Several aquatic top predators suffer from deficiency in vitamin B1 (thiamine), sometimes combined with low levels of carotenoid pigments, e.g., astaxanthin. The mechanisms leading to correlations between carotenoid pigmentation and thiamine status are not known. These substances and their precursors are produced by single-celled organisms and transferred to higher trophic levels via zooplankton. However, little is known about the factors regulating this transfer process and how it is affected by environmental stressors and zooplankton diet. We therefore exposed a common copepod, Temora longicornis, to ultraviolet radiation (UVR), which is an important environmental stressor, and to food items of different quality in terms of carotenoid profile. Astaxanthin was the most abundant carotenoid found in copepods. Its concentrations were negatively affected by UVR regardless of diet type, and the availability of an astaxanthin precursor (β-carotene) in the diet did not affect the response. Thiamine, on the other hand, showed a varying response, with elevated levels in copepods exposed to UVR at low β-carotene diet and lower levels in copepods exposed to UVR and high β-carotene diet. Altogether, this indicates that astaxanthin was consumed for photoprotection in the zooplankton and that thiamine dynamics might be modulated by UVR under certain dietary conditions. Hence, the concentrations of astaxanthin and thiamine in copepods are dynamic and to some extent regulated by exposure to UVR. Thus, the ability of zooplankton to transfer these substances to higher trophic levels depends, to some extent, on the exposure to environmental stressors.

Nitrogen fixation, the microbial reduction of dinitrogen to ammonia, is increasingly recognized to occur in the Arctic Ocean. However, knowledge about the composition, biogeography, abundance, and ecology of nitrogen-fixing organisms (diazotrophs) is poor. This ultimately hinders the prediction of ecosystem productivity fueled by nitrogen fixation in this rapidly changing and predominantly nitrogen-limited ocean. We assessed the composition and abundance of total and nifH-expressing diazotrophs in subsurface water (8 m; amplicon sequencing and quantification of the marker gene nifH) over ~ 3400 km from the mouth of the brackish Baltic Sea to the sea ice edge in the Arctic Ocean. Upon entering nutrient-rich waters in the Atlantic gateway to the Arctic, we discovered an abrupt transition from autotrophic to heterotrophic diazotrophy (nifH expression). Our findings therefore suggest that diazotrophy is functionally distinct in the Arctic Ocean compared to adjacent temperate-boreal waters—a difference likely driven by inorganic nutrients, salinity, and temperature. We identify three key non-cyanobacterial diazotroph groups in the Arctic Ocean with Arctic-specific (Rhodocyclales and Oceanospirillales) or more widespread (unknown Gammaproteobacterium) distribution patterns and report their nifH gene transcription levels (up to 103 nifH transcripts L−1). In contrast, nifH expression in the warmer and more nutrient-poor Norwegian Sea with coastal-influenced water was dominated by sublineages of Candidatus Atelocyanobacterium thalassa (UCYN-A1, UCYN-A2, UCYN-A4; up to 104 nifH transcripts L−1). With ongoing atlantification of the Arctic pushing oceanic provinces and biogeographical ranges poleward, we predict a future displacement of the transition from autotrophic to heterotrophic diazotrophy with likely significant changes in nitrogen fixation.

Due to climate change, sea ice more commonly retreats over the shelf breaks in the Arctic Ocean, impacting sea ice-pelagic-benthic coupling in the deeper basins. Nitrogen fixation (the reduction of dinitrogen gas to bioavailable ammonia by microorganisms called diazotrophs) is reported from Arctic shelf sediments but is unknown from the Arctic deep sea. We sampled five locations of deep-sea (900–1500 m) surface sediments in the central ice-covered Arctic Ocean to measure potential nitrogen fixation through long-term (> 280 days) stable-isotope (15N2) incubations and to study diazotroph community composition through amplicon sequencing of the functional marker gene nifH. We measured low but detectable nitrogen fixation rates at the Lomonosov Ridge (0.6 pmol N g−1 day−1) and the Morris Jessup Rise (0.4 pmol N g−1 day−1). Nitrogen fixation was observed in sediments with the lowest organic matter content and bacterial abundance, and where sulphate-reducers like Desulfuromonadia and Desulfosporosinus sp. were prominent. Most nifH genes were distantly related to known diazotrophs. In this study, we show a potential for nitrogen fixation in Arctic bathypelagic sediments, considerably extending the known biome of marine nitrogen fixation. It raises the question of the significance of low but potentially widespread nitrogen fixation in deep-sea sediments.

