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The Multi-disciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), was conducted from October 2019 to September 2020. The Research Vessel Polarstern was frozen into the ice in the Central Arctic Ocean north of Norway and drifted with the prevailing currents from north to south, traversing multiple Arctic basins and regimes and serving as a floating laboratory for an international, multidisciplinary program focusing on multiple facets of ice, ocean, atmosphere, biogeochemistry, and ecosystem responses to ongoing changing environmental conditions. Zooplankton ecology was investigated as part of the ecosystem team program. Abundance data from the area has been limited, particularly for the winter season. Weekly net tows were scheduled with multiple nets (mesh sizes 53, 150 and 1000 microns (µm)) to add to that data. This data set contains zooplankton abundance data (individuals per cubic meter) for the 53 µm ring net samples, sampling location and net depth information. Abundant zooplankton were identified to species and stage, including nauplii. Less abundant specimens were identified to subgroup, genus, group or family. 

The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition was conducted from October 2019-September 2020. During this ~1 year period, the Research Vessel (R/V) Polarstern was frozen into the ice in the Central Arctic Ocean north of Norway and drifted with the prevailing currents from north to south, traversing multiple Arctic basins and regimes, and was re-located in late July to near the North Pole after drifting through Fram Strait. The ship served as a floating laboratory for an international, multidisciplinary program focusing on multiple facets of ice, ocean, atmosphere, biogeochemistry, and ecosystem responses to ongoing changing environmental conditions. Zooplankton ecology was investigated as part of the ecosystem team program. Here we present data on key zooplankton morphological and compositional parameters collected over the period of the drift. This data set contains the carbon and nitrogen content (micrograms [µg]) and lengths for individuals or groups of calanoid copepods and other taxa (e.g., amphipods, chaetognaths), width (micrometers [µm]) for copepods, and body area (micrometers squared [µm2]) and lipid sac area (µm2) for Calanus spp. copepods collected in different water depth intervals at approximately weekly intervals during the period of the drift. 

Abstract Polar cod (Boreogadus saida) is an endemic key species of the Arctic Ocean ecosystem. The ecology of this forage fish is well studied in Arctic shelf habitats where a large part of its population lives. However, knowledge about its ecology in the central Arctic Ocean (CAO), including its use of the sea‐ice habitat, is hitherto very limited. To increase this knowledge, samples were collected at the under‐ice surface during several expeditions to the CAO between 2012 and 2020, including the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The diet of immatureB. saidaand the taxonomic composition of their potential prey were analysed, showing that both sympagic and pelagic species were important prey items. Stomach contents included expected prey such as copepods and amphipods. Surprisingly, more rarely observed prey such as appendicularians, chaetognaths, and euphausiids were also found to be important. Comparisons of the fish stomach contents with prey distribution data suggests opportunistic feeding. However, relative prey density and catchability are important factors that determine which type of prey is ingested. Prey that ensures limited energy expenditure on hunting and feeding is often found in the stomach contents even though it is not the dominant species present in the environment. To investigate the importance of prey quality and quantity for the growth ofB. saidain this area, we measured energy content of dominant prey species and used a bioenergetic model to quantify the effect of variations in diet on growth rate potential. The modeling results suggest that diet variability was largely explained by stomach fullness and, to a lesser degree, the energetic content of the prey. Our results suggest that under climate change, immatureB. saidamay be at least equally sensitive to a loss in the number of efficiently hunted prey than to a reduction in the prey's energy content. Consequences for the growth and survival ofB. saidawill not depend on prey presence alone, but also on prey catchability, digestibility, and energy content. 

The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition was conducted from October 2019-September 2020. During this ~1 year period, the Research Vessel (R/V) Polarstern was frozen into the ice in the Central Arctic Ocean north of Norway and drifted with the prevailing currents from north to south, traversing multiple Arctic basins and regimes, and was re-located in late July to near the North Pole after drifting through Fram Strait. The ship served as a floating laboratory for an international, multidisciplinary program focusing on multiple facets of ice, ocean, atmosphere, biogeochemistry, and ecosystem responses to ongoing changing environmental conditions. Zooplankton ecology was investigated as part of the ecosystem team program. Here we present data on key zooplankton rate processes collected over the period of the drift including: respiration, feeding, and reproduction. 

Abundance (ind. m-3) of zooplankton taxa was calculated from samples of Polarstern cruise PS122 (MOSAiC). Samples were taken with a Ring net with an opening area of 0.79 m2 and a mesh size of 1000 µm. Samples were analysed via image-based ZooScan analysis. The classified images are available at the web application EcoTaxa: https://ecotaxa.obs-vlfr.fr/prj/9966. 

The apparently obligate symbiosis between the diazotroph Candidatus Atelocyanobacterium thalassa (UCYN-A) and its haptophyte host, Braarudosphaera bigelowii , has recently been found to fix dinitrogen (N 2 ) in polar waters at rates (per cell) comparable to those observed in the tropical/subtropical oligotrophic ocean basins. This study presents the novel observation that this symbiosis increased in abundance during a wind-driven upwelling event along the Alaskan Beaufort shelfbreak. As upwelling relaxed, the relative abundance of B. bigelowii among eukaryotic phytoplankton increased most significantly in waters over the upper slope. As the host’s nitrogen demands are believed to be supplied primarily by UCYN-A, this response suggests that upwelling may enhance N 2 fixation as displaced coastal waters are advected offshore, potentially extending the duration of upwelling-induced phytoplankton blooms. Given that such events are projected to increase in intensity and number with ocean warming, upwelling-driven N 2 fixation as a feedback on climate merits investigation.