Description: Sea ice continues to decline across many regions of the Arctic, with remaining ice becoming increasingly younger and more dynamic. These changes alter the habitats of microbial life that live within the sea ice, which support healthy functioning of the marine ecosystem and provision of resources for human-consumption, in addition to influencing biogeochemical cycles (e.g. air–sea CO2 exchange). With the susceptibility of sea ice ecosystems to climate change, there is a pressing need to fill knowledge gaps surrounding sea ice habitats and their microbial communities. Of fundamental importance to this goal is the development of new methodologies that permit effective study of them. Based on outcomes from the DiatomARCTIC project, this paper integrates existing knowledge with case studies to provide insight on how to best document sea ice microbial communities, which contributes to the sustainable use and protection of Arctic marine and coastal ecosystems in a time of environmental change.

Description: Ocean warming and acidification will be most pronounced in the Arctic. Both phenomena severely threaten thecosome pteropods (holoplanktonic marine gastropods) by reducing their survival (warming) and causing the dissolution of their aragonitic shell (acidification). Lipids, particularly phospholipids, play a major role in veligers and juveniles of the polar thecosome pteropod Limacina helicina comprising more than two-thirds of their total lipids. Membrane lipids (phospholipids) are important for the temperature acclimation of ectotherms. Hence, we experimentally investigated ocean warming and acidification effects on total lipids, lipid classes, and fatty acids of Arctic early-stage L. helicina. The temperature and pCO2 treatments chosen resembled Representative Concentration Pathway model scenarios for this century. We found a massive decrease in total lipids at elevated temperatures and at the highest CO2 concentration (1,100 matm) of the in situ temperature. Clearly, temperature was the overriding factor. Total lipids were reduced by 47%–70%, mainly caused by a reduction of phospholipids by up to 60%. Further, based on pHT development in the incubation water of pteropods during the experiment, some evidence exists for metabolic downregulation in pteropods at high factor levels of temperature and pCO2. Consequently, the cell differentiation and energy balance of early-stage larvae were probably severely compromised. Comparison of our experimental with ‘wild’ organisms suggests phospholipid reduction to values clearly outside natural variability. Based on the well-known significance of phospholipids for membranogenesis, early development, and reproduction, negative warming effects on such a basal metabolic function may be a much more immediate threat for pteropods than so far anticipated shell dissolution effects due to acidification. KEYWORDS ocean warming, ocean acidification, arctic, pteropods, lipids

Description: 〈jats:p〉Consumer regulation of lipid composition during assimilation of dietary items is related to their ecology, habitat, and life cycle, and may lead to extra energetic costs associated with the conversion of dietary material into the fatty acids (FAs) necessary to meet metabolic requirements. For example, lipid-rich copepods from temperate and polar latitudes must convert assimilated dietary FAs into wax esters, an efficient type of energy storage which enables them to cope with seasonal food shortages and buoyancy requirements. Lipid-poor copepods, however, tend to not be as constrained by food availability as their lipid-rich counterparts and, thus, should have no need for modifying dietary FAs. Our objective was to test the assumption that 〈jats:italic〉Temora longicornis〈/jats:italic〉, a proxy species for lipid-poor copepods, does not regulate its lipid composition. Isotopically-enriched (〈jats:sup〉13〈/jats:sup〉C) diatoms were fed to copepods during a 5-day laboratory experiment. Compound-specific stable isotope analysis of algae and copepod samples was performed in order to calculate dietary FA assimilation, turnover, and assimilation efficiency into copepod FAs. Approximately 65% of the total dietary lipid carbon (C) assimilated (913 ± 68 ng C ind〈jats:sup〉-1〈/jats:sup〉 at the end of the experiment) was recorded as polyunsaturated FAs, with 20 and 15% recorded as saturated and monounsaturated FAs, respectively. As expected, 〈jats:italic〉T. longicornis〈/jats:italic〉 assimilated dietary FAs in an unregulated, non-homeostatic manner, as evidenced by the changes in its FA profile, which became more similar to that of their diet. Copepods assimilated 11% of the total dietary C (or 40% of the dietary lipid C) ingested in the first two days of the experiment. In addition, 34% of their somatic growth (in C) after two days was due to the assimilation of dietary C in FAs. Global warming may lead to increased proportions of smaller copepods in the oceans, and to a lower availability of algae-produced essential FAs. In order for changes in the energy transfer in marine food webs to be better understood, it is important that future investigations assess a broader range of diets as well as lipid-poor zooplankton from oceanographic areas throughout the world’s oceans.〈/jats:p〉

