General biological features of the South Atlantic
Phytoplankton

Our understanding of phytoplankton communities and processes in the oligotrophic Agulhas Current is very limited. Rates of primary production are low (99 mg C mö-2 hö-1) by comparison with those on the Agulhas Bank itself (Probyn et al., 1994) and biomass is concentrated at the thermocline. The influence of shelf-edge upwelling on production and biomass is of greatest significance over the eastern Agulhas Bank where it results in a doming of the isotherms over the outer shelf and leads to high subsurface chlorophyll maxima.

Chlorophyll biomass and production over the Agulhas Bank is strongly linked to the structure of the water column, the depth and intensity of the thermocline and to the forcing processes. Wind mixing in winter results in low production over the whole Agulhas Bank and chlorophyll biomass is distributed throughout the upper mixed layer (generally greater than 70 m but between 30-40 m in the east) (Gb32 D). On the western Agulhas Bank, the thermoclines may be very deep in winter because of large amplitude internal waves and tidal stirring (Largier and Swart, 1987). As the water column begins to stabilise in spring surface blooms develop which gradually deepen and decrease with stratification during summer. Chlorophyll biomass is generally shallower and higher in the east than the west (Probyn et al., 1994) because of the influence of the cold ridge and the strong, shallow thermocline induced by the shelf-edge upwelling there (Gb32 ).

Both phytoplankton biomass and production are higher inshore (Gb32 C) and downstream from the upwelling foci (downwind of the prominent capes) than elsewhere on the Agulhas Bank and are lowest in the central regions where water circulation is sluggish. The phytoplankton communities influenced by coastal upwelling are dominated by large cells (as in the Benguela system) but these are thought to give way to smaller cells elsewhere. Those of the Agulhas Current are likely to be dominated by small cells and are thus unlikely to contribute much in either the way of biomass or production to the west coast (via filaments).