General biological features of the South Atlantic
Zooplankton communities 1

Dessier (1981) identified a number of distinct zooplankton communities over the Gabonese-Congolese-Angolan shelf from studies conducted over several years. These communities were generally associated with either different temperatures or hydrological regimes, and their presence in the region varied on a seasonal basis. In essence, the region is characterized by two quite different communities associated with either oceanic or neritic waters. The warm oceanic group is persistently present north of Cape Lopez and, during the warm season, across the region beyond the 100 m isobath. During summer it encroaches onto the shelf, south of Cape Lopez.

The structure of frontal neritic communities is more complex and reflects their diverse origin and the seasonal variability of their environment. The composition of these communities varies with changes in hydrology and latitude, and seems to accurately reflect warming and cooling events (Dessier, 1981). For example, austral summer assemblages of the inner shelf south of Cape Lopez may be supplemented by those of Guinean waters (tropical waters with low salinity), owing to the southwards movement of shelf water then. These warm waters push the winter populations offshore and neritic communities are found in the middle of the shelf or southwards. Highest biomasses are situated near the coast ( Gb23), due in part to the regression of the outer shelf species and the development of inner shelf communities dominated by asexually reproducing cladocerans and thaliaceans.

During the cold season, the neritic communities can be divided up into those of the inner (depth <50 m) or outer shelf region. Upwelling mainly occurs at the shelf break, and as the shelf is 25 nautical miles wide inner and outer communities can develop simultaneously, aided by probable coastal divergence. Although both regions obviously share a number of species, the numerical structure of the two neritic communities differs markedly. The winter communities may be augmented either by offshore subsurface species which have been advected into the region by water of Equatorial Undercurrent origin during June, or by coastal and upwelling species from the Benguela Current in August due to the northwards movement of the surface current then. The most thermophilic superficial species are driven offshore and north of the front at Cape Lopez. Other less thermophilic species are found in abundance just south of the front. On the outer shelf, they are mainly replaced by subsurface species from off the shelf break, of which only Calanoides carinatus and Eucalanus monachus become abundant owing to their particular maintenance strategies (Binet, 1979; Petit and Courties, 1976). A neritic winter community develops inshore and Paracalanus scotti and Oithona nana overwhelm the plankton numbers.