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, provides a source of new nitrogen to marine ecosystems and has been increasingly recognized as a potential contributor to nitrogen supply in the Arctic Ocean. 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. We observed N2 fixation rates ranging from 0.16 to 2.71 nmol N L−1 d−1, notably higher than many previously reported rates for Arctic waters. The diazotrophic community was predominantly composed of UCYN-A. The highest N2 fixation rates co-occurred with peaks in chlorophyll a and primary production at a station in the Vaigat Strait, likely influenced by glacial meltwater input. On average, N2 fixation contributed 1.6 % of the estimated nitrogen requirement of primary production, indicating that while its role is modest, it may still represent a nitrogen source in certain conditions. These findings illustrate the potential 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.