General biological features of the South Atlantic
The tropical and subtropical eastern South Atlantic: Eastern tropical South Atlantic

South of the equator, the oceanic areas of the Gulf of Guinea can be divided into zones on the basis of hydrology and currents (Dufour and Stretta, 1973a; Stretta, 1975; Voituriez and Herbland, 1977; Herbland and Voituriez, 1977; Gallardo et al., 1974; Voituriez and Herbland, 1982). At the equatorial divergence, between 1°N and 3°S, there is a pronounced seasonality in the structure of the water column (Voituriez and Herbland, 1977). During the warm period (October-June) a thermal ridge develops due to the eastward flowing Equatorial Undercurrent ( Gb18a) under the westward flowing South Equatorial Current.

This ridge effectively separates a warm, shallow, nutrient-poor upper layer from a cooler, nutrient-rich deeper layer. Chlorophyll levels (up to >0.4 mg Chl a mö-3) and primary production (70 mg C mö-2 hö-1) are high and are strongly stratified as a result of the interplay between light and nutrient limitation, and peak in the region of the thermocline. During the cool period (July-September) the flow of the South Equatorial Current is accelerated and the Equatorial Undercurrent becomes much shallower ( Gb18b) ( Gb18c). Consequently, high vertical shear develops between the two currents which results in upwelling and the thermocline breaks the surface. During this period, chlorophyll (>0.8 mg mö-3) and primary production (up to 100 mg C mö-2 hö-1) are high, especially at the surface, but they may be no higher than during the warm period as a result of increased turbulence (Voituriez and Herbland, 1977).

Zooplankton biomass tends to be low at the core of the divergence zone (0.2 ml mö-3, settled volume) but increases with the latitude as populations and communities mature (0.56 ml mö-3) (Dufour and Stretta, 1973a). The biomass of zooplankton in this region may be over 2.5 times greater than that of the surrounding area, and only 1.5 times less than that observed in the Mauritanian upwelling region (Le Borgne, 1977). The vertical distributions of chlorophyll and mesozooplankton (200-500 m) are similar (Le Borgne, 1977). The depth of the thermocline deepens to the south of the equatorial divergence.

There is a surface movement of the upwelled water to 5°S, especially during the cold season, which is reflected by fairly high phytoplankton biomass and production at the surface (Herbland and Voituriez, 1977; Voituriez and Herbland, 1977). This may be weakly augmented to 8°S during the warm period by the advection of nutrient rich waters of Benguela current origin from the east (Dufour and Stretta, 1973a). Zooplankton biomass decreases from north to south (0.39 ml mö-3 settled volume) across this area.

Between 9-13°S, the surface water flow is weak and of variable direction, although this is the theoretical area of the South Equatorial Countercurrent. The thermocline may occur at depths in excess of 70 m. It is a region of low phytoplankton biomass (chlorophyll a between 0.25 and 0.50 mg mö-3) and production (21 mg C mö-2 hö-1), which peak in the region of the thermocline in the warm season and is scattered in the upper mixed layer in the cold season ( Gb18a), ( Gb18b), ( Gb18c). Production is largely based on regenerated nutrients. Zooplankton biomass is low (0.2 ml mö-3, settled volume) (Dufour and Stretta, 1973a; Voituriez and Herbland, 1977).

Between 13-16°S, in the southern branch of the South Equatorial Current (or Benguela Drift), the surface water flow is generally to the NW. Temperatures are generally low and the water has salinity characteristics of Benguela (and coastal upwelled) waters mixed with Subtropical Convergence waters. Thus, phytoplankton biomass (chlorophyll a: 0.3 to 0.8 mg mö-3) and primary production (around 50 mg C mö-2 hö-1) are rather high. They are distributed throughout the mixed layer. Zooplankton biomass is low in the core of this region (0.2 ml mö-3) but increases to the north and south (0.3 ml mö-3) (Dufour and Stretta, 1973a).

Between 16 and 23°S, in the Subtropical Convergence area the nutrient rich layer is too deep (100 m) to fuel high productivity. Chlorophyll a varies between 0.2 and 0.8 mg mö-2, and primary production is less than 13 mg C mö-2 hö-1 (Dufour and Stretta, 1973a).
( Gb19)

Phytoplankton and zooplankton biomasses are strongly correlated over much of the equatorial south Atlantic during the warm period, both horizontally ( Gb19) and vertically ( Gb20) (Dufour and Stretta, 1973a; Dandonneau, 1975; Le Borgne, 1977). This agreement tends to break down during the cold period in the enrichment areas, but improves to either side. Zooplankton communities comprise a greater number of higher trophic levels both during the warm (than cold) period and at distances from the divergence and enrichment zones. Such communities are typical of oceanic waters which are generally species rich, trophically diverse and said to be balanced. Those of the divergence and enrichment zones are unusual, in the sense that they tend to be relatively poor in species (mostly herbivores), and are probably unbalanced with their environment (Dufour and Stretta, 1973a; Dandonneau, 1975; Le Borgne, 1977).
( Gb20)

Although there are obvious differences in the zooplankton communities associated with waters inside and outside of the equatorial divergence, evidence to support faunal regions associated with the clear zones of biomass and production is ambiguous. Boltovskoy et al. (1996) failed to identify distinct polycystine radiolarian assemblages with any of the zones, and noted a relative homogeneity of the assemblage across the region between 10°N and 17°S irrespective of temperature, latitude, salinity or chlorophyll a. They did observe distinct vertical stratification, however, and as their study was conducted during the warm period, it is possible that different assemblages might have become clear during the cold season.

On approaching the African continental land mass, the South Equatorial Countercurrent and South Equatorial Underrcurrent are deflected southwards. This polewards deflection induces an uplift (or doming) of the isotherms and results in the formation of the Angola Dome centred on 10°S-9°E (Mazeika, 1968; Gallardo et al., 1974). Although the dome is a permanent structure at depth, it only has an influence on primary production when the thermocline and nutrient rich waters are uplifted to the euphotic zone. This is seasonal and occurs when the trade winds decrease and facilitate the eastwards flow of surface waters (Voituriez and Herbland, 1982). Gallardo et al. (1974) showed that the thermocline rises to 10 m in February-March, and although it does not induce any surface cooling or nutrient enrichment, production on the side of the structure seems slightly higher than in the surrounding oceanic waters. This contrasts with the situation north of the equator, where the Guinea dome (which arises due to the northwards deflection of the North Equatorial Countercurrent, which is itself a stronger feature of the superficial tropical circulation than the South Equatorial Countercurrent) is associated with highly elevated phytoplankton biomass and production (Voituriez and Herbland, 1982). The effect of the Angola Dome on zooplankton populations, processes and communities is unknown, although secondary production probably balances phytoplanktonic growth.