Late-season nitrogen uptake across the western coastal Alaskan Arctic

Productivity in the Arctic is expected to increase as temperatures and the number of open water days rise. With this increased productivity, the coastal shelves of the Arctic Ocean may act as a sink for atmospheric carbon. However, this storage is dependent on a sufficient nitrogen (N) supply and current literature on biogeochemical rates of N uptake in this region is severely limited. Here, we report the spatial extent and rate at which the aquatic microbial community utilizes inorganic and organic N substrates in the Alaskan Arctic during late summer. Uptake rates (> 0.3 mu m) were measured in 2016 and 2017 using isotopically labeled ammonium (NH4+$$ {\mathrm{NH}}_4<^>{+} $$), nitrate (NO3-$$ {\mathrm{NO}}_3<^>{-} $$), urea, and mixed algal amino acids. Rates of NH4+$$ {\mathrm{NH}}_4<^>{+} $$ regeneration were also measured to investigate the contribution of remineralized N to primary production. Primary production was estimated using isotopically labeled bicarbonate. We found that N species uptake varied by location. Although NH4+$$ {\mathrm{NH}}_4<^>{+} $$ was the form of N taken up at the greatest rate at most sites, we also found that uptake rates of urea could be greater than NO3-$$ {\mathrm{NO}}_3<^>{-} $$ and that amino acid uptake was widespread. Supporting previous studies, NO3-$$ {\mathrm{NO}}_3<^>{-} $$ uptake rates were correlated with primary production rates. Variability in nutrient reservoirs and sampling conditions in the Chukchi Sea between the 2 yr were responsible for some of the variances observed in estimated NH4+$$ {\mathrm{NH}}_4<^>{+} $$ recycling rates and primary production. Understanding which N sources support this late-season primary production requires obtaining uptake rates for a diverse range of inorganic and organic N substrates.