General biological features of the South Atlantic
General remarks on seasonality, biomass and diversity in the pelagic domain of the South Atlantic

The quantitative distribution of pelagic life in the South Atlantic parallels those found in the other oceans: a large area of poor central waters bound to the north and south by richer equatorial and subpolar bands, respectively, with the biologically richest sectors circumscribed to the coastal regions, especially along Africa (Gb3a, Gb3b, Gb3c).

Over half of the South Atlantic is occupied by the South Atlantic Central Gyre or South Atlantic Central water mass, a tropical/subtropical area with extremely low nutrient loadings and, in consequence, with very low phytoplanktonic and zooplanktonic populations. Primary production values range here around >0.1-0.2 g C mö-2 dö-1 (Koblentz-Mishke, 1977; Longhurst et al., 1995; see Table Gb1), with phytoplankton concentrations below 10ö3 cells lö-1 (Semina, 1977). This area shows clearly both in chlorophyll a and in primary production maps, as well as in those depicting the distribution of zooplanktonic biomass (Gb3c). In the vicinity of the equator biological richness is enhanced by equatorial divergence and by seaward advection of nutrient- and biomass-rich Benguela upwelling waters. On the western side, the Amazon plume can also seasonally contribute to the enhancement of plankton along the equator. Specific compositions between central and equatorial waters differ little, although the richer equatorial ones tend to show lower equitabilities (i.e., higher degrees of dominance of a few species over the rest of the assemblage). A few organisms, however, seem to be especially adapted to these oligotrophic waters. Such is probably the case of the colonial polycystine Radiolaria (see “Radiolaria Polycystina” in this volume), which host symbiotic algae in their protoplasm, presumably to compensate for the scarcity of food in the medium. In some areas the production of these symbiotic algae has been found to exceed that of the free-living phytoplankton (e.g., in the Gulf of Aden, according to Khmeleva, 1967). Mesozooplanktonic wet weight (0.2-20 mm size range) in the Central Waters is below 25-50 mg mö-3 (0-100 m; cf. Bogorov, 1974; Rudjakov, pers. comm.; (Gb3c).

The southern boundary of the Central Gyre is represented by the West Wind Drift (WWD), a strong circumglobal current carrying subantarctic and antarctic waters from west to east. In the area of the study, the gradient between the warmer waters north of the WWD and those to the south of it is probably the most conspicuous in biological terms. Indeed, it was the first (and sometimes the only) biogeographic limit proposed in early studies (e.g., Meisenheimer, 1905a), insofar as planktonic species can be roughly divided into warm water and cold water organisms, the former associated with the tropics and subtropics, and the latter with the subantarctic and the Antarctic (Gb8a, Gb8b).

Besides temperature per se, several environmental parameters are strongly modified south of 40-45°S, in particular the concentration of nutrients, which gradually increase from north to south, the growing importance of sunlight as a limiting factor for phytoplankton development, and the seasonality (varying temperature and insolation throughout the year). Typical P-PO4 concentrations below 0.25 g at P lö-1 (0-50 m) in the Central Waters increase to >1.5 g at P lö-1 around 45-50°S. Nitrates and silicates follow similar trends as well. At 60°S sunlight limits phytoplankton growth between May and August, and from April to July at 70°S (Voronina, 1977). The onset of spring autotrophic growth is determined by the increasing availability of light and by the stratification (and consequent stabilization) of the upper water layer; the steep density-gradient precludes phytoplankton cells from being drawn down away from the photic layer, thus favoring the buildup of plant biomass. In subantarctic waters spring and summer stratification of this upper layer is due to heating. Further south, however, where winter to summer temperature gradients are minor, meltwater from the retreating ice-pack, which spreads along the ice-edge as a large 20-50 m thick superficial lens, plays a determining role (Smith and Nelson, 1986).

Seasonality is probably a major factor in the structuring of pelagic communities, and it also may play a key role in the biogeochemistry of the oceans. Regardless of absolute values, more or less constant primary production levels throughout the year would permit a better coupling between production and consumption, thus allowing for less organic matter to go unused in the trophogenic layer and sink toward the sea floor. In contrast, sudden bursts in plant biomass, as those characteristic of high-latitude and some coastal upwelling systems (such as the Benguela system), have probably less chances to be used fully by the slower-reacting consumers, therefore exporting higher proportions of their matter to depth. These concepts, discussed in detail in a paleoceanographic context by Berger and Wefer (1990), are quite relevant to biogeography insofar as they may help explain some of the differences between warm water and cold water zooplanktonic assemblages. Thus, differences in species richness between tropics and poles are probably as much a result of extreme temperature conditions in the latter, as they are of the need to be able to adapt to long periods of starvation characteristic of polar régimes.