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Between 3rd February and 30th March 2021 the research icebreaker RV Polarstern conducted an extensive multidisciplinary research expedition across the southern Weddell Sea, Antarctica. During the Continental Shelf Multidisciplinary Flux Study (COSMUS) expedition (designated research expedition PS124) the Ocean Floor Observation and Bathymetry System (OFOBS) was deployed 20 times to collect high resolution seafloor image and acoustic data with a 26-megapixel resolution camera, HD (high definition) video camera, forward facing acoustic camera and multibeam side-scan system from heights of between 1.5 and 4.5 m above seafloor. To localise the collected data the OFOBS was equipped with a Posidonia transponder for ultra- short baseline position triangulation, an Inertial Navigation System (INS) and a seafloor tracking Dynamic Velocity Logger (DVL). The 20 deployments were made across contrasting areas of the Weddell Sea, Filchner Trough, Filchner Sill and Filchner Shelf. Here, we present the full image data set collected from across the various deployments, as well as raw side-scan data, and derived high resolution bathymetric maps produced from this acoustic data after the cruise. All images are available from: https://doi.pangaea.de/10.1594/PANGAEA.936205 (Purser et al., 2021b) and https://doi.pangaea.de/10.1594/PANGAEA.932827 (Purser et al., 2021c). Acoustic data for all deployments is also available from PANGAEA, with the raw side scan data available at: https://doi.pangaea.de/10.1594/PANGAEA.939322 (Hehemann et al., 2021b), forward facing acoustic camera data at: https://doi.pangaea.de/10.1594/PANGAEA.939341 (Purser et al., 2021a). Seafloor topographic maps derived from the side scan data are available at: https://doi.pangaea.de/10.1594/PANGAEA.939087 (Hehemann et al., 2021a).

Deep-sea regions provide vast ecosystem services such as biological habitat and nutrient cycling. Even though being threatened by climate change and facing possible biodiversity loss, these deep-sea ecosystems are poorly understood. So are macrobenthic communities and their functions within these ecosystems. Biodiversity and ecosystem function relationships as well as their link to environmental drivers can be assessed with the biological trait analysis. We used this approach for the first time for macrofauna assemblages across the deep Fram Strait between Greenland and Svalbard (1000–5500 m water depth) to evaluate their community-specific function from the upper continental slope down to the deepest known Arctic depression, the Molloy Deep. We aimed to investigate whether there are changes in benthic functioning along the bathymetric gradient and if so, which environmental stressors may drive these changes. In total, 16 stations were sampled with a giant box corer (0.25 m2) in 2016 and 2018. Sediments were sieved through a 0.5 mm mesh size sieve and fauna was identified to lowest possible taxonomic entity. Functions of species were characterized by using six traits split in 24 modalities gathered in a fuzzy coded species × traits array. Environmental parameters shaping the benthic habitat and reflecting food availability were gathered from remote sensing, mooring deployments, and sediment sampling. A distance-based redundancy analysis indicated near-bottom water temperature, seabed inclination, water depth as well as phytodetritial matter at the sea surface and seafloor (indicating food availability) to be the best variables explaining the trait and station distribution. Stations clustered into three groups based on their trait composition. Shallower stations characterized by high chlorophyll a concentration with large organisms, living within the sediment as well as predating specimens clustered in one group. A second group was characterized by stations with low chlorophyll a concentration and medium-sized, suspension feeding, epifaunal living macrofauna. A third group comprised stations with water depths ≥ 3000 m and was dominated by medium sized, surface deposit feeding and infaunal living specimens. Overall, the functional structure of macrofauna communities in the Fram Strait followed a food availability-driven gradient. Based on the relationship between sea ice, surface water primary production and food availability at the seafloor, these results point to macrobenthos being sensible to predicted anthropogenically generated environmental variations in polar regions. Alterations in benthic ecosystem functions might be expected when environmental conditions change.