The under-ice habitat constitutes a critical survival hub for zooplankton in the Central Arctic Ocean

The consequences of sea-ice decline for zooplankton survival in the Central Arctic Ocean (CAO) remain poorly understood, limiting predictions of ecosystem resilience. During the MOSAiC expedition (November 2019 - September 2020), we conducted the first year-round sampling of under-ice zooplankton (Ice-water interface and 10 m depth) along the Transpolar Drift using a remotely operated vehicle equipped with a zooplankton net (ROVnet). Multivariate analyses identified three spatio-seasonal community regimes: autumn and winter regimes in the Amundsen Basin, and a spring regime in both the Amundsen and Nansen Basin. Variations in taxon-specific abundances and biomass patterns indicate that the under-ice habitat shifts in function from a feeding and sheltering habitat in autumn and winter to a nursery habitat in spring. During the polar night, estimated particulate carbon supply, likely originating from sea ice, only marginally met community carbon demand. This deficit was partly offset by high trophic transfer efficiency inferred by normalized biomass size spectra (NBSS). This implies that the under-ice habitat enables zooplankton survival during the polar night at their very energetic limit, suggesting that changes in the duration and extent of sea-ice coverage will be critical for sustaining the future ecosystem functionality of the CAO.