FRESH-WATER OUTFLOW AND SUBTROPICAL CONVERGENCE INFLUENCE ON PHYTOPLANKTON BIOMASS ON THE SOUTHERN BRAZILIAN CONTINENTAL-SHELF

The present study discusses the role of different continental shelf wa ter masses on inorganic nutrient levels and on phytoplankton biomass a long the southern Brazilian coast during October 1987 and September 19 88. In this productive area, variability of phytoplankton biomass has been related to the seasonal latitudinal displacement of the Subtropic al Convergence and to the freshwater outflow of La Plata River and Pat os Lagoon. Very distinct precipitation rates, as a consequence of the ENSO (El Nino-Southern Oscillation) cycle, preceding the two sampled p eriods of this study, allowed a first evaluation of the impact of this event on freshwater outflow and shelf phytoplankton biomass. The amou nt of chlorophyll in shelf waters was directly related to nutrients su pplied by the Coastal Water (i.e. freshwater outflow), Subtropical and Subantarctic Waters. Comparing the results of this study with previou s cruises performed in the area, it was found that high chi a concentr ations are common features during periods of elevated precipitation ra tes. Apart from nutrient input by freshwater and the consequent change s in the vertical profiles of density, the extreme precipitation rates are also associated with different wind patterns, which, in turn, con trol the presence of particular water masses in the euphotic zone. Our results suggest that ENSO events have an important impact on the vari ability of phytoplankton production, and thus should affect biogeochem ical cycles in the southern Brazilian coastal areas.