the whole CINECA Cape Blanc, where they are subject to surfacing with area. This region has been surveyed during several the ascending movement. It is conceivable that such a Spanish cruises (Sahara I, Atlor I, II, III), and has “bank water’7 type, rich in organics, could act as a been analysed by Cruzado (1974 and 1975). These biological conditioning agent and influence the source works describe the physical, chemical, and biological waters, along the Banc d’Arguin, as well as in the Cape features of the upwelling. T-S characteristics are essen­ Blanc-Cape Corveiro area. Such mixing between up­ tially those of NACW waters, as proved by the nutrient welling waters and bank waters, which could be benefi­ concentrations’ being lower than in the south. Cruzado cial to production, could be at the origin of the high indicates a peculiar current pattern on the continental phytoplankton biomass belt (Chi a > 20 (xg/1) found shelf which takes place most frequently during the fall along the Banc d’Arguin (Groupe Médiprod, 1974a,b) and in the absence of local winds. The upwelling waters and in the immediate surroundings (Voituriez, 1974; reach the continental shelf from the north and surface Weichart, 1974; Gillbrjcht, 1977). against the Pena Grande coast, due to the shallowing of the shelf; from there, the waters flow toward the south­ west and meet warm waters, thus creating a pro­ nounced thermal front. This water mass, on the conti­ Cape Barbas to Cape Juby area (~ 22° to 28° N) nental shelf, reminds one of the cold-water “cushion” This zone exhibits features intermediate between the encountered by Tréguer and Le Corre (1979) in the northern system (Cape Ghir-Cape Dra) and the Cape Morocco area. However, it is not known whether this Blanc-Cape Corveiro system, without forming a sepa­ water mass contains an extra supply of locally recycled rate entity. Less studied than the preceding zones, the nutrients. This current pattern represents a special case shore portion has been surveyed during cruises and relative to those encountered on other continental some publications have already been cited (Jones and shelves, but its influence on production, ecosystem Folkard, 1970; Hughes and Barton, 1974). Various structure, and dynamics is not yet known. So far, the physical aspects have been detailed by Hughes and only significant productivity data that appear to be Barton (1974); according to these authors, the Cape available are the 14C incubator measurements of sur­ Bojador upwelling is stronger in July-August (1972) face waters carried out by Velasquez and Cruzado than that off Cape Blanc; the undercurrent going north (1974).

171 30' to ' 10' IO*W SO' 40' so' to ' 10 ' IO*W BO' 40' Figure 150. Surface temperature distribution between Cape Sim and Agadir, during two different periods in September 1972 (from Grail et al., 1974).

near the coast is between 180 mg C /m2/d (start of The Morocco area from Cape Juby to Gibraltar cruise) and 1720 mg C /m2/d (end of cruise). In March, upwelling becomes stronger and leads to slightly higher (~ 28° to 36° N) production values. According to the seasonal study of hydrology by Fur- Hydrological mapping near the coast (Groupe nestin (1959) in the area from Gibraltar to the Canary Médiprod, 1974b) has outlined the upwelling activity Strait, upwelling is more active in spring and summer from Cape Sim southwards. Temperature minima (especially summer) than in winter. Three zones of occur near Cape Tafelney and especially Cape Ghir intensified upwelling can be outlined: Cape Spartel, (14-98° C). Hydrological transects (Minas et al., 1974; Cape Blanc (El Jadida) to Cape Ghir, and the south­ Groupe Médiprod, 1974b) indicate that the upwelling ern area from the Massa Oued to Puerto Cansado waters rise from about 200 m (150 to 175 m in winter (28-2°N). The better part of our knowledge of produc­ and ~ 250 m in summer) according to Le Floch (1974). tion and upwelling comes from some “Charcot” cruises One of the main features of this upwelling phenome­ (Grail et al., 1974; Groupe Médiprod, 1974b; Minas et non during winter is the extent of the mixed layer al., 1974; Le Corre et Tréguer, 1976; Thiriot, 1976; (100 m). Huber et al., 1977; Tréguer et Le Corre, 1979). More From a few stations where measurements were car­ recent hydrological and chemical observations could ried out (end of March 1971), primary production not be taken into consideration. ranges from 0-27 to 2-4 g C/m2/d inshore, with a value The winter situation in this region has been discussed of 1-6 g C/m2/d at 36 miles offshore. by Le Floch (1974). From January to February 1971 One of the features of the summer Moroccan upwell­ (CINECA-Charcot I) the upwelling is weak from the ing is that it is locally intensified; in certain areas hydrological point of view and from a photosynthetic upwelling is strong, while in other regions, it becomes point of view: surface chlorophyll concentrations range less important than in winter (e.g., Cape Ghir). Fol­ between 0.14 and 1-75 ng/1, and primary production lowing Cape Sim, the region of Cape Tafelney and its

