Rapp. P.-v. Réun. Cons. int. Explor. Mer, 180: 280-288. 1982. Some features of zooplankton distribution in the upper 200 m in the upwelling region off Northwest Africa H. Weikert Institut für Hydrobiologie und Fischereiwissenschaft der Universität Hamburg Zeiseweg 9, 2000 Hamburg 50, Bundesrepublik Deutschland In May 1968 significant quantitative differences existed in the distribution of biomass, abundance, and dominance relationships of zooplankton off Mauritania in the upper 200 m. South of Cape Blanc in the South Atlantic Central Water (SACW) more zooplankton was found than in the subregion north of Cap Blanc, where North Atlantic Central Water (NACW) predominated (132 no./m3, 92 mg wet weight/m3, and 72 no./m3, 56 mg wet weight/m3, respectively). In the upper 30 m of the 200 m depth range sampled, 65 % to 80 % of the invertebrates were collected. The de­ crease in plankton observed in a series of samples taken while travelling north was comparable to the marked decrease observed while moving from the shelf and slope into the open sea. The bulk of the plankton was contributed by the copepods (60 % north and 81 % south of Cape Blanc). Of this group four families were of ecological importance: Calanidae, Eucalanidae, Metridiidae and Temoridae; however, the abundant species of copepods were not limited to the waters off Cape Blanc. Differ­ ences in water quality associated with the NACW and SACW affected the relative and absolute abundances of dominant species. Off Mauritania composition and spatial distribution of the plankton showed an excellent correlation between the data obtained from the upper 200 m of the water column and those from the upper 10 cm surface layer (neuston). Further compari­ sons were made between neuston samples from off Mauritania and off Morocco during the winter and spring seasons of several years. From the results on the distribution of the dominant calanoid species it is concluded that, in these seasons, the copepod community between Gibraltar and Cape Timiris (Cape Mirik) is rela­ tively homogeneous as it is characteristic of the Canary Current System. Introduction Within the vast area of the Northwest African coastal distribution and taxonomic composition of the secon­ upwelling, the zooplankton of the region off Morocco dary producers, the zooplankton, is still fragmentary. was given special attention. Most of the principal infor­ In the Cape Verde region, the composition of zoo­ mation on the faunistic and biomass characteristics of plankton and its seasonal changes were studied by the zooplankton as well as on its relationship to the Gaudy and Seguin (1964) and Seguin (1966). Khromov environment was contributed by French investigators (1962; 1965) provided additional data on the biomass such as Furnestin (1957; 1959a, b, c; 1961; 1964), Goy distribution. Off Mauritania, some information is avail­ and Thiriot (1974) and Grail et al. (1974). able on the relative abundance of the main taxonomic In the late sixties, new investigations were begun, groups in the zooplankton (Wiktor, 1969) and the dis­ focusing on the neuston community (Weikert, 1972; tribution of its biomass (Pavlov, 1968; Casanova, 1973) and, in the course of the CINECA programme, 1974). Hargreaves (1978) was the first to present a on physiological processes of planktonic invertebrates coherent faunistic and numerical analysis of the zoo­ (Champalbert and Gaudy, 1972; Boucher and Samain, plankton and some of its principal groups between the 1974; 1975; Nival et al., 1974; Packard et al., 1974). Canary Islands and Cape Blanc by comparing stations Extensive sampling during expeditions by the Ger­ on and off the shelf. Weigmann-Haass (1976) did the man Democratic Republic (Postel, 1978, unpublished, same for the euphausiids off Cape Blanc, and Weikert in Schulz et al., 1978) has revealed that the waters off (1977) reported on mass mortality in the stocks of Mauritania (Cape Blanc) and Senegal (Cape Verde) dominant copepod species caused by upwelling events. contain a greater zooplankton biomass than do the The first part of the present paper deals with the northern parts of the upwelling area, i.e. off Morocco. vertical and horizontal distribution of zooplankton In spite of the importance of the southern regions as biomass and its composition in the 0-200 m layer and fishing grounds, the information on the quantitative the 0-10 cm microlayer off Mauritania. The influence 280 C. Barbas !2°N IC. Bi a n i » * Tim iri s Figure 205. Zooplankton stations (neuston, zooplankton) of RV “Meteor” on three upwelling expeditions. Open squares, 1977. Open triangles, 1972. Full circles, 1968. of the main regional water masses on the structures of March 1972 off Mauritania (between Cape Blanc and zooplankton is investigated, with special reference to Cape Timiris), the calanoid copepods. The material was collected by February 1973 off Morocco (between Gibraltar and RV “Meteor” in 1968 between Cape Barbas and Cape 30°N), Timiris (Fig. 205). In the second part, special emphasis