General biological features of the South Atlantic
Phytoplankton

Phytoplankton biomass and productivity in the Benguela region are enhanced by the wind-driven upwelling over the shelf. Brown et al. (1991) and Brown (1992) have described the distribution and productivity of phytoplankton on the west coast, while Probyn et al. (1994) describe primary production and related processes on the Agulhas Bank. Highest chlorophyll a concentrations (5-10 mg mö-3) occur close to the coast, associated with upwelling centres, with declining concentrations to less than 1 mg mö-3 offshore in the southern Benguela, but still greater than 2 mg mö-3 beyond the 200 m isobath off Namibia, indicating a wider productive zone in the northern Benguela. Diatoms and dinoflagellates dominate the biomass near upwelling centres, while small flagellates and other nanoplanktonic organisms dominate in aged upwelled waters. Clear successional changes occur as upwelled waters stabilize, mix with surrounding waters and mature ( Gb27) (Mitchell-Innes and Walker, 1991; Pitcher, 1988; Pitcher et al., 1991).

Red tides are common throughout the Benguela Current and have caused numerous mortalities in sheltered bays when aggregations of toxic species develop or when oxygen is depleted during the decay of the bloom. In the southern Benguela there are distinct seasonal patterns in the development of dinoflagellate populations, with dense aggregations occurring whenever physical conditions are favorable (Pitcher et al., 1995; Pitcher and Boyd, 1996). The affect of these red-tides on zooplankton communities is unknown.

Primary production measurements are relatively rare in the Benguela region, being limited to a small region around the SW Cape where more than 10 measurements have been made within a limited area. Elsewhere only 1-2 measurements have been documented in the literature, and are certainly inadequate to describe seasonal, areal and interannual variations in primary productivity in the Benguela region. The measurements that have been made indicate that the southern Benguela is more productive (2.0 g C mö-2 dö-1) than the northern Benguela (1.2 g C mö-2 dö-1), despite the higher mean biomass of phytoplankton in the north (4.8 vs 2.2 mg chlorophyll a mö-3, respectively). On the south coast, east of Cape Point, biomass is lower (1.5 mg chlorophyll a mö-3), and productivity is intermediate (1.8 g C mö-2 dö-1), compared with the west coast. The high biomass, long turnover time and low productivity off Namibia is somewhat surprising, but as these data are based on very few measurements future research may yield different results. Seasonal changes in productivity were reported by Brown et al. (1991) for a very limited area in the southern Benguela. Nitrogen uptake experiments (Probyn 1985, 1987, 1992) indicate that much of the production on the continental shelf, downstream from the upwelling centres, is fuelled by recycled nutrients from the microbial foodweb, rather than by “new” nutrients advected from elsewhere. The high organic content of the shelf sediments indicates considerable deposition of carbon. Part of the organic matter is recycled into the upwelling source water and part is sequestered in the sediments, resulting in the Benguela system being both a source and a sink for carbon dioxide (Hutchings et al., 1995). Grazing control of the initial blooms in newly upwelled water is relatively poor in the southern Benguela, due in part to the poor spatial and temporal matching between phytoplankton and mesozooplankton in the strongly pulsed upwelling regime (Verheye, 1991; Hutchings et al., 1995).