Nitrogen fixation in Arctic coastal waters (Qeqertarsuaq, West Greenland): influence of glacial melt on diazotrophs, nutrient availability, and seasonal blooms

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 (N 2 ) 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 N 2 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 N 2 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 N 2 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 N 2 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, N 2 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 N 2 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.