Rich to ridges – Diversity and composition of benthic Isopoda along a bathymetric gradient from shelf to abyss into the Norwegian Sea

The Nordic Seas, bounded by the North Atlantic and the Arctic Ocean, are pivotal for global ocean circulation and climate regulation. The region’s unique geological features, particularly the Greenland-Iceland-Faroe (GIF) Ridge, create distinct environmental conditions characterized by varying water masses and seabed temperatures. This study investigates benthic isopod crustaceans of the Nordic Seas along a bathymetric gradient from the continental shelf to the abyss, with a particular focus on the Ægir Ridge. During the IceAGE-3 (SO276/MerMet17-06) expedition onboard RV Sonne in boreal summer 2020 samples were collected using a modified epibenthic sledge at depths ranging from 681 to 3,702 m along the ridge. These data were complemented by previously published data from the region to create a comprehensive transect spanning from the crest of the GIF Ridge to abyssal depths of the Ægir Ridge. Our results indicate that isopod diversity was highest at 400–800 m, corresponding to the thermocline, and decreases toward the abyssal Nordic Seas, likely due to the region’s energy-poor and isolated nature. Distance-based redundancy analysis (dbRDA) indicated that community composition was primarily structured by depth and bottom temperature, while other environmental variables had weaker effects. The study also underscores persistent taxonomic challenges in identifying deep-sea species, such as presence of species complexes, potentially leading to underestimated biodiversity. This highlights the pressing need for improved taxonomic resources in deep-sea biodiversity research. Overall, this research enhances our understanding of deep-sea biodiversity and sheds light on the complex interplay of environmental factors shaping these remote ecosystems.