Description: The data has been collected during the expedition "Multidisciplinary drifting Observatory for the Study of Arctic Climate" (MOSAiC) from September 2019 to September 2020 on research vessel Polarstern. The dataset contains abundance of sea ice protists, including ice algae (autotrophic) and protozoa (heterotrophic). Protists were identified and counted with light microscopy using the Utermöhl method and the result are given as cells per liter (cells/L) called Abundance. Sea ice samples were collected with a 9 cm diameter ice corer (Kovacs Enterprise) from both level and ridge ice. The samples were collected from the bottom part of the ice core and generally sectioned from 0-3 cm, 3-10 cm and in 10 cm intervals thereafter. With some exceptions, ice core sections were melted in filtered sea water at 4°C. Melted samples were preserved using Lugol-formaldehyde mixture with a few drops of acidic Lugol solution and hexamethylenetetramine-buffered formalin at a final concentrations of 1%.

Description: The data has been collected during the the year-long drift expedition "Multidisciplinary drifting Observatory for the Study of Arctic Climate" (MOSAiC) from September 2019 to September 2020 on research vessel Polarstern. The samples were collected with Niskin bottles attached to a CTD rosette, an Apstein net with 20 µm mesh size, a hand pump or a pump mounted on a ROV. The samples were preserved using a few drops of Lugol and hexamethylenetetramine-buffered formalin at a final concentration of 1%. The samples were collected with Niskin bottles attached to a CTD rosette at the following depths: 5, 10, 30, 60, 90 m and deep chlorophyll max (DCM). Protists were identified and counted with light microscopy using the Utermöhl method and the result are given as cells per liter (cells/L) called Abundance.

Description: The data has been collected during the year-long drift expedition "Multidisciplinary drifting Observatory for the Study of Arctic Climate" (MOSAiC) from September 2019 to September 2020 on research vessel Polarstern. The dataset contains abundance of pelagic marine and sea ice protists, including algae (autotrophic) and protzoa (heterotrophic). Protists were identified and counted with light microscopy using the Utermöhl method and the result are given as cells per liter (cells/L) called Abundance. The samples were collected with short-term sediment traps deployed at 3-4 depths (1, 5, 15 and 50 m) below level ice and near sea-ice ridges. The samples were preserved with a few drops of Lugol and hexamethylenetetramine-buffered formalin at a final concentrations of 1%.

Description: We investigated diets of 24 Barents Sea zooplankton taxa to understand pelagic food-web processes during late summer, including the importance of sea ice algae-produced carbon. This was achieved by combining insights derived from multiple and complementary trophic marker approaches to construct individual aspects of feeding. Specifically, we determined proportions of algal-produced fatty acids (FAs) to reflect the reliance on diatom- versus dinoflagellate-derived carbon, highly branched isoprenoid (HBI) lipids that distinguish between ice-associated and pelagic carbon sources, and sterols to indicate the degree of carnivory. Copepods had the strongest diatom signal based on FAs, while a lack of sea ice algae-associated HBIs (IP25, IPSO25) suggested that they fed on pelagic rather than ice-associated diatoms. The amphipod Themisto libellula and the ctenophores Beroë cucumis and Mertensia ovum had a higher contribution of dinoflagellate-produced FAs. There was a high degree of carnivory in this food web, as indicated by the FA carnivory index 18:1(n−9)/18:1(n−7) (mean value 〈 1 only in the pteropod Clione limacina), the presence of copepod-associated FAs in most of the taxa, and the absence of algal-produced HBIs in small copepod taxa, such as Oithona similis and Pseudocalanus spp. The coherence between concentrations of HBIs and phytosterols within individuals suggested that phytosterols provide a good additional indication for algal ingestion. Sea ice algae-associated HBIs were detected in six zooplankton species (occurring in krill, amphipods, pteropods, and appendicularians), indicating an overall low to moderate contribution of ice-associated carbon from late-summer sea ice to pelagic consumption. The unexpected occurrence of ice-derived HBIs in pteropods and appendicularians, however, suggests an importance of sedimenting ice-derived material at least for filter feeders within the water column at this time of year.