General biological features of the South Atlantic
The western South Atlantic: Northern Brazil

Along the Brazilian coast, the area toward the northwest of Cabo San Roque (5°S) differs from that to the southwest of this site in that it hosts the mouth of the Amazon drainage basin. This river discharges about 190,000 mö3 sö-1 of fresh water into the sea, its plume extending far into the open ocean where it can be traced up to 1000 km to the north and east of the river mouth (Lentz, 1995; Limeburner et al., 1995). The nutrients thus exported are responsible for a permanent diatom bloom between the 10 and 30 m isobaths (up to 30 mg Chl a mö-3; Muller-Karger et al., 1988). During February-May Amazon water flows into the Caribbean at ca. 90 cm sö-1 in a broad (150-200 km) continuous band of high pigment concentrations (>1.5 mg mö-3; as compared with background concentrations in oceanic waters of 0.15 mg mö-3). Between January and June this plume is carried northward, around a retroflection of the Guiana Current, and then turns east, toward Africa, joining the North Equatorial Countercurrent (Muller-Karger et al., 1988). This mechanism seems to be a major contributor to the low salinities observed across the tropical Atlantic between 5-10°N, to enhanced zooplanktonic biomass (Greze et al., 1969), and to high and very variable rates of sedimentation of organic aggregates in the northwestern Atlantic (Deuser et al., 1988). In view of its enormous discharge, the biological and biogeochemical significance of the Amazon plume (primary production, organic carbon burial, sediment accumulation) is such that it has been reported to affect not only regional, but also global mass budgets (Nittrouer and DeMaster, 1996). Another effect of this freshwater plume is the rather dramatic local drop in the diversity of most oceanic planktonic groups (e.g., chaetognaths, siphonophores and medusae; Alvariño, 1968).

Coastal circulation off north-eastern and central Brazil is chiefly defined by the southward-flowing oligotrophic Brazil Current; because the shelf over this entire area is narrow, typically neritic populations are loosely delimited and pelagic flora and fauna invade the coastal domain regularly. Along the northern coast of Brazil, both phytoplanktonic biomass (<1000 cells mlö-1, <0.2 g Chl a lö-1; cf. Valentin, 1989), and primary production (ca. 300 g C mö-2 yö-1, cf. Longhurst, 1995) are low, with the exception of some restricted areas (e.g., off Cabo Frio, at 23°S) where quasi-permanent wind-driven coastal upwelling conditions can enhance primary production values and foster the development of colder- and deeper-water planktonic assemblages at the surface (Valentin, 1980, 1989).