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 1994 in the South Atlantic off Brazil by performing 15N tracer experiments on microbial populations from two depths (50% and 1% of surface light penetration). Carbon uptake rates were determined by 14C tracer experiments at the same depths. At each sampling site temperature and salinity profiles were obtained and ambient nutrient and phytoplankton biomass were measured. Concentrations of chlorophyll a were < 0.3 μg 1−1 in the oceanic region and varied from 0.12 to 1.41 μg 1−1 in the inshore region. On average NO3− was the main form of nitrogen available for phytoplankton (64–86% of total nitrogen) in oceanic waters, while at the inshore station, NO3− represented less than 32% of total nitrogen supply. Nonetheless, primary production was dominated by regenerated production. Uptake of reduced forms of nitrogen represented on average 74% of total nitrogen utilized by phytoplankton in the oceanic region and 96% at the inshore station. In oceanic waters, the mean f-ratio ratio (contribution of new to total production) was 0.16 and 0.30 at subsurface (50% Io and bottom (1% Io) of the euphotic zone, respectively. 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 zone, resulting from the intrusion of the South Atlantic Central Water on the continental shelf. Episodic enrichment of the base of the euphotic zone with NO3− enhanced the contribution of new to total production and led to a maximum f-ratio of 0.86 in oceanic waters and 0.26 in coastal waters.