172 32“

C.S CS es

CT C.T

/ . 'C r / C G C.G CG

^ JAgadir Agadir

CD C.D "CD 0 m 10 m 3 0 m

ln°w io°w

Figure 151. Horizontal distribution of nitrate in p.gat/1 at 0,10, and 30 m near Agadir in July-August 1972 (from Grall et al., 1974).

south-southwestern continental shelf seems to be influ­ are much higher than those prevailing at the original enced by upwelled waters (Fig. 150). Between Cape depth. In comparing the nutrient values and the oxygen Ghir and the cold upwelling plume originating from in the “cushion” with those present offshore at depth, Cape Tafelney, a poleward coastal current transports Le Corre and Tréguer (1976) have computed the warm waters that come from the Agadir coastal area, a amount of regenerated nutrients, as well as the oxygen zone that is affected by upwelling only at depth. Figure consumed. Figure 155 gives an example of a regenera­ 151 shows that at 30 m, the upwelling is clearly outlined tion pocket (silicates). by the horizontal nitrate distribution. The summer situ­ In general, the regeneration becomes obvious in the ation may well repeat itself annually, because similar Morocco summer upwelling, even at greater depths. values were observed during two different years. The Figure 156 shows, at the 200 m depth, an increase in circulation, which has not been studied in as much nitrates, which reach a maximum in the immediate detail as that in the Cape Corveiro area, except for proximity of the slope. some measurements by Le Saos and Talarmin (1976), The winds are notably higher in the north than in the seems to be influenced by the shape of the continental south. A time-series station (August 1974) near the 50 shelf, and the shore topography. According to Le m isobath off Cape Tafelney illustrates the wind action Corre and Tréguer (1976), the circulation belongs to on the upwelling. A 24 h diel variation in wind action the one-cell type, although a few horizontal distribu­ (30 knots at the end of the day, 10 knots at the end of tion maps (Fig. 152) show an upwelling effect off Cape the night) is accompanied, with a few hours' lag, by the Ghir near the 200 m isobath. appearance of nitrates at the surface. The weaker The vertical distributions in the northern Morocco winds in the southern area probably account for the area (Sim-Ghir) and the southern area between Ifni absence of direct surface upwelling of cold waters. and Cape Dra are different. In the south, the continen­ The planktonic biomass distribution exhibits rela­ tal shelf becomes wider and exhibits a cool water tively low values in the north (1 to 3 [ig Chi a/1; Fig. “cushion”, with hydrological features corresponding to 157) while in the south, plankton blooms develop those found offshore at 250-300 m (Figs. 153 and above regeneration pockets and reach 14 ug Chi a/1. 154a,b). The nutrient concentrations of this “cushion” The hydrological analysis shows that the rich waters are

173 2 0 19

30°N

Figure 152. From left to right, „ . surface temperature . (°C), . nitrate (ngat/1), and Chi a ((ig/1) in the Moroccan upwelling region during summer 1973 (from Le Corre and Tréguer, 1976; and Grail et al., 1978).

