Nitrogen uptake by heterotrophic bacteria and phytoplankton in Arctic surface waters

We estimated rates of heterotrophic bacterial and phytoplankton uptake of nitrate, ammonium, and urea using N-15-labelled nitrogen and specific metabolic inhibitors of prokaryote and eukaryote nitrogen metabolism in the surface waters of the North Water (northern Baffin Bay) during autumn that were characterized by the absence of cyanobacteria (comprising prochlorophytes). The percentage of nitrate + ammonium uptake by heterotrophic bacteria ranged between 44 and 78% of the measured total uptake and was the highest when the phytoplankton biomass was relatively low (< 2 mu g Chlorophyll a L-1). Phytoplankton accounted for a larger fraction (e.g., 58-95%) of urea uptake than heterotrophic bacteria. When our results are combined with those from previous studies carried out in diverse temperate and polar areas, it appears that heterotrophic bacteria account for similar to 25% (14-40%; median and interquartile range) of the total nitrate uptake in surface waters with chlorophyll biomass < 2 mu g L-1. Estimates of new production computed from phytoplankton carbon uptake and f-ratios may be strongly overestimated in regions where nitrate uptake by heterotrophic bacteria is high and the biomass of phytoplankton is low.