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

At 25-28°S the axis of the Brazil Current moves even farther offshore and the neritic area is seasonally influenced by Río de la Plata and Lagoa dos Patos outflow waters which are carried northwards by the coastal, Malvinas (=Falkland)-driven flow (Castello and Müller, 1977; Hubold, 1980a, b; Brandini, 1990). Chlorophyll a values are around 0.2-3.0 mg mö-3, but isolated peaks of up to 25 mg mö-3 can be recorded in association with the fertilizing effects of freshwater runoff (Hubold, 1980a, b) and colder waters of subantarctic origin, especially during the late winter and spring (Odebrecht and García, 1997). Data on particulate primary production rates off southern Brazil have been summarized by Odebrecht and García (1997), reporting between 12.3 and 120.3 mg C mö-2 hö-1, with peaks in excess of 340 mg C mö-2 hö-1 (yearly average: approximately 160 g C mö-2). In general terms, planktonic production and biomass are highest inshore and decrease oceanward, as one enters the oligotrophic waters of the Brazil Current (Vannucci and Almeida Prado, 1959; Vannucci, 1961; Hubold, 1980a, b; Brandini, 1990; Montú et al., 1997). However, occasional upwellings of nutrient-rich South Atlantic Central Water along the continental slope, especially during the summer, may strongly enhance the productivity of this offshore sector (Brandini, 1990). The smaller plankton of areas strongly influenced by the Brazil Current, generally north of 25-26°S and off the slope, is dominated by nanoflagellates, cocolithophorids, blue-greens and small (<20 µm) naked ciliates; while large naked and thecate heterotrophic dinoflagellates, diatoms, silicoflagellates, large (<40-80 µm) naked ciliates and tintinnids are more abundant in areas influenced by coastal and subantarctic waters (Brandini, 1988; Odebrecht and Djurfeldt, 1996; Odebrecht and García, 1997). A roughly similar transition is also observed between winter communities (characterized by large-sized producers and relatively short and highly efficient food webs) and summer associations (dominated by small producers and bacteria, which in turn give rise to more complex trophic relationships where mucus-feeding mesozooplankters, like Thaliacea and Pteropoda, play a dominant role) (Castello et al., 1997).

Representative images of the distribution of water masses off the southern Brazilian, Uruguayan and northern Argentinian coasts, and the associated phyto- and zooplanktonic abundances, are nicely illustrated by the results of Hubold (1980a, b), partially reproduced in (Gb10).

Resgalla (1993) produced an interesting analysis of the zooplanktonic assemblages (Pteropoda, Cladocera and Chaetognatha) off the Brazilian coast between 31 and 35°S (Gb11). During the summer, the area is dominated by neritic warm water fauna exclusively (several Cladocera, Sagitta tenuis, Creseis virgula). In winter, on the other hand, tropical-subtropical species are circumscribed to the seaward sector, coastal ones stretch some 50-70 miles offshore, while the band separating these 2 assemblages is dominated by cold water plankters (Sagitta tasmanica, Limacina retroversa) swept in by the Malvinas (= Falkland) Current (Gb11). These subantarctic influences can be traced as far north as 31-32°S, but during the summer the subtropical-subantarctic front retreats southwards beyond the Río de la Plata estuary. However, Malvinas (=Falkland)-related waters and the associated benthic and planktonic biota are present year-round at depth along the Brazilian coast as far north as 20°S (E. Boltovskoy et al., 1996 and references therein).