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.