old upwelled waters that have warmed. The question in the south (4-22 g C/m2/d, Cape Tafelney; and 4-03 remains whether these come from the northern upwell­ g C/m2/d, south of Sidi Ifni), according to Grail et al. ing or from the cold cushion. (1978). Fig. 157 shows the distribution of chlorophyll Production reaches high values in the north as well as and of relative primary production rates during the summer in this principal Moroccan upwelling zone. Although recycling seems to play a very important role in the upwelling off Morocco, the influence of ammonia could not be studied as intensively as off Cape Corveiro. Nevertheless, its distribution relative to the wind regime is worth comparing with that of k. SIM other CINECA areas. Offshore of Cape Tafelney, with low winds, the ammonia forms a subsurface layer, with a maximum below the euphotic zone, 40 to 50 m (Figs. AGADIR 158 and 159). During high winds (30 knots), the inter­

30° mediate maximum tends to disappear owing to homo­ genizing, and higher values appear at the surface. With

SID I IFNI low winds, phytoplankton assimilation probably helps to keep the ammonia concentration at a low level in the 29» surface layer. This seems analogous to some of the features observed in the Cape Corveiro area. 10° Perhaps the reader will find it useful to consult Table 23 (modified from Le Corre and Tréguer, 1976), which Figure 153. Location of the hydrographic stations correspond­ ing to the distribution of parameters shown in Figure 154 summarizes many of the inter-regional differences (from Le Corre and Tréguer, 1976, modified). described above.

174 0 m

14.Î

100

200

4.8

300 m l / l 300-

W 4.2

0m 12 13 wy m Figure 154b. Temperature section off Cape Dra taken during summer 1972 (from Le Corre and Tréguer, 1976).

corresponds roughly to the ratio in NACW waters. This leads them to think that, in general, in Morocco, the nitrogen, phosphorus, and silicate supplies are exhausted simultaneously. A comparative study by the same authors of the variation of the AN/ASi ratio in the 7.0 Bay of Biscay and the offshore zone off Cape Blanc- Cape Corveiro shows that the silicon supply becomes exhausted for both regions while nitrogen and phos­ phorus concentrations are still high (Fig. 160). It is important to point out that in some cases off Cape -N03 pg-at/l Corveiro, and generally within inshore areas, silicate may not be limiting (Groupe Médiprod, 1976; Le Corre and Tréguer, 1976; Friederich and Codispoti, 1979). There is enough agreement among observations by several authors to claim that silicate is recycled 4 0 0 . rapidly over the shelf. Nelson and Goering (1977), Figure 154a. Oxygen and nitrate sections off Cape Dra taken using isotopic measurements (28Si), have shown that during summer 1972 (from Tréguer and Le Corre, 1979). the dissolution rate is important in the upper 50 m off Cape Corveiro, but their suggestion that dissolution might have actually exceeded uptake could be due to a bias introduced by their sampling locations (Friederich and Codispoti, 1979; 1982). According to Rowe et al. (1977), the nearshore sediments near Cape Corveiro Nutrient assimilation and regeneration also make a significant contribution to silicon cycling, ratios the Si/N/P ratios in the nutrients released from the sedi­ ments being 11/8/1 (by atoms). Friederich and Codis­ Tréguer and Le Corre (1979), using a model of the type poti (1979) have made a detailed analysis of dissolved proposed by Broenkow (1965), but perfected by using silicon regeneration over the shelf off Cape Corveiro. a mixing system with three water masses, have esti­ They show that the dissolved silicon regeneration (0-5 mated assimilation ratios in the Moroccan upwelling. (igat/l/d) in the upwelling water moving onto the shelf These do not significantly deviate from the classical exceeds the inorganic nitrogen regeneration rate (0-3 values of Redfield et al. (1963) for the AN/AP ratio. jigat/l/d) even though the initial dissolved Si/N03 ratio The AN/ASi is slightly greater than 2 (by atoms) which Cont’d on p. 179.

175 St 47 _ J}£ L Figure 155. Dissolved silicon distribution (|xgat/l) along a section off Cape Dra during summer 1972 (from Grail et al., 1974).

Figure 156. Regeneration of nitrate (Hgat/1) at the 200 m level in the region between Cape Sim and Cape Dra (from Tréguer and Le Corre, j 10 °w 1979). -200m

176 »af i

C.Sim

.C Ghir

I AGADIR

3000

Ifni

Lanzarote

12'.

