No other region has warmed as rapidly in the past decades as the Arctic. Funded by the British Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF), the Changing Arctic Carbon cycle in the Coastal Ocean Near-shore (CACOON) project investigates how this warming influences Arctic coastal-marine ecosystems. Arctic rivers annually carry around 13% of the globally transported dissolved organic carbon (despite the Arctic Ocean making up only approx. 1% of the Earth's ocean volume). Arctic shelf waters are therefore dominated by terrestrial organic carbon pools, so that shelf ecosystems are intimately linked to freshwater supplies. Arctic ecosystems also contain permafrost organic carbon that may be released with warming. Climate change already thaws permafrost, reduces sea ice and increases riverine discharge, triggering important feedbacks. The importance of the near-shore region, consisting of several tightly connected ecosystems that include rivers, deltas, and the shelf, is however often overlooked. Year-round studies are scarce but needed to predict the impact of shifting seasonality, fresher water, changing nutrient supply and greater proportions of permafrost-derived organic carbon on coastal waters. The aims of the CACOON project are to quantify the effect of changing freshwater export and permafrost thaw on the type and fate of river-borne organic matter (OM) delivered to the Arctic shore and resulting changes on ecosystem functioning in the coastal Arctic Ocean. We are achieving this through a combined observational, experimental, and modelling approach. We conduct laboratory experiments to parameterise the susceptibility of terrigenous organic carbon to abiotic and biotic transformation and losses, then use the results from these to deliver a marine ecosystem model capable of representing major biogeochemical cycles. We apply this model to assess how future changes to freshwater runoff and carbon fluxes alter the ecosystems. To reach these aims, we conducted 4 field campaigns in 2019 in the Lena (see https://epic.awi.de/id/eprint/53575/) and Kolyma Delta region. In the Lena Delta, during spring we were using a mobile camp on sledges to collect water samples, ice cores, surface sediments, gas samples as well as CTD profiles. A permafrost cliff (Sobo-Sise) was sampled to analyse terrestrial endmembers of organic matter entering the deltaic and eventually marine system following erosion and transport. During the summer campaign we retrieved samples along a 200 km transect from the centre of the Delta to the Laptev Sea covering the fresh-salt water transition. The aim of Kolyma field sampling was to capture the open water season from the ice breakup to re-freezing and sample the Kolyma River and the near shore area. The lab work on these samples is currently ongoing with first papers lead by CACOON or with project contributions being published already (available here: https://www.researchgate.net/project/CACOON-Changing-Arctic-Carbon-cycle-in-the-coastal-ocean-near-shore).