On May 19, 2015, the German research icebreaker Polarstern began a six-week expedition to the Arctic Ocean initiated by the “ART” team, which stands for “Arctic in Rapid Transition”. The expedition PS92 (ARK XXIX/1) “TRANSSIZ” (Transitions in the Arctic Seasonal Sea Ice Zone, Fig. 1.1) conducted ecological and biogeochemical early spring process studies from the shelf to the basins of the European Arctic margin and on the Yermak Plateau, in order to link past and present sea-ice transitions in the Arctic Ocean. The cruise involved scientists from eleven countries in collaboration with research groups from divisions of the Alfred Wegener InstituteHelmholtz Centre for Polar and Marine Research together with scientists from the German BMBF-project ‘Transdrift’, as well as from the French-Canadian projects ‘GreenEdge’. Overall the vessel travelled 3,665 sm and carried out 68 stations with a total of 242 casts. Some groups also used the transit to the research area to study meridional variability of trace gases, algae species and nutrients from temperate regions of the North Atlantic and into the ice-covered Arctic Ocean by using the surface online water system. Once reaching the investigation area, the science parties conducted process studies for rate measurements of productivity, ecosystem interactions and carbon- and nitrogen cycling. By comparing data from the shelf, data from across the shelf-break into the deep basin it was possible to compare carbon export from plankton and sea ice communities as well as identifying the potential characteristics in carbon production, the fate and export, and to identify similarities and differences in ecosystem functioning along topography-, sea ice- and water mass-related gradients. The ice stations (Fig. 1.2) involved coring of a standard set of sea-ice cores for biological, physical and chemical variables as well as for trace gases and geological proxy validation. It further involved the study of sea ice properties and under-ice water and covered the study of trace and greenhouse gases, biodiversity, primary and bacterial production as well as a detailed study of the nitrogen cycle. Short-term moorings were deployed under the ice to determine the vertical carbon flux. A small Remotely Operated Vehicle (ROV) was operated under the ice to focus on spectral radiation measurements, but also to record environmental parameters (e.g. ice thickness, salinity, temperature) and video imaging of the under-ice environment. Light transmission measured with the ROV showed an increase of light penetration during the course of our expedition. The under-ice fauna and other environmental parameters were investigated by using the towed “Surface Under Ice Trawl” (SUIT). Helicopter flights were used to determine the large-scale distribution of sea-ice thickness with an EM-bird along the cruise track, which overall revealed that the average sea-ice thickness was 1.4 m, that is, comparatively thin and similar to summer vales previously observed in Fram Strait. During the sea-ice stations, parallel sampling of pelagic and benthic ecosystems and geological cores were conducted. Light spectra of hyper-spectral radiometers were used to establish the penetration depth of ultraviolet radiation into the different types of oceanic waters from the zodiac and under the ice. Water samples were taken from the water bottles of the CTD rosette to study the chemistry, biology and various geological proxies. A UVP (Underwater Video Profiler System) was deployed to provide detailed vertical profiles of particle distribution, size composition and the zooplankton community. Quantitative sampling of the mesozooplankton, and also foraminifera, which are used as paleo-proxies, were carried out by using multi-nets. For macrozooplankton and nekton, a Multiple-closing Rectangular Midwater Trawl (MRMT) was used. The distribution of macrozooplankton and pelagic fish was monitored continuously on selected transects with Polarstern’s EK60 echosounder. Benthic communities were collected by box corers and the material was further used for experimental and biogeochemical analyses of the benthic surface sediment layers, including sea ice- and paleo proxies. Benthic communities show a clear shelf to basin decrease in the oxygen demand. Overall, the communities also show a clear North South gradient in our study area. TV-Multi-corers for geological measurements were used to get undisturbed core tops of near-surface sediments (Fig. 1.3). For the geological coring, detailed bathymetric mapping and sub-bottom profiling systems (Hydrosweep and Parasound) were used to find suitable coring positions for Kastenlot and Gravity cores. Special emphasis on this cruise was taken to quantify the environmental preconditions for productivity (e.g. nutrients, stratification) which will allow us to be able to improve predictions of the potential annual primary production in a future ice-free Arctic Ocean, as well as improving reconstructions of productivity, sea ice and ocean circulation across the last 1-2 last glacial cycles. Results from the cruise will further improve the understanding of ecosystem functioning and biogeochemical cycles during the transition from spring to summer. The expedition ended on the morning of the 28th June 2015 in Longyearbyen.