C.Sim

31 N

(AGADIR

Ifni

Lanzarotø

Figure 157. Top: Chlorophyll a (lig/i) isolines at the surface. The size of the dots indicates relative primary production rates at the surface. Bottom: Chlorophyll a (mg/m2) integrated over the euphotic zone. The size of the dots indicates relative primary production rates integrated over the euphotic zone (from Grail et al., 1978).

12 Rapports et Procès-Verbaux 177 7 1 -1 7 1 -2 7 1 -3 7 1 -4 7 1 -5 Om /0 ,10 0,20 0,10

/0 ,30 0,40

0,50

0 ,0 4 opz 50m '0,50

0,40

0,00 ox>o

100-■

Figure 158. Distribution of ammonia (ngat/1) during moderate winds off Cape Tafelney (from Le Corre and Tréguer, 1976).

6 1 - 2 61-1 Om

0,10 0,20 0,30 P ,40

50-•

100

Figure 159. Distribution of ammonia (ngat/1) during strong winds off Cape Tafelney (from Le Corre and Tréguer, 1976). Table 23. Main characteristics of the most studied areas. Completion of a table established by Le Corre and Tréguer (1976), indicating an order of magnitude and the range of data taken from various sources.

Cape Timiris- Cape Blanc- Morocco (summer) Morocco (winter) Nouakchott Cape Corveiro Cape Sim, Cape Ghir Cape Sim, Cape Ghir

Wind (knots): 10-15 0-30 0-45 No data' “Source waters” T(°C) 14-0 15-5 14-5 15-0 NO , |xgat/l 20 14-15-5 a-9 (~ 6 )a NH4 ngat/1 < 0-3 0-2-0 0-0-5 P 0 4 ngat/1 1-5 0-9-1-0 0-6-0-7 (0-3-0-4)3 S i0 4 (igat/1 10 6-5-7-5 4-5 (2-3)“

Characteristics and fer­ tility of coastal waters Euphotic layer 32-12 m 15—21b m 18-25 m 19-66 m Chi a mg/m 1-5-20 1-9C 1-4C 0-15-1-75 Exceptionally 30-50 Primary production 1-6-4-3 0-8-5-0b 0-4-22 0-18-2-45 g C/nr/d 3-9 2-4 2-4 AN mg C/mg Chi a/h 1-0-9-7 2-4—10-9bf 4-2—15-7c,f 6-11-1

Phytoplankton species Thalassiosira rotula Coscinosira sp. Schroederella delicatula Schroederella sp. Rhizosolenia delicatula Nitzschia sp. Nitzschia “delicatissima” Dinoflagellate: Thalassiosira Prorocentrum sp. partheneia (less abundant)11 (Grail, 1978)

a Estimated values from diagrams of Le Corre and Tréguer (1976). b Values of Huntsman and Barber (1975). ' Values of “CC3”, “CC4”, and “Capricorne 7403” (Grail, personal communication). Jacques and Grail (1976). For a detailed analysis of phytoplankton communities, see Margalef (1978). c Huber et al. (1977) indicate a mean force of 5 Bft. AN has been obtained from AN, by using the relationship of Figure 139 and by dividing the daily production rate by 12.

