General biological features of the South Atlantic
Zooplankton processes and communities

Zooplankton communities over the western Agulhas Bank have strong Benguela affinities (especially during summer) while those of the eastern area are characterised by coastal communities dominated by Calanus agulhensis (Verheye et al., 1994). Communities over the Agulhas Bank are distinct from those of the Agulhas Current (e.g. De Decker, 1984) and tend to comprise typically coastal species such as small calanoids and cladocerans. The communities in the Agulhas Current are typically tropical in structure and contain a preponderance of carnivores. The common herbivores are those which are capable of surviving in oligotrophic waters and which feed primarily on small cells. Their persistance outside these regions is thus unlikely given the altered structure of the food webs elsewhere. Having said that, the Agulhas Current may seed the shelf with oceanic species such as Thalia democratica during summer and these can develop to dominate assemblages over the central Bank where circulation is sluggish and waters oligotrophic (e.g. Gibbons, 1997a).

Zooplankton biomass is generally greater over the eastern Agulhas Bank (3.07 g dry wt mö-2) than the western Agulhas Bank (1.21 g dry wt mö-2), though whether this is due to differences in primary production associated with the cold ridge or to differences in fish predation is largely unknown. Fluctuations in zooplankton biomass over the western Agulhas Bank are strongly linked to fluctuations in phytoplankton standing stock and coastal upwelling (Hutchings and Nelson, 1985). Although zooplankton communities over the Agulhas Bank may consume in excess of 50% of daily primary production, the food chain efficiency is thought to be low (3%) and much of the primary production may move through microheterotrophs (Verheye et al., 1994).

Our understanding of the zooplankton in the Agulhas Current is confined largely to taxonomy and we remain ignorant of community processes. Zooplankton communities in the current are much more diverse than those of the surrounding water masses and species richness tends to decrease from east to west (Gibbons and Hutchings, 1996). Indicator species of the Agulhas Current have been identified amongst euphausiids (Talbot, 1974), and chaetognaths (Stone, 1969; Schleyer, 1977), as well as copepods (De Decker, 1984). These species have been detected in the nearshore waters of the Benguela system and at the front to Hondeklip Bay up the west coast (Gibbons et al., 1995). Their presence there must largely reflect the influence of surface filaments. Further north along the western seaboard of the African continent species attributed to the Agulhas Current have been observed by Ducret (1968), Frontier (1968) and Godeaux and Goffinet (1968). If these species really are of Agulhas Current origin then their presence so far north must reflect entrainment in the northward flowing shelf-edge current. It would also suggest that conditions at the front must be suitable for their continued survival, if not reproduction. Certainly feeding conditions in this region are thought to be good (Hutchings, 1992) and it does have a stable thermal structure (Barange, 1990) which may favour their persistence. Unlike conditions on the Agulhas Bank, the survival of Agulhas Current expatriates in the nearshore waters of the west coast is unlikely due to the variability of the nearshore environment. Records of Agulhas Current species have been made on the western side of the South Atlantic, especially in the tropical latitudes (Ponomareva, 1990). If valid, these records can only suggest passage by Agulhas rings, and this has major implications for our understanding of pelagic biogeography as a whole.