March 1977 off Cape Blanc (21°30'N). is given to comparison of the region off Mauritania with the region off Morocco. The distribution patterns The plankton samples were taken on a station grid or of zooneuston biomass and species, including daily and along transects, extending from coastal (upwelled) seasonal changes, are presented. For this purpose, data waters to oceanic waters. In 1968, additional zooplank­ on the neuston were evaluated which were obtained ton stations were located in the oceanic region between during five cruises of RV “Meteor” in the aforemen­ Cape Barbas and Cape Bojador (25°N). The transects tioned regions during the winter and spring seasons of ran more or less parallel to the shelf edge. various years. The numbers of samples of the different cruises are distributed as follows: Material and methods Morocco, 1967 - neuston: 19 Morocco, 1973 - neuston: 43 The plankton samples were gathered in winter and late Mauritania, 1968 - neuston: 62, upper 200 m: spring of several years and in various main regions of 270 samples from investigation (see Figs. 205, 206): 45 stations Mauritania, 1972 - neuston: 23 June 1967 off Morocco (between Gibraltar and Mauritania, 1977 - neuston: 16. 30°N), May/June 1968 off Mauritania (between Cape Bar­ Most of the zooplankton was collected with a neus­ bas and Cape Timiris), ton net of 300 ßm mesh size (Hempel and Weikert, 281 6°W 35° ’Agadir 30° 30° 1 1 ° Figure 206. Zooplankton stations (neuston) of RV “Meteor” on two upwelling expeditions. Full circles, 1973. Open triangles, 1967. 1972) from the uppermost 10 cm of the water and the studied by several authors (Mittelstaedt, 1972; 1976; adjacent 13-38 cm layer. In 1968, additional vertical Meincke et al., 1975; Richert, 1975; Peters, 1976; Shaf­ tows were made off Mauritania with a Helgoland Lar­ fer, 1976; and Huber et al., 1977). Upwelling occurred vae Net (HLN) of 500 um mesh size, equipped with a regularly off Mauritania, where in May 1968 two sub- changing bucket device (Kinzer and Hempel, 1970). regions could be clearly distinguished which were The water column was sampled in five 30 m intervals designated according to the two predominant water from 150 m to the surface; an additional sample was masses, the North Atlantic Central Water (NACW) taken at a depth of 200-150 m. At a few stations, the and the South Atlantic Central Water (SACW). The top 30 m were subsampled at 10 m intervals. boundary between the two was located in the vicinity of Each sample or subsample was preserved in buffered Cape Blanc. The upwelled SACW was characterized 4 % formaldehyde and sorted into major taxonomic by lower temperatures and salinities, but higher nu­ categories, but radiolarians, coelenterates, tunicates, trient concentrations, than the NACW (Weichart, and fishes were excluded from numerical analysis. 1974). No definite indications of upwelling were found Some lower categories, including certain families, gen­ off Morocco in winter 1973 nor in late spring 1967. era, and species of various invertebrates, especially copepods, were given special attention. The total plankton biomass (wet weight), including a small number of radiolarians, tunicates, and fish larvae, was Results determined from the vertical tows with the HLN using The zooplankton off Mauritania in May/June the method described by Tranter (1962). No weight 1968 data were taken from the neuston samples, since these contained large numbers of phytoplankton and radio­ Zooplankton standing stock larians, especially in the regions of upwelling. In May 1968, the presence of the NACW and SACW caused qualitative and quantitative differences in the plankton distribution in the region off Mauritania. Chlorophyll values were higher in the subregion south Hydrography of Cape Blanc than in its northern counterpart The hydrography in the regions off Morocco and (Weichart, 1974; Gillbricht, 1977). Mauritania during the periods of investigation has been Like the phytoplankton, the zooplankton showed 282 Table 53. Zooplankton (0—200 m) and the most important varied between 15 and 25 m. Here also the highest copepod families in the subregions north and south of Cape Blanc in 1968. Mean values for relative abundances of cope­ chlorophyll concentrations were recorded (Weichart, pods are derived from relative values for each sample: total 1974). In the southern subregion, the bottom of the copepods relative to total zooplankton; copepod families and euphotic zone was strongly shifted upward (to about 15 species relative to total copepods (=100 %). m), and about 80 % of the plankton within the consi­ dered 200 m depth range was collected in the upper 30 North of South of Cape Blanc Cape Blanc m (717 no./m3). This proportion corresponds to 75 % (C. Barbas - (C. Blanc - in terms of biomass (485 mg/m3). In the northern C. Blanc) C. Timiris) subregion, the euphotic layer had moved downward to No./m3 Abun­ No./m3 Abun­ some extent (to about 25 m), and no definite dance dance (%) (% ) chlorophyll peak was formed (Weichart, 1974).