General biological features of the South Atlantic
The western South Atlantic: Rio de la Plata estuary

At appoximately 35°S, the mouth of the Río de la Plata defines a major hydrographic and biogeographic boundary. Fresh waters of the Paraná-Uruguay drainage basin extend hundreds of miles into the sea thus forming an effective dispersal barrier for many planktonic species. The estuary of the Río de la Plata has a total surface of about 30,000 kmö2 (Gb12, a), with a mean feshwater runoff of 20,000 mö3 sö-1. In spite of the very high nutrient loadings associated with the freshwater plume, primary production in the inner sections of the estuary is low due to the strong light-limiting conditions imposed by the high concentrations of suspended inorganic particulates. Farther oceanward, however, beyond the area where most of the suspended silt and clay sediments, primary production peaks noticeably, exceeding conspicuously values typical of the neighboring Malvinas (=Falkland) and Brazil waters. Both the estuary itself and the adjacent oceanic waters influenced by its plume are subject to ample temperature and salinity fluctuations, partly due to seasonal changes, and also to a large extent as a result of prevailing wind, precipitation (and consequent runoff), and tide conditions (Guerrero et al., 1997). Surface salinities vary widely within the area of influence of estuarine waters; yearly changes off Montevideo and Punta del Este (Gb12, a), for example, range from 13 to 35°/°°, with spans up to 20°/°° in a few days being common (Ferrando, 1962; Milstein and Juanico, 1985). Usually there also is a strong vertical salinity stratification, with marine waters (saltier and denser) penetrating deeper into the estuary along the bottom, while fresh waters extend farther oceanward on the surface (Gb12, b). Planktonic populations in the estuary are almost exclusively represented by marine eurihaline species, whereas freshwater representatives are practically absent (Boschi, 1988). Among the former, several tintinnids (Souto, 1974), appendicularians (Esnal, 1973), ctenophores, medusae, cladocerans, copepods and a few other holo- and meroplankters have been identified (Boschi, 1988; Milstein and Juanico, 1985), including both species closely associated with subtropical waters, and those which dwell best farther south, in the Malvinas (=Falkland)-influenced subantarctic ones. Off Punta del Este (Gb12, a) mean yearly zooplanktonic densities (180 µm-mesh net) around 3,900 ind. mö-2 have been recorded, with very conspicuous qualitative and quantitaive variations in very short periods (Milstein and Juanico, 1985).