Comparative rates of nitrogen uptake and NH4+ regeneration by plankton of <153 and <5 m in size were determined in the Sargasso Sea and Gulf Stream, and by plankton associated with marine snow in the Gulf Stream during May 1982. Rates of total nitrogen uptake of Sargasso Sea phytoplankton exceeded those of the Gulf Stream phytoplankton by factors ranging from 1.8 to 5.6. Rates of microplankton NH4+ regeneration equaled or exceeded rates of NH4+ uptake in the Sargasso Sea, but in the Gulf Stream were negligible in all but one case. Significant rates of NH4+ regeneration were measured for Gulf Stream marine snow, and, in all but one case, exceeded those of NH4+ uptake. Rates of NO3– and urea uptake by the snow were less than half those of NH4+. Protozoan densities were enumerated on aliquots of the same snow particles and compared with previously reported bacterial estimates; enrichment factors of the cultivable ciliates and flagellates were 6500–9000 relative to ambient seawater. These organisms were also grazing and reproducing rapidly. Bacterial densities were also moderately enriched, but their productivity was lower than surrounding seawater bacteria. Thus, the large bacterivorous population associated with marine snow may have accounted for a substantial fraction of the observed NH4+ regeneration.
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