Biological- and physical-induced oxygen dynamics in melting sea ice of the Fram Strait

Abstract

We investigated the production, consumption, and exchange of O2 in melting sea ice to assess the biological- and 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 m-2 d-1, which was 4.5 times higher than the primary production of sea ice-encrusted microalgae (0.11 mmol C m-2 d-1). The phototrophic biomass of the aggregates (2.94 mg chlorophyll a m-2) 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 O2 consumption of 0.32 mmol m-2 d-1. 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-2 d-1, 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 m-2 d-1. 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. © 2014, by the Association for the Sciences of Limnology and Oceanography, Inc.