NEW AND REGENERATED PRODUCTION IN THE SOUTH-ATLANTIC OFF BRAZIL

Rates of assimilation of NH4+, NO3- and urea were determined at eight oceanic stations in March 1994 and one inshore station in December 199 4 in the South Atlantic off Brazil by performing N-15 tracer experimen ts on microbial populations from two depths (50% and 1% of surface lig ht penetration). Carbon uptake rates were determined by C-14 tracer ex periments at the same depths. At each sampling site temperature and sa linity profiles were obtained and ambient nutrient and phytoplankton b iomass were measured. Concentrations of chlorophyll a were <0.3 mu gl( -1) in the oceanic region and varied from 0.12 to 1.41 mu gl(-1) in th e inshore region. On average NO3- was the main form of nitrogen availa ble for phytoplankton (64-86% of total nitrogen) in oceanic waters, wh ile at the inshore station, NO3- represented less than 32% of total ni trogen supply. Nonetheless, primary production was dominated by regene rated production. Uptake of reduced forms of nitrogen represented on a verage 74% of total nitrogen utilized by phytoplankton in the oceanic region and 96% at the inshore station. In oceanic waters, the mean f-r atio (contribution of new to total production) was 0.16 and 0.30 at su bsurface (50% I-o) and bottom (1% I,) of the euphotic zone, respective ly. At the inshore station the mean f-ratio was 0.05 at the subsurface and 0.10 at the bottom of the euphotic zone. Temperature and salinity measurements showed occasional injections of NO3- into the euphotic z one, resulting from the intrusion of the South Atlantic Central Water on the continental shelf. Episodic enrichment of the base of the eupho tic zone with NO3- enhanced the contribution of new to total productio n and led to a maximum f-ratio of 0.86 in oceanic waters and 0.26 in c oastal waters. (C) 1997 Elsevier Science Ltd.