With climate change-induced sea ice decline in the Arctic Ocean, nitrogen is expected to become an increasingly important determinant of primary productivity. Nitrogen fixation is the conversion of molecular nitrogen to bioavailable ammonium by microorganisms called diazotrophs. Here, we report nitrogen fixation rates, diazotroph composition, and expression under different stages of declining sea ice in the Central Arctic Ocean (multiyear ice, five stations) and the Eurasian Arctic (marginal ice zone, seven stations). Nitrogen fixation in the Central Arctic Ocean was positively correlated with primary production, ranging from 0.4 ± 0.1 to 2.5 ± 0.87 nmol N L⁻¹ d⁻¹. Along two transects across the marginal ice zone, nitrogen fixation varied between days and ice regime from below detection up to 5.3 ± 3.65 nmol N L⁻¹ d⁻¹ associated with an ice-edge phytoplankton bloom. We show nitrogen fixation in sea ice-covered waters of the Arctic Ocean and provide insight into present and active non-cyanobacterial diazotrophs in the region.

Information on connectivity and genetic structure of marine organisms remains sparse in frontier ecosystems such as the Arctic Ocean. Filling these knowledge gaps becomes increasingly urgent, as the Arctic is undergoing rapid physical, ecological and socio-economic changes. The abundant and widely distributed polar cod (Boreogadus saida) is highly adapted to Arctic waters, and its larvae and juveniles live in close association with sea ice. Through a reduced-representation sequencing approach, this study explored the spatial genetic structure of polar cod at a circum-Arctic scale. Genomic variation was partitioned into neutral and adaptive components to respectively investigate genetic connectivity and local adaptation. Based on 922 high-quality single nucleotide polymorphism (SNP) markers genotyped in 611 polar cod, broad-scale differentiation was detected among three groups: (i) Beaufort –Chukchi seas, (ii) all regions connected by the Transpolar Drift, ranging from the Laptev Sea to Iceland, including the European Arctic and (iii) West Greenland. Patterns of neutral genetic structure suggested broadscale oceanographic and sea ice drift features (i.e., Beaufort Gyre and Transpolar Drift) as important drivers of connectivity. Genomic variation at 35 outlier loci indicated adaptive divergence of the West Greenland and the Beaufort–Chukchi Seas populations, possibly driven by environmental conditions. Sea ice decline and changing ocean currents can alter or disrupt connectivity between polar cod from the three genetic groups, potentially undermining their resilience to climate change, even in putative refugia, such as the Central Arctic Ocean and the Arctic Archipelago.

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 immature B. saida and 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 of B. saida in 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, immature B. saida may 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 of B. saida will not depend on prey presence alone, but also on prey catchability, digestibility, and energy content.

Knowledge about seafloor depth, or bathymetry, is crucial for various marine activities, including scientific research, offshore industry, safety of navigation, and ocean exploration. Mapping the central Arctic Ocean is challenging due to the presence of perennial sea ice, which limits data collection to icebreakers, submarines, and drifting ice stations. The International Bathymetric Chart of the Arctic Ocean (IBCAO) was initiated in 1997 with the goal of updating the Arctic Ocean bathymetric portrayal. The project team has since released four versions, each improving resolution and accuracy. Here, we present IBCAO Version 5.0, which offers a resolution four times as high as Version 4.0, with 100 × 100 m grid cells compared to 200 × 200 m. Over 25% of the Arctic Ocean is now mapped with individual depth soundings, based on a criterion that considers water depth. Version 5.0 also represents significant advancements in data compilation and computing techniques. Despite these improvements, challenges such as sea-ice cover and political dynamics still hinder comprehensive mapping.

As a result of global warming, the marine ecosystem around the North Pole, the Central Arctic Ocean (CAO), is in fast transition from a permanently to a seasonally ice-covered ocean. The sea-ice loss is expected to enable summer access to the CAO for non-icebreaking ships, including fishery vessels, in the near future. However, the lack of knowledge on the CAO ecosystem impedes any assessment of the sustainability of potential future fisheries in the CAO. Taking a precautionary approach, the EU and nine countries in October 2018 signed the Agreement to Prevent Unregulated High Seas Fisheries in the Central Arctic Ocean. This agreement entered into force in June 2021 and a.o. requires the establishment of a joint scientific program to improve the understanding of the CAO ecosystem, including mapping and monitoring. To reduce the existing lack of knowledge, 12 scientists from the EFICA Consortium participated, together with 26 other on-board scientists, in sampling and data collection of ecosystem data during the Swedish SAS-Oden expedition in summer 2021. This report describes the field work performed by the EFICA scientists using water-column acoustics, deep-sea optical observations, and fish, zooplankton, sediment otolith and eDNA sampling for targeting fish, zooplankton and mammals. Further ecosystem data (physical, chemical and biological) were collected by the EFICA scientists in collaboration with other scientists on-board. Together with this report, a metadata database containing lists of all collected samples and data that are relevant for future fish-stock modelling and assessment studies was delivered to the European Commission.