Sea-ice ridges - an understudied yet key component of the Arctic sea-ice system

Sea-ice ridges (or more precisely, deformed ice) constitute a large fraction of the Arctic ice pack, however, estimates range broadly from 30 to 70%. Yet, we know disproportionally little about their role in the Arctic sea-ice system, a system that is in rapid change. In situ studies of ridges are logistically challenging, and as a result, most research has focused on level sea ice, despite the significant proportion of sea ice residing in ridges.Ridges evolve over time and provide an environment for ice growth very different than the typical level ice. Observations during the year-long MOSAiC drift expedition revealed several processes that could contribute to ice growth, even in summer, when most of the ice pack is melting. Indirectly ridges also affect, for example, melt pond formation, and also under-ice spreading of meltwater (under-ice ponds), that  in turn affect the melt rates of level ice. However, these indirect effects of ridges have virtually never been quantified.Rare observations from the MOSAiC expedition explored the unique habitats within sea-ice 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. In summer, ridges can be hotspots of protist biomass and can contain the majority (up to 80%) of sea ice algal biomass - an element not yet considered in Arctic assessments of sea-ice biomass.We argue that sea-ice ridges should 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 can we predict how the changes in the Arctic icescape will affect atmosphere-ice-ocean-ecosystem interactions and how the ongoing changes of the ice pack will affect ridging in the future.