General biological features of the South Atlantic
Coastal area between Cape Lopez and Cape Frio

The area, delimited to the north and the south by seasonal thermohaline fronts, is characterized by a superficial layer of warm water with low salinity which is periodically pushed away by upliftment of the thermocline and upwelling.

—The four seasons cycle
During the warm season, the water column is stratified and the thermocline is relatively deep (the 18°C isotherm is between 50 and 120 m off Pointe-Noire, Dessier, 1979). As a consequence, nutrient concentrations (phosphate 0.3 atg lö-1), phytoplankton biomass (0.5 mg mö-3) and production (35 mg C mö-2 hö-1) can be low and are generally situated in the region of the thermocline.

( Gb22a), Temperature at 17m.
( Gb22b), Chlorophyll a.
( Gb22c), Phosphate at 5m.
( Gb22d), Primary production.
( Gb22e), Zooplankton biomass.
( Gb23), Distribution of zooplankton.

The low phytoplankton biomass observed during the warm period is reflected by low zooplankton biomass (dry weight: 1 g mö-2, displacement volume <0.1 ml mö-3), across the shelf ( Gb22e) and ( Gb23), although this may be elevated when the thermocline shoals or (locally) in response to river runoff (Dessier, 1981).

At the end of May, the thermocline shoals and breaks the surface. Subsurface cold waters with high salinity (from the Equatorial Undercurrent) replace the tropical warm waters with low salinity. A first peak in nutrients and phytoplankton occurs in June. After a decrease in July, a second (and greater) maximum is reached during August, when Benguela waters occupy the whole column. On average, nutrient concentrations are high (phosphate: 0.7 atg lö-1, ( Gb22c). This in turn is reflected by a seasonal peak in coastal chlorophyll a (2 mg mö-3) and phytoplankton production (75 mg C mö-2 hö-1) at 5 m depth. Pigment diversity (D430/D665) decreases from March to August indicating that phytoplankton populations are the youngest when the nutrient input is the largest (Dessier, 1981). Zooplankton dry weight (2.5 g mö-2) and displacement volume (0.25 ml mö-3) also tend to be higher during the cold period across the shelf ( Gb23). At the start of the cold season a marked increase in zooplankton biomass to the edge of the shelf is related to a maximum in divergence and upwelling there (as indicated by the high salinities), and to the development of two large copepods species.