General biological features of the South Atlantic
The Cape Frio thermohaline front

South of the area, in the vicinity of Cape Frio, the intensity of the southward flowing Angola Current is seasonal and its convergent boundary with the Benguela Current fluctuates between 16-20°S (sometimes to 22°S) (Dias, 1983; Shannon and Agenbag, 1987; Shannon et al., 1987; Fig 26). The more northerly position of the front occurs during the cold period when upwelling over the Namibian shelf is strongest and northward advection greatest. By contrast, it is furthest south during the warm period when southerly winds are weakest and the South Atlantic Anticyclone is furthest south. Thermal and salinity fronts between these two systems do not coincide because the maximum of salinity is not in the cold waters upwelled from the Benguela Current, but in the warm waters from the Equatorial Undercurrent, South Equatorial Countercurrent and South Equatorial Undercurrent. There is a strong suggestion that two thermal fronts may exist in the region at times, one associated with Benguela upwelling and the other with a tongue of surface tropical and equatorial water: the Angola Tongue.

The Angola-Benguela Front ( Gb26) is dynamic and can display quite rapid changes in latitudinal position in response to changes in wind stress (Shannon et al., 1987). The front is most clearly observed in the surface waters and strong, shallow thermal stratification is observed north of 16°S throughout the year. Stratification is weaker to the south (especially during the cold season) and the mixed layer can be fairly deep. The front may be pushed offshore in northern Namibia during the warm period by coastal upwelling. During the cold season upwelling occurs all along the coast and the thermal gradient is weaker.

Zooplankton communities to either side of the Benguela-Angola Front are very different (Olivar, 1987; MacPherson, 1991; Barange et al., 1992). Those to the north comprise many species with tropical affinities which occur at low abundance, whilst those to south comprise fewer species with coastal and cold water affinities, at high abundance. Southward movement of Angola Current water can occur along the shelf edge, favored by an abattment of the coastal Namibian upwelling. Such an unusual intrusion is probably more frequent during the so called “Benguela Niños”. It is marked by an inflow of gelatinous carnivorous species, siphonophores and hydromedusae into Namibian waters. The number of species in both groups decrease southward and the siphonophores exhibit a weak inshore-offshore gradient (Pagès and Gili, 1991). This illustrates the fact that fronts are boundaries between ecosystems of different trophic structure. The cold side is the lower level with high productivity and herbivores, whereas the warm side is characterized by planktonic predators such as cnidarians (north of Cape Frio), or even tunas (north of Cape Lopez; Dufour and Stretta, 1973b).