Contrasting effects of substrate and grazer manipulations on picoplankton in oceanic and coastal waters off Brazil

Abstract

In two contrasting regions off the coast of Brazil, picoplankton (<1 μm) responses to removal of larger grazers and to the additions of glucose and amino acids were determined. Effects of glucose and amino acid additions (1 μM) on particulate nitrogen and chlorophyll a concentrations, and on rates of NH4+ uptake and regeneration, were observed after 5 h pre-incubation. In the oceanic waters, removal of the >1 μm fraction had no significant effect on the chlorophyll a of the picoplankton after 5 h. However, the addition of glucose stimulated both uptake and regeneration by a mean of 27%, and the addition of arginine led to significant decreases in the rates of NH4+ uptake and regeneration. In contrast, in the coastal waters, significant increases in chlorophyll and particulate nitrogen concentrations were found after 5 h incubation in both the amended samples and in the controls, and mean rates of NH4+ uptake and regeneration were affected to a lesser degree by the additions of either glucose or amino acids than in the oceanic waters. The oceanic responses were suggestive of carbon limitation of heterotrophic bacteria. In the coastal region, on the other hand, the supply of organic carbon and nitrogen was likely to have been sufficient to meet the nutritional requirements of the heterotrophic bacteria and cyanobacteria. Grazing by larger organisms on the picoplankton appeared to play a more significant role in the nitrogen cycle in the coastal waters than in the oceanic waters.