in the upwelling source waters is < 1. In our view, Of all the CINECA areas, it is in Morocco that sub­ silicon limitation seems to be a function of geomor­ surface regeneration becomes the most apparent. phology and dynamics as well as the initial silicon con­ Therefore, it is well suited for a study of regeneration tent of the upwelling source waters. Silicon limitation processes. Tréguer and Le Corre (1979) discuss re­ does not appear to occur where topography and/or cur­ generation in the cold cushion found off south rent patterns favor the formation of silicon “traps” and Morocco. They found the following regeneration ratios the re-entry of the regenerated silicon in the ascending (by atoms): AN/AP = 11-3, AN/ASi = 0-95, and circulation. AP/ASi = 0-08; it turns out that phosphorus and silicon In the Mauritanian upwelling zone, between Cape are recycled with a ratio close to their concentration Timiris and Nouakchott, Herbland and Voituriez ratios in the offshore source waters, and both are re­ (1974) have shown that silicon is the first nutrient to be cycled faster than nitrogen. Ammonia is not taken into exhausted. Here, the upwelling source water exhibits account here, but the AN/AP regeneration ratios from an N/Si ratio of 2/1 (by atoms) and the assimilation the north Morocco area (Cape Sim to Cape Ghir) in­ ratio, AN/ASi, is only 1-4 (by atoms). Apparently, clude ammonia, and exhibit the same order of mag­ regeneration on this relatively narrow shelf cannot nitude (12). These results are in agreement with obser­ compensate for such a high demand. Thus this region vations by Jones (1971) in the Benguela Current appears to differ from some zones with wide shelves (AN/AP = 9). such as off Cape Dra (Le Corre and Tréguer, 1976) and The Moroccan upwelling illustrates very well that the Cape Corveiro (Friederich and Codispoti, 1978) where role of regeneration on the continental shelf is most regeneration compensates for the silicon deficiencies of important. A compound like silicon, which should in the upwelling source waters. Off Nouakchott where theory be limiting because it is present in low concen­ coastal upwelling interacts with the rising waters of the trations in the source waters, is frequently in excess in small “dome” that is found farther offshore (see those areas owing to the nature of the nutrient regener­ above), the rate of silicon regeneration is not very well ation processes. understood. This region deserves to be thoroughly The continental shelf topography also seems to play investigated. a most important role. In the South African upwelling

12* 179 O m N inorg Si inorg

A A

* a a A

150m

t * ..***. . *

^ a ’^ . - V . A A A a , * ^ **# * A> A/ * * •• * « ^ • * * 1 - Om . ? S*

26!6 27 ^ 2 8

Fig. 160. Inorganic nitrogen/dissolved silicon ratios vi ot. Stars, Bay of Biscay. Triangles, off Cape Blanc. Dots, Morocco. (From Tréguer and Le Corre, 1979).

region (such as Walvis Bay) where the shelf is very jected to several time-series stations, is the most pro­ wide, the work of Calvert and Price (1971), Jones ductive (12 months a year) and reaches, according to (1971), and Carmack and Aagaard (1977) has shown the authors, 365x0-59 g C/m2/d = 215 g C/m2/year. On regeneration ratios that are similar to those found off the continental shelves, production was estimated to be the wide portion of the Northwest African shelf, rela­ only 0-443 g C/m2/d for the Northwest African coasts. tively low for AN/AP and often high for ASi/AP. This value is somewhat lower than the 0-5 g C men­ tioned on the large-scale map by Koblentz-Mishke et al. (1970), which includes offshore areas where rela­ tively low production rates are to be expected. In addi­ tion, when the values of Schulz and Kaiser (1975) are Problems relative to the production compared with other results taken at similar locations and times of the year (Lloyd, 1971; Voituriez, 1974; budget in the CINECA areas Barber and Huntsman, 1975; and Minas, 1976), the The only large-scale study on the distribution of prim­ latter values are abolit four times higher. If the same ary production rates has been undertaken by Schulz discrepancy exists for all the areas, the mean daily pro­ and Kaiser (1975), who proposed production budgets duction for the Northwest African coast would be for the whole area surveyed by RV “Alexander von about 2 gC /m 2/d, which means 730 gC/m 2/year. Per­ Humboldt”, i.e. covering Cape Vega to Bahia de Gar­ haps the world production map of Koblentz-Mishke et net (Fig. 120). Using planimetry of production maps al. should be corrected on the basis of this result by and taking into account the number of months during including a narrow high-production band close to the which upwelling takes place, an estimate of annual pro­ coast. A major problem worthy of investigation is the duction has been proposed for the various coastal sec­ appropriate area of the high productivity upwelling sec­ tors. The area off Cape Blanc, which has been sub­ tor, i.e. its spreading offshore.

