Year-round observations of the physical snow and ice properties and processes that govern the ice pack evolution and its interaction with the atmosphere and the ocean were conducted during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition of the research vessel Polarstern in the Arctic Ocean from October 2019 to September 2020. This work was embedded into the interdisciplinary design of the 5 MOSAiC teams, studying the atmosphere, the sea ice, the ocean, the ecosystem, and biogeochemical processes. The overall aim of the snow and sea ice observations during MOSAiC was to characterize the physical properties of the snow and ice cover comprehensively in the central Arctic over an entire annual cycle. This objective was achieved by detailed observations of physical properties and of energy and mass balance of snow and ice. By studying snow and sea ice dynamics over nested spatial scales from centimeters to tens of kilometers, the variability across scales can be considered. On-ice observations of in situ and remote sensing properties of the different surface types over all seasons will help to improve numerical process and climate models and to establish and validate novel satellite remote sensing methods; the linkages to accompanying airborne measurements, satellite observations, and results of numerical models are discussed. We found large spatial variabilities of snow metamorphism and thermal regimes impacting sea ice growth. We conclude that the highly variable snow cover needs to be considered in more detail (in observations, remote sensing, and models) to better understand snow-related feedback processes. The ice pack revealed rapid transformations and motions along the drift in all seasons. The number of coupled ice–ocean interface processes observed in detail are expected to guide upcoming research with respect to the changing Arctic sea ice.

This review provides an overview of prey preferences of seven core Arctic marine mammal species (AMM) and four non‐core species on a pan‐Arctic scale with regional examples. Arctic marine mammal species exploit prey resources close to the sea ice, in the water column, and at the sea floor, including lipid‐rich pelagic and benthic crustaceans and pelagic and ice‐associated schooling fishes such as capelin and Arctic cod. Prey preferred by individual species range from cephalopods and benthic bivalves to Greenland halibut. A few AMM are very prey‐, habitat‐, and/or depth‐specific (e.g., walrus, polar bear), while others are rather opportunistic and, therefore, likely less vulnerable to change (e.g., beluga, bearded seal). In the second section, we review prey distribution patterns and current biomass hotspots in the three major physical realms (sea ice, water column, and seafloor), highlighting relations to environmental parameters such as advection patterns and the sea ice regime. The third part of the contribution presents examples of documented changes in AMM prey distribution and biomass and, subsequently, suggests three potential scenarios of large‐scale biotic change, based on published observations and predictions of environmental change. These scenarios discuss (1) increased pelagic primary and, hence, secondary production, particularly in the central Arctic, during open‐water conditions in the summer (based on surplus nutrients currently unutilized); (2) reduced benthic and pelagic biomass in coastal/shelf areas (due to increased river runoff and, hence, changed salinity and turbidity conditions); and (3) increased pelagic grazing and recycling in open‐water conditions at the expense of the current tight benthic–pelagic coupling in part of the ice‐covered shelf regions (due to increased pelagic consumption vs. vertical flux). Should those scenarios hold true, pelagic‐feeding and generalist AMM might be advantaged, while the range for benthic shelf‐feeding, ice‐dependent AMM such as walrus would decrease. New pelagic feeding grounds may open up to AMM and subarctic marine mammal species in the High Arctic basins while nearshore waters might provide less abundant food in the future.