The SAS-Oden 2021 expedition (SO21) with icebreaker Oden (IB Oden) is the Swedish contribution to the international scientist-driven initiative ”Synoptic Arctic Survey” (SAS). SAS will collect primary ecosystem data in the Arctic Ocean in 2020-2022 from both icebreaking and non-icebreaking research vessels. The goal of SAS is to generate a comprehensive dataset that allows for an improved characterisation of the Arctic Ocean with respect to its (1) physical oceanography, (2) marine ecosystems and (3) carbon cycle. The complete SAS dataset will provide a unique baseline that will allow for tracking climate change and its impacts as they unfold in the Arctic region over the coming years, decades and centuries.The marine ecosystems of the Arctic Ocean are experiencing rapid change. This includes the Large Marine Ecosystem (LME) in the middle, the Central Arctic Ocean (CAO) as defined by the Arctic Council, i.e., the deep basins and ridges around the North Pole that until recently were permanently covered by 2-3 m thick sea ice. In the past two decades, up to 40% of the 3.3. km² large CAO has been ice-free for a short period in September. This reduction in sea-ice coverage of the CAO is transforming a basically inaccessible marine ecosystem into a new type of ecosystem with seasonal changes in sea ice cover. The CAO is a poorly investigated corner of the World Ocean, especially from an ecosystem perspective, i.e., integrating biological, chemical and physical data. Since the SAS-Oden 2021 expedition was designed as a joint ecosystem study with data collection in an integrated way, the results from this expedition will contribute significantly to the knowledge on ecological baselines of the CAO as well as on ecosystem change. The latter is achieved by comparing areas along the expedition route with different types of sea ice, including both steady and heavily melting multi-year ice.The SAS-Oden 2021 expedition reached further west on the Greenland shelf than any other research expedition has ever done before. Closest was the geological Lomrog III expedition with IB Oden in 2007, but then ice-breaking assistance was needed from the accompanying Russian atomic-driven icebreaker “50 Let Pobedy” to get this far west. Between SO21 stations 42 and 53 (Figure 1.1), the SAS-Oden 2021 expedition was in a completely unexplored area. This is, a.o., illustrated by the fact that the seabed map of this area now needs revision; in one place we recorded 900 m depth instead of 300 m on the existing map while in another place it was 1200 m shallower than indicated on the map. We now have the first on-site measurements of physical environment, carbon cycle and nutrients combined with prokaryote, photosynthetic, zooplankton and fish production and diversity, as well as many other ecosystem parameters, from the area between SO21 stations 30 and 53. The SO21 omics samples form the basis for a unique biodiversity dataset, covering water column, ice habitats, and sediments. These samples include the metagenomes and metatranscriptomes of viruses, archaea, bacteria, protists, and eDNA of multicellular organisms such as zooplankton, fish, squid and mammals.The SO21 joint ecosystem study was carried out by 38 scientists assisted by 15 persons from the Swedish Polar Research Secretariat (SPRS) and the 22-person IB Oden crew. Altogether, the expedition visited 60 sampling stations, of which 36 were ship stations and 24 were ice stations accessed by helicopter (Figure 1.1, Table 1.1). Since the main research project on board (EFICA) included nine “EFICA Master Stations”, lasting for on average 33 hours during which the ship was lying still, there were ample possibilities for ice work from the ship as well.