Nitrogen fixation – the conversion of nitrogen gas to bioavailable ammonia by a group of prokaryotes called diazotrophs – was previously not thought to occur in the Arctic Ocean. However, studies have now reported considerable nitrogen fixation rates from western Arctic shelf seas. This is of relevance as the often nitrogen-limited Arctic is undergoing rapid change with unknown consequences for nutrient availability and thus primary productivity. In this study, nitrogen fixation and the underpinning diazotroph communities were mapped over contrasting hydrographical and sea ice regimes in the unexplored central and eastern Arctic Ocean. Non-cyanobacterial diazotrophs are putative key players in the Arctic and may take up dissolved organic carbon. We therefore targeted the chlorophyll a maximum to achieve highest biomass and experimentally investigate organic carbon limitation. Strikingly, we detected nitrogen fixation below sea ice in the central Arctic Ocean ranging from 0.5-2.6 nmol N L^-1 d^-1. Across the marginal ice zone, rates varied with regime and time, reaching up to 6.0 ± 1.4 nmol N L^-1 d^-1 in the area between open water and pack ice. Very low rates of 0.4 ± 0.2 nmol N L^-1 d^-1 were detected below land-fast sea ice in northeast Greenland. The response to organic carbon amendments was strongest in the central Arctic, indicating carbon limitation of diazotrophs in this region. The surprisingly high rates of nitrogen fixation in the low-productive central Arctic Ocean suggest it could be a regionally significant process partially supporting local primary production. 

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

Authors:

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

Hanna Farnelid,   ()

Wilken von Appen,   ()

Mar Benavides,   ()

Olivier Grosso,   ()

Christien Laber,   ()

Johanna Schüttler,   ()

Marcus Sundbom,   ()

Sinhué Torres-Valdés,   ()

Stefan Bertilsson,   ()

Ilka Peeken,   ()

Pauline Snoeijs-Leijonmalm,   ()

Lasse Riemann,   ()