Description: While the Arctic Ocean is changing rapidly, we have barely begun to understand the dynamics of animal life under the permanent sea ice, as it exists today. During the MOSAiC expedition, RV Polarstern was moored to an ice floe and drifted more than 3,000 km across the central Arctic Ocean, enabling multidisciplinary observations of the inter-linked processes in atmosphere, sea ice, ocean and ecosystem. We studied year-round changes in diversity, abundance, vertical distribution, physiology, and ontogeny of Arctic ectotherms from the pack ice at the surface into the deep ocean. Imaging profilers show the fine-scale distribution of zooplankton at high resolution, how the vertical distribution and aggregation of different species change with season, and how zooplankton species prepare for reproduction already in the deepest winter. Systematic sampling with nets shows that the pelagic food web was active from the under-ice habitat down to bathypelagic depths throughout the winter, supporting a variety of predators, such as amphipods, polar cod (Boreogadus saida), and the understudied diversity of jellyfish. The first-ever hydroacoustic survey of the Transpolar Drift recorded the change in pelagic biomass in time and space, and highlights brief periods of diel vertical migration in spring and autumn. Video surveys with a Remotely Operated Vehicle (ROV) documented the use of the under-ice habitat by polar cod and various jellyfishes. A new under-ice net mounted on the ROV can provide new insights in the connection of the life cycles of sympagic amphipods with the seasonal change of sea-ice- and water column properties. We collected thousands of samples for the analysis of condition, physiological parameters, food preference, microplastic, and trophic biomarkers, and conducted numerous rate process measurements including respiration, feeding and reproduction, for key species with the goal of unravelling the sources and fate of carbon in the food web. The first results demonstrate how sampling techniques from the days of Nansen in combination with modern technology can unfold a comprehensive picture of the contribution of Arctic fauna to ecosystem functioning and biogeochemical cycles, as well as their resilience and potential changes in a future seasonally ice-covered Arctic Ocean. We will present and discuss first results and conclusions emerging from our data with regard to the scientific objectives of MOSAiC.

Description: Microalgae growing within and attached to the bottom of Arctic sea ice (sympagic algae) can serve as a nutritious food resource for animals inhabiting the sea-ice water interface (under-ice fauna), particularly during the bottom ice-algal bloom in spring. As a consequence, under-ice fauna is likely impacted by sea-ice decline and changes in ice-algal primary production. To investigate this, samples of pelagic (=PPOM) and ice-associated particulate organic matter (=IPOM) and the ice-associated amphipods Apherusa glacialis and Eusirus holmii, and polar cod (Boreogadus saida), collected below ridged sea ice at two locations with pronounced differences in productivity in the northern Barents Sea during May 2021, were assessed for their trophic marker content. Specifically, we investigated the composition of diatom- and dinoflagellate-produced fatty acids (FAs), pelagic and sympagic highly branched isoprenoid (HBI) lipids as well as sterols to determine the animals’ dietary preferences and trophic association to the sea-ice habitat during spring. Relative proportions of FAs differed strongly between PPOM and IPOM, indicating differences in species composition and degradation state between pelagic and sympagic habitats, respectively. FA signatures and sterol content of the consumers largely resembled known diet compositions with a strong reliance on diatom-derived carbon in A. glacialis, a higher degree of carnivory in E. holmii and evidence of Calanus-feeding in polar cod. Sympagic HBIs were detected at either low concentrations or not at all, in both producers and consumers, likely as a result of the very low abundance of their source diatoms. Pronounced trophic marker variability in A. glacialis collected at the highly productive shelf slope station versus the less productive central Arctic Basin station suggests a surprisingly high flexibility in carbon-source composition with a stronger reliance on pelagic food when available versus a higher importance of ice algal carbon when pelagic production is low. Nevertheless and despite the general lack (below detection limit) of sympagic HBIs in our dataset, high ice-algal biomass and elevated proportions of polyunsaturated FAs in IPOM compared to other seasons indicate that ice algae constitute a valuable nutritional carbon source as alternative to pelagic carbon during spring.