Denitrification activity and oxygen dynamics in Arctic sea ice

Denitrification and oxygen dynamics were investigated in the sea ice of Franklin Bay (70 degrees N), Canada. These investigations were complemented with measurements of denitrification rates in sea ice from different parts of the Arctic (69 degrees N-85 degrees N). Potential for bacterial denitrification activity (5-194 mu mol N m(-2) day(-1)) and anammox activity (3-5 mu mol N m(-2) day(-1)) in melt water from both first-year and multi-year sea ice was found. These values correspond to 27 and 7%, respectively, of the benthic denitrification and anammox activities in Arctic sediments. Although we report only potential denitrification and anammox rates, we present several indications that active denitrification in sea ice may occur in Franklin Bay (and elsewhere): (1) despite sea ice-algal primary production in the lower sea ice layers, heterotrophic activity resulted in net oxygen consumption in the sea ice of 1-3 mu mol l(-1) sea ice per day at in situ light conditions, suggesting that O-2 depletion may Occur prior to the spring bloom. (2) The ample organic carbon (DOC) and NO3- present in sea ice may support an active denitrification population. (3) Measurements of O-2 conditions in melted sea ice cores showed very low bulk concentrations, and in some cases anoxic conditions prevailed. (4) Laboratory studies using planar optodes for measuring the high-resolution two-dimensional O-2 distributions in sea ice confirmed the very dynamic and heterogeneous O-2 distribution in sea ice, displaying a mosaic of microsites of high and low O-2 concentrations. Brine enclosures and channels were strongly O-2 depleted in actively melting sea ice, and anoxic conditions in parts of the brine system would favour anaerobic processes.