Zooplankton communities across the Nansen and Amundsen Basins of the Arctic Ocean

Abstract

Abstract The Arctic Ocean remains one of the least studied ecosystems due to its remoteness and persistent sea ice. Ongoing warming and Arctic amplification may lead to an ice-free 'blue ocean' by 2050. In the Eurasian sector, the Nansen and Amundsen Basins, separated by the Gakkel Ridge, host distinct water masses that shape pelagic habitats. We examined zooplankton diversity across both basins, sampling a broad size range from meso- to macrozooplankton at five depths. Mesozooplankton diversity did not differ significantly between stations but varied by depth in relation to water-mass structure, with generally higher diversity at depth, especially in the Amundsen Basin. Abundance peaked in the southern Nansen Basin near the continental slope and Atlantic inflow. Small cyclopoid copepods, particularly Oithona similis and Oncaeidae, dominated across basins. Calanus copepods accounted for over 50% of mesozooplankton biomass, with the Arctic species Calanus hyperboreus and Calanus glacialis contributing far more than the North Atlantic Calanus finmarchicus, which was largely confined to the Nansen Basin. The northern Amundsen Basin supported fewer, less biomass-rich mesozooplankton but showed greater taxonomic diversity. Macrozooplankton communities also differed: chaetognaths were more abundant in the Nansen Basin, while amphipods and euphausiids dominated the Amundsen Basin. Gelatinous zooplankton biomass was similar between basins, though species composition varied. Historical comparisons (1975–2007) indicate small shifts in community structure, including the presence of previously unobserved Atlantic taxa such as Themisto compressa and Triconia conifera, and the absence of ice-associated species such as Gammarus wilkitzkii and Eusirogenes arctica. Our results provide a snapshot of present-day zooplankton communities in a rapidly changing Arctic gateway, highlighting the influence of water-mass origin and shelf–slope transitions on community structure.