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

In two contrasting regions off the coast of Brazil, picoplankton(<1 µm) responses to removal of larger grazers andto the additions of glucose and amino acids were determined.Effects of glucose and amino acid additions (1 µM) onparticulate nitrogen and chlorophyll a concentrations, and onrates of NH₄⁺ uptake and regeneration, were observed after 5h pre-incubation. In the oceanic waters, removal of the >1µm fraction had no significant effect on the chlorophylla of the picoplankton after 5 h. However, the addition of glucosestimulated both uptake and regeneration by a mean of 27%, andthe addition of arginine led to significant decreases in therates of NH₄⁺ uptake and regeneration. In contrast, in the coastalwaters, significant increases in chlorophyll and particulatenitrogen concentrations were found after 5 h incubation in boththe amended samples and in the controls, and mean rates of NH₄⁺uptake and regeneration were affected to a lesser degree bythe additions of either glucose or amino acids than in the oceanicwaters. The oceanic responses were suggestive of carbon limitationof heterotrophic bacteria. In the coastal region, on the otherhand, the supply of organic carbon and nitrogen was likely tohave been sufficient to meet the nutritional requirements ofthe heterotrophic bacteria and cyanobacteria. Grazing by largerorganisms on the picoplankton appeared to play a more significantrole in the nitrogen cycle in the coastal waters than in theoceanic waters.