Abstract. In September 2019, the research icebreaker Polarstern started the largest multidisciplinary Arctic expedition to date, the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) drift experiment. Being moored to an ice floe for a whole year, thus including the winter season, the declared goal of the expedition is to better understand and quantify relevant processes within the atmosphere–ice–ocean system that impact the sea ice mass and energy budget, ultimately leading to much improved climate models. Satellite observations, atmospheric reanalysis data, and readings from a nearby meteorological station indicate that the interplay of high ice export in late winter and exceptionally high air temperatures resulted in the longest ice-free summer period since reliable instrumental records began. We show, using a Lagrangian tracking tool and a thermodynamic sea ice model, that the MOSAiC floe carrying the Central Observatory (CO) formed in a polynya event north of the New Siberian Islands at the beginning of December 2018. The results further indicate that sea ice in the vicinity of the CO (<40 km distance) was younger and 36 % thinner than the surrounding ice with potential consequences for ice dynamics and momentum and heat transfer between ocean and atmosphere. Sea ice surveys carried out on various reference floes in autumn 2019 verify this gradient in ice thickness, and sediments discovered in ice cores (so-called dirty sea ice) around the CO confirm contact with shallow waters in an early phase of growth, consistent with the tracking analysis. Since less and less ice from the Siberian shelves survives its first summer (Krumpen et al., 2019), the MOSAiC experiment provides the unique opportunity to study the role of sea ice as a transport medium for gases, macronutrients, iron, organic matter, sediments and pollutants from shelf areas to the central Arctic Ocean and beyond. Compared to data for the past 26 years, the sea ice encountered at the end of September 2019 can already be classified as exceptionally thin, and further predicted changes towards a seasonally ice-free ocean will likely cut off the long-range transport of ice-rafted materials by the Transpolar Drift in the future. A reduced long-range transport of sea ice would have strong implications for the redistribution of biogeochemical matter in the central Arctic Ocean, with consequences for the balance of climate-relevant trace gases, primary production and biodiversity in the Arctic Ocean.

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Abstract. Climate change is having profound impacts on Arctic ecosystems with important implicationsfor coastal productivity and food web dynamics. We investigated seasonal variations in resource use of 16 invertebrate taxa in lagoon ecosystems along the Alaska Beaufort Sea coast using a combination of fatty acid (FA) biomarkers, bulk stable carbon isotope measurements of whole animals, and compound-specific stable carbon isotope measurements of total lipid extracts and individual FAs. Invertebrates were collected during full-ice cover (April), ice breakup (June), and open water (August) periods. Amphipods (Onisimus glacialis) had higher proportions of 18:2n-6 and 18:3n-3 FAs in April than in the other months. These elevated markers were accompanied by relatively low bulk and 18:2n-6 δ 13 C values, indicating proportionally higher contributions from terrestrial/freshwater sources in April. A wider range of invertebrates examined during June and August showed increases in algae-specific markers and higher proportions of essential FAs (e.g., 22:6n-3 [docosahexaenoic acid] and 20:5n-3 [eicosapentaenoic acid]) later in the summer. There were also marked differences in FA characteristics among invertebrates that highlighted differential feeding modes. For example, proportions of bacterial FAs were generally higher in deposit-feeding invertebrates than in suspension feeders. These results highlight the current role of diverse carbon sources to Arctic coastal food webs, which may change with future warming.