General biological features of the South Atlantic
The tropical and subtropical eastern South Atlantic

By comparison with the western South Atlantic, dedicated biogeographic studies of the eastern region are far and few between and mostly of a patchy and taxa-specific nature. For example, Thiede (1975), Robson (1983), Boltovskoy et al. (1995), have examined the distribution of some shell- and skeleton-producing microplankton in the tropical and subtropical east Atlantic; Gibbons et al. (1995) have looked at the biogeography of euphausiids around southern Africa, and Pakhomov et al. (1994a) have studied communities in the region between the Subtropical Convergence and the Polar Front. However, fully synoptic studies of multi-taxa communities are largely absent and therefore discussions of biogeography have to be chiefly deductive.

There is quite a bit of evidence to suggest that neritic zooplankton communities in this area are distinct from oceanic ones (Dessier, 1981; Barange et al., 1992; Gibbons et al., 1995) although biogeographic affinities amongst them are less clear. Certainly there are three assemblages along the African coastline associated with the Gulf of Guinea (Bainbridge, 1972; Binet, 1993), the Benguela Current and the Agulhas Current, respectively (Gb1), and within each there are some permanent and seasonal boundaries to "sub-communities". The vicinity of the African continent affects the hydrology and the climate (circulation, coastal upwellings, river run-off) and imparts a strong seasonal signal over the shelf. At the coastal station off Pointe-Noire (4°47’S; see (Gb17), the mean seasonal variation of surface temperature reaches 15°C, while 150 nautical miles offshore it is only 7°C, and 3°C at 50 m (Dessier, 1981). Except in the equatorial belt, the seasonal variability becomes attenuated offshore. Oceanic provinces seem less clearly associated with water mass type, although there are obvious latitudinal, thermal associations with the Polar Front and the Subtropical Convergence.

Unlike the western South Atlantic, where the South American continent essentially extends from the equator to the pole and strongly limits the movement of water from the Pacific Ocean Basin into the Atlantic, the eastern boundary is open to the Indian Ocean south of Africa. Thus, superimposed upon the north-south provincial make of the region are patterns induced by contact with Indian Ocean water. Given the general west-east flow of water at higher latitudes, the effects of this contact would be negligible were it not for the fact that Indian Ocean water is actually "fed" into the South Atlantic via the fast flowing Agulhas Current. Although strictly speaking, the Agulhas Current retroflects to the east around 38°S, 15°E (Hutchings and Boyd, 1992; see (Gb17), its water enters the region via eddies (which move to the south) and rings (which move lazily across the entire region from the SE to the NW), and it also gets caught up in the northward flowing frontal jet system of the Benguela ecosystem. Because subtropical Agulhas Current waters enter the South Atlantic in the transition zone between the Southern Ocean and the subtropical South Atlantic, the transition zone itself is exaggerated, and communities there are far more species rich than those at lower latitudes. The northward extent of the transition zone in the eastern South Atlantic is likely to be far greater than that on the west owing to the strong northward flow in both the coastal and oceanic areas. Indeed, Agulhas Current species have been recorded off the Gabonese-Congolese-Angolan shelf (roughly 0 to 15°S; Ducret, 1968; probably caught up in the shelf edge jet), as well as in waters off northern Brazil (Ponomareva, 1990).

For the sake of convenience we have divided the coastal eastern Atlantic into two areas around the Angola-Benguela front (at 16-20°S, also called the Cape Frio front, see (Gb17). North of the Angola-Benguela front (Gb26), along the Gabonese and Congolese coast, the physical and biological processes are markedly seasonal, as they are too in the region of the southern Benguela south of Luderitz (ca. 25°S). By contrast, more or less perennial conditions exist over the Namibian shelf, where coastal upwelling is quite permanent. While these divisions are largely artificial because of considerable interaction between them (especially south of 20°S) they conform to the biogeochemical provinces of the region proposed by Longhurst (1995; see (Gb1). Having said that, a lack of congruence between the biogeography and biogeochemistry has been observed for the coastal domains off Africa (Gibbons, 1997b) but future studies will undoubtedly change this.