Características biogeográficas del Atlántico Sudoeste: plancton.

BOLTOVSKOY DEMETRIO

PHYSIS - A

ASOCIACIÓN ARGENTINA DE CIENCIAS NATURALES

Lugar: Buenos Aires; Año: 1978 vol. 38 p. 67 - 90

0325-0342

Several causes have determined that the southern half of the Atlantic Ocean (Antarctica excluded) is much more poorly known than many other areas of the World Ocean. However, during the last years numerous papers by South-americ-an (Argentine and Brazilian) and Russian authors dealing with different planktonic groups of this part of the Atlantic have appeared; this enables us to make an attempt to characterize several recognizable biogeographical zones within the area under study. The main hydrological surface currents of the Southwestern Atlantic are (from south to north): East Wind Drift, West Wind Drift, Cape Horn Current, Malvinas (= Falkland) C., Patagonian Coastal c., Brazil C., and Southequatorial C. These currents define the following hydrological fronts: Antarctic Divergence, Antarctic Convergence, Subantarctic Divergence (?), Subtropical Convergence, and Tropical Convergence ( ?), plus several minor areas such as the ones influenced by the outflow of major rivers (Rio de la Plata and Amazonas), the Central Southatlantic Gyre, and a few distinct coastal zones. In relation to these water-masses there are four major planktonic complexes: 1) Antarctic Complex, characterized by a relatively high primary productivity, the phytoplankton being dominated by diatoms and the zooplankton by copepods, euphausiids and chaetognaths. 2) Subantarctic Complex, with relatively low primary productivity in the pelagic zone, and somewhat higher in the neritic one, a larger proportion of dinoflagellates in the phytoplankton, and of several different groups in the zooplankton, the diatoms and copepods being still the dominant taxa. It has a large neritic region with self properties. 3) Subtropical Complex, very low productivity. The dominant phytoplankters are dinoflagellates and coccoliths, and the zooplankters-copepods, although in the latter the share of gelatinous forms, molluscs, microplankters, etc., becomes quite important. 4) Tropical Complex, on the West Atlantic coast the overall primary productivity is low, but can increase in certain restricted areas such as the continental shelf and the estuary of the Amazon River. Dominant phytoplankters: dinoflagellates and blue-green algae, and, perhaps, coccoliths; zooplankters: copepods and many other taxa. All these biogeographical areas and subareas can be characterized biologically and every taxon has a number (usually low) of cosmopolitan species inhabiting more than one complex, either in the surface or in subsurface waters. The boundaries between complexes are given by interphases coincident with hydrological fronts. Antarctic and Subantarctic Complexes are separated by a sharp limit: the Antarctic Convergence; there is a wide zone of overlap and mixture between the Subantarctic and Subtropical Complexes: the Subtropical Convergence (on the surface). The Antarctic Divergence (which defines a "High Antarctic Zone" and a "Low Antarctic Zone"), the Subantarctic Divergence (i') and the Tropical Convergence (?) are not important barriers from a biological point of view and usually can not be planktologically detected.