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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.

This study was carried out in the Yenisei estuary and in the adjacent part of the Kara Sea shelf. We analyzed and compared the distribution patterns of the different benthic taxa and size classes (micro-, meio-, macro- and megabenthos) along a pronounced gradient of environmental factors. Four ecological zones were distinguished based on certain environmental parameters, from the upper riverine area to the mouth of the estuary and the Kara Sea slope, which had normal marine conditions. The distribution of the metazoan assemblages generally corresponded to the divisions of these zones and represented a series of species assemblages with coincident boundaries that changed successively along the estuarine gradient. For all three size classes of metazoan benthos, we detected three main species assemblages: brackish-water, transitional and marine. Each taxon showed, however, its own particular spatial pattern and, therefore, has a set of environmental variables that is responsible for its fine-scale distribution. The distribution of unicellular organisms showed a worse match with the division of the estuary into environmental zones. Foraminifers established a single association in most of the estuary but were absent in the upper desalinated part. Diatom algae were patchily distributed, with a number of local communities and many rare species. We conclude that the estuarine habitat appears to drive the distribution of the various benthic taxa in taxa-specific ways. The distribution of size classes (i.e., mega-, macro-, or meiofauna) that combine several high-level taxa demonstrates a better match with the multivariate environmental zonation than the distribution of the separate taxa. The role of different benthic taxa and size classes in biomonitoring studies is discussed.