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

Most of the biogeographic schemes proposed for this area agree in the presence of a discontinuity around 20-23°S, which is generally interpreted as a boundary between the tropical and the subtropical domains (Gb9); see reviews in Dadon and Boltovskoy, 1982; Palacio, 1982; Bisbal, 1995). This break was already suggested almost 150 years ago on the basis of SST data (Dana, 1853; (Gb9), and was subsequently confirmed on the basis of floral and faunal data. It is especially noticeable in benthic assemblages (benthic foraminifers, hydrozoans, echinoderms, molluscs, crustaceans) and fishes, where a large proportion of the north-Brazilian species is shared with the Caribbean (Vannucci, 1964; Semenov and Berman, 1977; Semenov, 1978). Because the Brazilian coast from 5°S to ca. 30-35°S is dominated by the southbound Brazil Current, changes in the plankton at 20-23°S are minor and gradual; nevertheless, modifications in the specific composition of various pelagic groups (e.g., planktonic foraminifers, cephalopods) have been reported to occur at or slightly south of Cabo Frio (23°S). (Gb9)

South of 20°S the Brazil Current strengthens and moves somewhat farther offshore. Pires-Vanin et al. (1993) summarized some outstanding features of the physical and biological oceanography around Ubatuba (approx. 24°S) (see also the chapter “Fish larvae” in this volume). Seasonal changes are mainly driven by transgressive movements of fresh South Atlantic Central Waters onto the middle and inner shelf. In late spring and summer these nutrient-rich waters extend coastward as a thick subsurface layer the top of which lies within the photic zone. Nannoplanktonic producers account for a major part of the chlorophyll a (around 1 mg mö-3), while salps are the dominant primary consumers. In the winter South Atlantic Central Waters retreat oceanward, thermic stratification disappears in the coastal region, and primary production is mainly fed by wind-driven mixing and sediment resuspension. Moderate faunal gradients occur vertically and perpendicularly to the coastline in association with the replacement of water masses of different origin (e.g., Björnberg, 1981; Christoffersen, 1982). Overall primary production is low, generally comparable to that of the northern sector; in oceanic areas it ranges around 0.45 µg C lö-1 hö-1, while in coastal waters it is somewhat higher and more variable (1.08-15.52 µg C lö-1 hö-1; cf. Metzler et al., 1997). As in most other oligotrophic areas, regenerated production is dominant over new production, with reduced forms of nitrogen accounting for 74% (offshore) to 96% (inshore) of total nitrogen utilized by phytoplankton (Metzler et al., 1997). Isolated coastal and shelf-break upwelling cells, like the ones at 24°S and 26°S (shear-related vortices, probably enhanced by topographic discontinuities) may host both higher production and unusually cold- and deep-water planktonic assemblages (Brandini et al., 1989; Brandini, 1990; Pires-Vanin et al., 1993); under these conditions the share of new production is probably enhanced.