Anaerobic N₂ production in Arctic sea ice

We quantified anaerobic N 2 production through bacterial denitrification and anaerobic NH 4 + oxidation (anammox) in first‐year ice from Young Sound (74°N) and in an ice floe off Northeast Greenland (79°N). Bacterial denitrification activity (100–300 nmol N L −1 sea ice d −1 ) occurred in the lower 0.5 m of the sea ice, which had high concentrations of NO 3 − , NH 4 + , and dissolved organic carbon (DOC). Despite sea‐ice algal production in the lower sea‐ice layers, heterotrophic activity resulted in a net O 2 consumption of 13 µmol O 2 L −1 sea ice d −1 in the lower 0.5‐m ice layers. Together with melting of deoxygenated ice crystals, this led to anoxic conditions in the brine system favoring conditions for anaerobic NO 3 − reduction. Numbers of anaerobic NO 3 − ‐reducing bacteria in the same ice layers were high (1.1 × 10 5 cells ml −1 sea ice, corresponding to 1.2 × 10 6 cells ml −1 brine). Area‐integrated denitrification rates were 10–45 µmol N m −2 sea ice d −1 , which corresponds to 7–50% of the sediment activity in the area. Although the proportion of anammox to total N 2 production was up to 19% in layers of the ice floe from the Greenland Sea, the integrated rate only accounted for 0–5% of total NO 3 − reduction at the investigated localities.