Anaerobic N2 production in Arctic sea ice

Abstract

We quantified anaerobic N2 production through bacterial denitrification and anaerobic NH4+ 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 NO3−, NH4+, and dissolved organic carbon (DOC). Despite sea‐ice algal production in the lower sea‐ice layers, heterotrophic activity resulted in a net O2 consumption of 13 µmol O2 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 NO3− reduction. Numbers of anaerobic NO3−‐reducing bacteria in the same ice layers were high (1.1 × 105 cells ml−1 sea ice, corresponding to 1.2 × 106 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 N2 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 NO3− reduction at the investigated localities.