Sea-ice ridges and their role in the Arctic sea-ice ecosystem – new findings from the MOSAiC expedition

Sea-ice ridges (or more precisely, deformed ice) constitute a large fraction of the Arctic sea-ice volume, probably order of 30-50%. Yet, we know disproportionally little about their role in the coupled system. In situ studies of ridges are logistically challenging, and as a result, most research has focused on level sea ice. Here, we highlight findings from the MOSAiC expedition that showcase the dynamic nature of the sea-ice physical enviroment and this the ridge as habitat and their potential contributions to sea-ice biomass and biodiversity.

Unique habitats within ridges, including surfaces of ice blocks in association with water-filled voids, showed distinct differences in biological properties compared to level ice bottom and pelagic biota. These ridge-specific habitats had unique protist and bacterial assemblages contributing to the high diversity and richness found in Arctic sea ice. Winter observations indicate that ridge formation can release organic material from level ice to the water column, a potential energy source for pelagic grazers during the polar night. In summer, ridges can be hotspots of protist biomass and can contain 80±20% of sea-ice algal biomass - a contribution not considered in Arctic assessments. During the melt season, consolidation (the freezing of water-filled voids), reduced habitable space inside ridges for algae and larger biota, ransforming species assemblages while simultaneously increasing bacterial activity with unknown consequences for the role of ridges in the Arctic carbon cycle.

We argue that sea-ice ridges must be included in assessments of Arctic Ocean biodiversity and biogeochemistry to fully understand the Arctic sea-ice ecosystem and its response to ongoing changes. Future efforts should not only investigate the complex physical and biological processes within sea-ice ridges but also integrate these processes into models. Only then we can predict how the changes in the Arctic icescape will affect atmosphere-ice-ocean-ecosystem interactions.