General biological features of the South Atlantic
Zooplankton communities 2

As with the coastal regions to the north of 20°S, there is a very pronounced frontal boundary between neritic and oceanic communities that approximates the edge of the continental shelf (Barange et al., 1992). Communities on the shelf across the region are characterized by low diversity and high abundance and contain species which are typical of upwelling areas or cold-temperate shelf waters world-wide (Gibbons and Hutchings, 1996). These include Calanoides carinatus, Euphausia lucens, Sagitta tasmanica, Muggiaea atlantica and Themisto gaudichaudii. Although the frontal region is generally composed of a mixture of neritic and oceanic species, it may contain species which reach peak abundances here, such as Euphausia recurva (Pillar et al., 1991; Barange, 1990). The frontal boundary may be intensified during the upwelling season (austral summer) in the south when Agulhas Bank and Agulhas Current water gets entrained in the northward flowing shelf-edge jet. Although perennial, the front may also become blurred at times, especially in areas where the shelf is narrow (e.g., off capes Columbine and Point), and where as a consequence frontal and oceanic water is advected shoreward during wind reversals. Under these circumstances, oceanic species may be introduced into the coastal area. While shelf communities south of Cape Columbine contain many of the characteristic neritic species of the region, they are extremely labile, and changes in their composition reflect the dynamic oceanography of this region. Although species richness in this area can be extremely high (De Decker, 1984; Gibbons and Hutchings, 1996), the communities are largely structured by physical processes and the region (to Hondeklip Bay, ca. 31°S) must be considered a transition zone. Just as the southern boundary to the Benguela ecosystem lies inside the physically defined Benguela region (Gibbons et al., 1995), so too does the northern boundary with the Angola Current (Barange et al., 1992). Both are essentially described by the extremes of advection of Agulhas Current and Angola Current water respectively. Shelf communities to the north of Cape Columbine (to the Angola-Benguela front) generally display a homogeneity in species composition, although cross shelf gradients that reflect mixing or community responses to the ageing of upwelled water are observed (Gibbons and Hutchings, 1996). There are, however, subtle differences in the numeric composition of zooplankton communities to either side of the perennial upwelling cell at Luderitz (Barange et al., 1992; Gibbons et al., 1995). It is clear that while the upwelling cell may not provide a permanent barrier to the longshore transport of holozooplankton from south to north (but see Emmanuel et al., 1992 for intertidal communities) there are differences in the seasonality of upwelling (Shannon, 1985), production and biomass (Brown et al., 1991; Pitcher et al., 1992), as well as differences in the circulation of water across the shelf systems (Barange and Pillar, 1992) that are reflected by the zooplankton communities.

Of particular interest here is the presence in the region of species which also occur in the transitional, south subtropical convergence ( Gb1), such as Euphausia lucens and Sagitta tasmanica. These species are thought of as characteristic zooplankters in the Benguela upwelling region and dominate their respective assemblages. It is thought that these species seed into the system from the south, presumably from South Atlantic Central Water, and undergoe population explosions in the productive waters of the South African west coast (Gibbons and Hutchings, 1996). The eventual fate of these populations is unknown. It is possible that these populations are self-perpetuating within the nearshore system (they have maintenance strategies which would favour this) but they could also be continullay lost to entrainment in the South Atlantic gyre in the north and replenished from the south. But whatever the case, these species which occur at low densities outside the Benguela ecosystem, thrive within it.