General biological features of the South Atlantic
The western South Atlantic: Argentinian coastal and offshore waters

Much of the literature dealing with the distribution of benthic coastal/neritic flora and fauna concludes that 2 biogeographic provinces can be recognized along the coasts of southern Brazil to Tierra del Fuego: an Argentine province, extending to 43°S, and a Magellanic province between 43°S and the Drake Passage (e.g., López Gappa and Lichtschein, 1988, and references therein; (Gb9). Planktonic groups do not seem to follow an identical pattern, yet a boundary at 43°S (or 47°S, see(Gb1) is also often identified. The neritic area located between Río de la Plata and Península Valdés (43°S), or even Cabo Dos Bahías (47°S), is of special interest insofar as, while generally dominated by subantarctic species, it periodically hosts significant proportions of subtropical plankters (Balech, 1949, 1964a, 1976; E. Boltovskoy, 1970; Lange, 1985). Benthic assemblages in this area also show clear relationships with those from Brazilian shelf waters (e.g., benthic Foraminifera, cf. E. Boltovskoy, 1964). These findings are interpreted as the result of the (summer) intrusion of Brazil Current waters which slip between the main axis of the Malvinas (=Falkland) Current and the coast and move southward along the shelf. Summer SST satellite images do indeed suggest a branching of the Brazil Current north of the estuary of the Río de la Plata, with a major arm deflecting to the southeast and a minor one invading the shelf in a southwest direction (Gb13). Alternatively, Balech (1986) suggested that this warmer water biota is the biological imprint of a backflow of Malvinas (=Falkland) Current waters which retroflect southward along the coast after having been heated and partially mixed with coastal subtropical waters at lower latitudes (the "Argentine Flux" sensu Severov, 1990). It should be pointed out, however, that at least as far as spore- or cyst-forming organisms (mainly phytoplankters) are concerned, local populations of warmer-water preferences could be maintained in dormant stage on the sea-floor, blooming with the onset of adequate ambient conditions in the water-column. This being the case, Brazil water intrusions would not be required to explain their presence in this area. (Gb13)

Mixing of subantarctic and subtropical waters off Southern Brazil down to ca. 45°S is nicely illustrated by results based on analyses of the provenance of planktonic organisms recorded in the samples (i.e., hydrological indicators or tracers). (Gb14) illustrates a selection of these showing the wedge-like intrusion of the cold Malvinas (=Falkland) flow into the subtropical domain (Gb14 A), (Gb14 B), (Gb14 D), ( Gb14 E); see also (Gb13), extensions of Brazil Current waters to the east of this intrusion (Gb14 A), ( Gb14 B), (Gb14 C), as well as the coastal tongue with warm-water organisms reaching as far as 46-47°S (Gb14 A); see also (Gb13). These maps also stress the irregularity and extremely high temporal instability of the front in question (e.g., Olson et al., 1988), and of the concomitant distribution patterns of the organisms concerned.

Off Buenos Aires Province (ca. 37-39°S) a clear cross-shore zonation is present (Fernández Aráoz et al., 1991; Carreto et al., 1995; see (Gb12 C):

(a) (Gb14) In the inner or costal system (to ca. 30 km offshore) wind- and tide-induced mixing yields a homogeneous water-column year-round. Phytoplankton growth is chiefly nitrogen-limited, but swift nutrient regeneration yields chlorophyll a values between 1 and 4 mg mö-3. Seasonal variations in phytoplanktonic abundance are low, with maxima in the spring and autumn, diatoms being the most conspicuous producers (Carreto et al., 1995). The zooplankton hosts typically coastal species, (e.g., the copepods Paracalanus parvus, Calanoides carinatus, and the cladocerans Evadne nordmanni, Podon spp., Penilia avirostris; Fernández Aráoz et al., 1991).

(b) (Gb14 ) The Subantarctic shelf waters system is separated from the coastal one by a coastal front, which breaks down during the winter. A very strong seasonal (summer) thermocline develops in this area at depths of ca. 30 m. Phytoplankton growth has two well defined maxima in spring and in autumn, with a conspicuous gradient of higher values seawards. This area is periodically influenced by Río de la Plata outflow waters.

(c) (Gb14 ) The Malvinas (=Falkland) system is dominated by the northeastward-flowing cool Malvinas (=Falkland) waters and separated from the shelf system by a well defined shelf-break front. Plants and animals are here represented by typically subantarctic species (Fernández Aráoz et al., 1991), although during the summer warmer-water forms are common as well (Lange, 1985).

The shelf-break front is a major oceanographic feature along the southern Southamerican Atlantic coast between ca. 37 and 50°S; active vertical water mixing along this boundary is responsible for enhanced primary production (350-450 g C mö-2 yö-1; Negri, 1993), and the concomitant concentrations of zooplankton (Ciechomski and Sánchez, 1983; Dadon, 1984a) (Gb3b, Gb3c, Gb15, Gb16). Interestingly, radio-tracked elephant seals from Península Valdés (ca. 43°S) have been observed to spend many months at a time precisely within this front, where the high concentration of animal biomass makes them advantageous feeding grounds (C. Campagna, pers. comm.). (Gb15)

In general terms, zooplanktonic abundances in this area down to 55°S are highest during the spring and early summer, decreasing noticeably during the winter (Ramírez, 1981b; Ciechomski and Sánchez, 1983; (Gb15). Typically cold water zooplankters (e.g., the pteropod Limacina retroversa), however, may be most abundant over the shelf and slope at other times of the year, like in the autumn (Dadon, 1990a).

It should be noted that the large-scale biogeographic divisions described and illustrated for the Argentine Sea (Gb1) reflect only marginally the strong longitudinal component derived from the influence of the Malvinas (=Falkland) Current. Indeed, at the same latitude inshore waters are usually warmer and host warmer-water organisms than those farther offshore, while the slope and adjacent oceanic area are dominated by the cold, subantarctic Malvinas (=Falkland) Current flow, where only subantarctic plankters dwell. Thus, detailed studies often show that boundaries between dissimilar assemblages run roughly paralell to the coast, rather than perpendicularly to it (Gb16 A), (Gb16 B), which is clearly matched by the north-south orientation of the isotherms (Gb16 C). Thus, northward dispersion of cold water species is favored by these advective longshore processes, which are further enhanced by the concomitant deformation of the associated temperature fields.

Additional complications to these geographic patterns arise when the vertical component is brought into the picture. While up to around 40°S the Malvinas (=Falkland) Current occupies the entire water-column, further north it submerges partially under the warmer Brazil Current and pogresses northward at increasingly higher depths. Waters of subantarctic origin are present at depths of ca. 300 m off the coasts of Brazil (Emilsson, 1961; Matsuura, 1986); and frequent records of subantarctic and even Antarctic plankters in upwelling cells as far north as 23°N (E. Boltovskoy, 1970; Valentin et al., 1987; Brandini, 1990) strongly suggest that these organisms have been advected from higher latitudes at subsurface depths. Typically subantarctic benthic organisms are recorded year round off Buenos Aires and Uruguay at depths of 150 to >200 m. E. Boltovskoy et al. (1996), for example, found that, between 30 and 60°S, along 55°W, at any given latitude the proportions of the cold water planktonic foraminifer Globoquadrina pachyderma (with respect to the entire foraminiferal assemblage) are noticeably higher at 30-50 m than higher up in the water column. This submersion and equatorward advection of cold water zooplanktonic species is probably a world-wide phenomenon with important biogeographic and, especially, paleoecologic implications (Boltovskoy, 1988, 1994; see chapter “Polycystine Radiolaria” in this volume).