Anaerobic [N.sub.2] production in Arctic sea ice

We quantified anaerobic [N.sub.2] production through bacterial denitrification and anaerobic N[H.sup.+.sub.4] oxidation (anammox) in first-year ice from Young Sound (74[degrees]N) and in an ice floe off Northeast Greenland (79[degrees]N). Bacterial denitrification activity (100-300 nmol N [L.sup.-1] sea ice [d.sup.-1]) occurred in the lower 0.5 m of the sea ice, which had high concentrations of N[O.sup.-. sub.3], N[H.sup.+.sub.4], and dissolved organic carbon (DOC). Despite sea-ice algal production in the lower sea-ice layers, heterotrophic activity resulted in a net [O.sub.2] consumption of 13 [micro]mol [O.sub.2] [L.sup.-1] sea ice [d.sup.-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 N[O. sup.-.sub.3] reduction. Numbers of anaerobic N[O.sup.-.sub.3]-reducing bacteria in the same ice layers were high (1.1 x [10.sup.5] cells [ml. sup.-1] sea ice, corresponding to 1.2 x [10.sup.6] cells [ml.sup.-1] brine). Area-integrated denitrification rates were 10-45 [micro]mol N [m.sup.-2] sea ice [d.sup.-1], which corresponds to 7-50% of the sediment activity in the area. Although the proportion of anammox to total [N.sub.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 N[O.sup.-.sub.3] reduction at the investigated localities.