Nitrogen Fixation in Arctic Coastal Waters (Qeqertarsuaq, West Greenland): Influence of Glacial Melt on Diazotrophs, Nutrient Availability, and Seasonal Blooms

Abstract. The Arctic Ocean is undergoing rapid transformation due to climate change, with decreasing sea ice contributing to a predicted increase in primary productivity. A critical factor determining future productivity in this region is the availability of nitrogen, a key nutrient that often limits biological growth in Arctic waters. The fixation of dinitrogen (N₂) gas, a biological process mediated by diazotrophs, not only supplies new nitrogen to the ecosystem but also plays a central role in shaping the biological productivity of the Arctic. Historically it was believed to be limited to oligotrophic tropical and subtropical oceans, Arctic N₂ fixation has only garnered significant attention over the past decade, leaving a gap in our understanding of its magnitude, the diazotrophic community, and potential environmental drivers. In this study, we investigated N₂ fixation rates and the diazotrophic community in Arctic coastal waters, using a combination of isotope labeling, genetic analyses and biogeochemical profiling, in order to explore its response to glacial meltwater, nutrient availability and its impact on primary productivity. Here we show, N₂ fixation rates, ranging from 0.16 to 2.71 nmol N L^-1 d^-1, to be notably higher than those observed in many other oceanic regions, suggesting a previously unrecognized significance of N₂ fixation in these high-latitude waters. The diazotrophic community is predominantly composed of UCYN-A. We found highest N₂ fixation rates co-occurring with maximum chlorophyll a concentrations and primary production rates at a station in the Vaigat Strait close impacted by glacier meltwater inflow, possibly providing otherwise limiting nutrients. Our findings illustrate the importance of N₂ fixation in an environment previously not considered important for this process and provide insights into its response to the projected melting of the polar ice cover.