No other region has warmed as much or as rapidly in the past decades as the Arctic. A new project, CACOON, investigates how the ecosystems are influenced by this warming. Funded by the British Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF), CACOON will help to better predict changes to the Arctic coastal-marine environment. Arctic rivers (Fig. 1) annually carry around 13% of all dissolved organic carbon transported globally from land to ocean, despite the Arctic Ocean making up only approximately 1% of the Earth's ocean volume. Arctic shelf waters are therefore dominated by terrestrial carbon pools, so that shelf ecosystems are intimately linked to freshwater supplies. Arctic ecosystems also contain perennially frozen carbon that may be released by further warming. Climate change already thaws permafrost, reduces sea-ice and increases riverine discharge over much of the pan-Arctic, triggering important feedbacks (Mann et al. 2015). The importance of the near-shore region, consisting of several tightly connected ecosystems that include rivers, deltas, estuaries and the continental shelf, is however often overlooked. We need year-round studies to be able to predict the impact of shifting seasonality, fresher water, changing nutrient supply and greater proportions of permafrost-derived carbon on coastal waters