Contrasting nitrogen fixation between Arctic and Atlantic waters in the Fram Strait

Nitrogen availability limits primary production in the Arctic Ocean, making it vital to understand its sources and sinks to predict future productivity. Although nitrogen fixation has been reported in the Arctic Ocean, data remain scarce, especially in the Atlantic sector. Here, we measured nitrogen fixation rates and examined diazotroph community composition across the Fram Strait, targeting Polar waters in the East Greenland Current, Atlantic waters in the West Spitsbergen Current, and their frontal zone. Nitrogen fixation was mainly low (< 1 nmol N L-1 d-1) in Polar waters, however, elevated at the one station in the Atlantic water sector (up to 10.15 nmol N L-1 d-1). Rates were only detectable in the epipelagic layer (0–100 m) across the strait and positively correlated with temperature, primary production, and chlorophyll-a fluorescence, and negatively correlated with coloured dissolved organic matter and silicate. The diazotrophs were dominated by non-cyanobacterial diazotrophs (NCDs; 77% of nifH amplicon reads), with an Arctic Betaproteobacterial group (order Rhodocyclales) accounting for 11% of sequence reads. This group was quantifiable (up to 6700 nifH gene copies L-1) within the West Spitsbergen Current and the frontal zone, where the highest nitrogen fixation and primary production occurred, and its prevalence was positively correlated with temperature. We propose that temperature and freshly produced dissolved organic matter influence the NCD-dominated nitrogen fixation in Fram Strait. Our study suggests that NCDs are key diazotrophs in Fram Strait, and that nitrogen fixation rates and their potential importance for primary production vary across the contrasting water masses entering and exiting the Arctic Ocean. We encourage future studies to quantify these nitrogen fluxes and evaluate their importance for productivity in the Arctic Ocean.