We investigated the production, consumption, and exchange of O2 in melting sea ice to assess the biologicaland physical-induced O2 turnover. The underside of the ice was covered with 5–20 cm3 large, buoyant algal aggregates. Their gross primary production amounted to 0.49 mmol C m22 d21, which was 4.5 times higher than the primary production of sea ice–encrusted microalgae (0.11 mmol C m22 d21). The phototrophic biomass of the aggregates (2.94 mg chlorophyll a m22) was six times higher than that encountered in the sea ice itself. Taxonospecific investigations strongly suggest that the aggregates were formed from agglutinated algae released from the melting ice. At the prevailing light conditions, the sea ice–encrusted communities were almost at metabolic balance, while the aggregates were net heterotrophic. Together, the two ommunities were responsible for an overall O2 consumption of 0.32 mmol m22 d21. The sea ice–associated communities thereby represent a southward-drifting carbon source that is being exhausted by sea ice–affiliated food webs. The sea ice volume decreased rapidly, releasing meltwater at a rate 25 L m22 d21, but no surface melt ponds were formed. Aquatic eddy correlation (EC) measurements on the underside of the ice revealed a light-dependent O2 exchange rate. However, the integrated signal resolved a net O2 uptake of 7.70 mmol m22 d21. The net O2 exchange was therefore dominated by the production of O2-depleted meltwater rather than biological activity. The EC technique represents a new non-invasive tool for O2 studies in sea ice communities.
Limnology and Oceanography, 2014, Vol 59, Issue 4, p. 1097-1111