We investigated the production, consumption, and exchange of O 2 in melting sea ice to assess the biological-and physical-induced O 2 turnover. The underside of the ice was covered with 5–20 cm 3 large, buoyant algal aggregates. Their gross primary production amounted to 0.49 mmol C m 22 d 21 , which was 4.5 times higher than the primary production of sea ice–encrusted microalgae (0.11 mmol C m 22 d 21). The phototrophic biomass of the aggregates (2.94 mg chlorophyll a m 22) was six times higher than that encountered in the sea ice itself. Taxono-specific 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 communities were responsible for an overall O 2 consumption of 0.32 mmol m 22 d 21 . 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 m 22 d 21 , but no surface melt ponds were formed. Aquatic eddy correlation (EC) measurements on the underside of the ice revealed a light-dependent O 2 exchange rate. However, the integrated signal resolved a net O 2 uptake of 7.70 mmol m 22 d 21 . The net O 2 exchange was therefore dominated by the production of O 2 -depleted meltwater rather than biological activity. The EC technique represents a new non-invasive tool for O 2 studies in sea ice communities.