180 Acknowledgements mesoscale influences on nutrient variability in the North­ west African upwelling region near Cabo Corbeiro. Deep- This study would not have been possible without the Sea Res., 25: 751-770. Coste, B., Minas, H. J., Minas, M., Collos, Y., and Slawyk, data base that resulted from the efforts of the large G. 1975. Oxygen and ammonia distribution in the upwelling group of scientists and mariners who participated in the area off Cape Blanc (Results from the CINECA-Charcot V CINECA program. While it is impossible to thank Cruise). Comm. no. 11, third int. Symp. Upwelling Ecosys­ them individually, we want each and every one of them tems, Kiel, 25-28 August 1975. Cruzado, A. 1974. Coastal upwelling between Cape Bojador to know that we are deeply grateful for their efforts. and Point Durnford (Spanish Sahara). Téthys, 6: 133-142. We are also most appreciative of the assistance given Cruzado, A. 1975. Is wind stress the main driving force of to us during the preparation of this manuscript by M. coastal upwelling? Comm. no. 12, third int. Symp. Upwell­ Minas, M. C. Bonin, R. Cromoga, D. Doyle, N. ing Ecosystems, Kiel, 25-28 August 1975. Curl, H., and Small, L. F. 1965. Variations in photosynthesis McGary, S. Patterson, and D. Wisegarver. assimilation ratios in natural marine phytoplankton com­ Financial support was provided by the Government munities. Limnol. Oceanogr., 10 (suppl.): 67-74. of France under grant numbers CNRS RCP-247 and Dugdale, R. C. 1967. Nutrient limitation in the sea: dynamics, LA-41, by the National Science Foundation’s Interna­ identification and significance. Limnol. Oceanogr., 12: 685-695. tional Decade of Ocean Exploration Program under Dugdale, R. C., and Goering, J. J. 1967. Uptake of new and grants ID072-06422, OCE76-00136, OCE76-04825, regenerated forms of nitrogen in primary productivity. Lim­ and OCE77-27128, and by the Office of Naval nol. Oceanogr., 12: 196-206. Research under contract N-00014-76-C-0271. Estrada, M. 1974. Photosynthetic pigments and productivity in the upwelling region off NW Africa. Téthys, 6: 247-260. Fraga, F. 1973. Oceanografia quimica de la region de afloramiento del noroeste de Africa. I. Res. Exp. cient. B/O Cornide, 2: 13-52. Fraga, F. 1974. Distribution des masses d’eau dans l’upwelling de Mauritanie. Téthys, 6: 5-10. Fraga, F., and Manriquez, M. 1975. Oceanografia quimica de la region de afloramiento del noroeste de Africa. II. Cam- References pana “Atlor II” Marzo 1973. Res. Exp. dent. B/O Cornide, 4: 185-217. Anderson, J. J. 1973. Silicate water mass analysis off the N. Friederich, G. E., and Codispoti, L. A. 1979. On some factors W. coast of Africa. Res. Exp. cient. B/O Cornide, 2: 53-64. influencing dissolved silicon distribution over the North­ Anon. 1970. Data report R/V T. G. Thompson Cruise 36 west African shelf. J. mar. Res., 37: 337-353. (PISCO). Part I. Hydrography and productivity. Biological Friederich, G. E., and Codispoti, L. A. 1982. Some factors production in upwelling ecosystems, special rep. 42, Uni­ influencing dissolved silicon distribution over the North­ versity of Washington, 97 pp. west African shelf (abstract). (This volume). Barber, R. T., Dugdale, R. C., Maclsaac, J. J., and Smith, R. Furnestin, J. 1959. Hydrologie du Maroc atlantique. Rev. L. 1971. Variations in phytoplankton growth associated Trav. Inst. Pêches mar., 23: 5-78. with the source and conditioning of upwelling water. Inv. Gardner, D. 1977. Nutrients as tracers of water mass structure Pesq., 35: 171-189. in the coastal upwelling off northwest Africa. In A voyage Barber, R. T., and Huntsman, S. A. 1975. JOINT-I carbon, of discovery. Ed. by M. Angel. Pergamon Press. Oxford chlorophyll and light extinction. R/V Atlantis Cruise 82. and New York, 712 pp. CUEA Data rep. 14: 165 pp. Gillbricht, M. 1977. Phytoplankton distribution in the upwell­ Barber, R. 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