Nitrogen is a key nutrient limiting primary production in the Arctic Ocean. Knowledge about sources and sinks of nitrogen is, therefore, essential for the understanding of current and future productivity in the region. Nitrogen fixation has been reported in the Arctic Ocean, but data remain sparse. Here, we measured nitrogen fixation rates and examined the diazotroph community composition across the deepest gateway to the Arctic Ocean - the Fram Strait. Samples were obtained from two major currents and their frontal zone: the southbound Arctic-influenced East Greenland Current, the northbound Atlantic-influenced West Spitsbergen Current, and the Polar Front. Nitrogen fixation rates were measurable in 9 of 20 samples (0.02 – 10.15 nmol N l^-1 d^-1), detected only above 100 m. Rates were highest in the Atlantic-influenced water, positively correlated with temperature and silicate. Amplicon sequencing of the nifH marker gene revealed that non-cyanobacterial diazotrophs dominated across Fram Strait, with Proteobacteria accounting for 66% of all sequences. For samples where nitrogen fixation rates were detected, the dominating classes (<96%) were affiliated with Beta-, Delta- and Gammaproteobacteria. Surprisingly, Cyanobacteria only accounted for 12% of the sequences, mainly affiliated with Nostocales. Our study shows a large potential for nitrogen fixation in this important gateway but with large differences between water masses. Our future work includes identification of the active diazotrophs (nifH gene expression) and the factors driving their activity in Fram Strait.

Primary Presenter: Stine Zander Hagen, University of Copenhagen (tdn480@alumni.ku.dk)

Authors:

Stine Zander Hagen, University of Copenhagen  (stine.hagen@bio.ku.dk)

Lisa Winberg von Friesen, University of Copenhagen  (lisa.vonfriesen@bio.ku.dk)

Olivier Grosso, Mediterranean Institute of Oceanography  (olivier.grosso@mio.osupytheas.fr)

Mar Benavides, Mediterranean Institute of Oceanography  (mar.benavides@ird.fr)

Rafael Gonçalves-Araujo, DTU-Aqua  (rafgo@aqua.dtu.dk)

Colin Stedmon, DTU-Aqua  (cost@aqua.dtu.dk)

Lasse Riemann, lriemann@bio.ku.dk  (lriemann@bio.ku.dk)