Multiannual trends in fronts and distribution of nutrients and chlorophyll in the southwestern Atlantic (30-62ºS)

Abstract

This paper presents the "rst data on the vertical distribution of chlorophyll and nutrients in the upper layers of the southern southwestern Atlantic and interprets it in relation to frontal systems and the general hydrographic features. The survey covered quasi-synoptically the area between 30 and 623S, at the beginning of the austral summers of three consecutive years (1993 through 1995). Our results show a rather consistent oceanographic structure, with similar patterns of chlorophyll and nutrient distributions with latitude. Outstanding features present throughout the three seasons are two zones of high phytoplankton production, where chlorophyll concentrations of'1 lg/l seem to occur frequently. The "rst is the upper euphotic zone of the Brazil}Malvinas Confluence Zone between approximately 36 and 503S. The second is centered on subsurface layers of the Antarctic Surface Water south of the Polar Front as far as 623S. The rest of the area covered shows low chlorophyll concentrations ((0.4 lg/l), due either to nutrient limitation in the oligotrophic subtropical waters of the Brazil Current or to excessive turbulence and upper-layer instability in the energetic Malvinas Current. The occurrence of a permanent meander around 383S coincided with the cyclonic retro#exion of the Malvinas Current, causing upwelling of deep water. The ecological implications of this eddy-induced upwelling are discussed.