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Arctic warming is causing ancient perennially frozen ground (permafrost) to thaw, resulting in ground collapse, and reshaping of landscapes. This threatens Arctic peoples' infrastructure, cultural sites, and land-based natural resources. Terrestrial permafrost thaw and ongoing intensification of hydrological cycles also enhance the amount and alter the type of organic carbon (OC) delivered from land to Arctic nearshore environments. These changes may affect coastal processes, food web dynamics and marine resources on which many traditional ways of life rely. Here, we examine how future projected increases in runoff and permafrost thaw from two permafrost-dominated Siberian watersheds—the Kolyma and Lena, may alter carbon turnover rates and OC distributions through river networks. We demonstrate that the unique composition of terrestrial permafrost-derived OC can cause significant increases to aquatic carbon degradation rates (20 to 60% faster rates with 1% permafrost OC). We compile results on aquatic OC degradation and examine how strengthening Arctic hydrological cycles may increase the connectivity between terrestrial landscapes and receiving nearshore ecosystems, with potential ramifications for coastal carbon budgets and ecosystem structure. To address the future challenges Arctic coastal communities will face, we argue that it will become essential to consider how nearshore ecosystems will respond to changing coastal inputs and identify how these may affect the resiliency and availability of essential food resources.

Forward looking analyses are needed in order to anticipate which policy/management options can deliver the very ambitious objectives of the Zero Pollution (ZP) action plan. Integrated and sophisticated numerical modelling tools are useful to generate future scenarios and ‘what if’ analysis as they allow the virtual manipulation of the anthropogenic pressures on ecological systems. JRC has been developing an integrated modelling framework covering the inland and marine waters of the EU, the Marine Modelling Framework (MMF) that follows the principle of the Digital Twins (DT) and that allow to test the impacts of diverse management strategies on the status of freshwater and marine ecosystems through the EU. In the present report, the JRC-DT for water and marine ecosystems is used to test how different policy options can help achieve some of the ZP objectives. From the six top ambitions of the ZP action plan, two are particularly relevant for the water/marine environments. First, the ZP action plan states that it aims at ‘improving water quality by reducing waste’ and in particular, it mentions the (reduction of) ‘plastic litter at sea (by 50%)’. The second relevant ambition refers to ‘improving soil quality by reducing nutrient losses and chemical pesticides’ use by 50%’, which does not only impact soil quality but also the receiving waters (rivers, lakes and seas).

Arctic rivers deliver ≈11% of global river discharge into the Arctic Ocean, while this ocean represents only ≈1% of the global ocean volume. Ongoing climate warming across the Arctic, and specifically Siberia, has led to regional-scale changes in precipitation patterns, greater rates of permafrost thaw and active layer deepening, as well as enhanced riverbank and coastal erosion. Combined, these climatic and cryospheric perturbations have already resulted in increased freshwater discharge and changes to constituent loads (e.g. dissolved organic carbon - OC) supplied from land to the Arctic Ocean. To date, the majority of studies examining terrestrial organic matter (OM) delivery to the Arctic Ocean have focused almost entirely on freshwater (riverine) or fully-marine environments and been conducted during late summer seasons – often due to logistical constraints. Despite this, an improved understanding of how OC is transformed, mineralised and released during transit through the highly reactive nearshore estuarine environment is critical for examining the fate and influence of terrestrial OM on the Arctic Ocean. Capturing seasonality over the open water period is also necessary to identify current OM fluxes to the ocean vs the atmosphere, and aid in constraining how future changes may modify them. Here we focus upon carbon dioxide (CO2) and methane (CH4) measurements collected during six repeated transects of the Kolyma River and nearshore zone (covering ~120 km) from 2019. Transects spanned almost the entirety of the riverine open water season (June to September). We use these results, in parallel with gas concentrations derived from prior studies, to develop and validate a simple box-model of gas emissions from the nearshore zone. Observations and model‐derived output data reveal that more than 50% of the cumulative gross delivery of CH4 and CO2 to the coastal ocean occurred during the freshet period with dissolved CH4 concentrations in surface water reaching 660

Massive Arctic rivers are feeding ≈11% of the global river discharge into the Arctic Ocean, while the ocean stores only ≈1% of the global ocean volume. The ongoing rapid climate warming has led to pronounced changes in precipitation, active layer thickening, increased air and soil temperatures, increased riverbank and coastal erosion rates, extensive permafrost thaw and increasing freshwater discharge to the Arctic Ocean. Since most studies have focused on rivers or oceans itself and mainly during the late summer, near-shore coastal regions are understudied and crucial in determining the amount of carbon transported and/or released into the Arctic Ocean. Here, we investigated river-derived carbon dioxide (CO2) and methane (CH4) emissions from seven repeated transects of the Kolyma River and nearshore (120 km between Cherskiy and Ambarchik) over the entire open water season between June and September 2019. We estimated the cumulative gross delivery of river-derived CH4 and CO2 to the coastal ocean to be around 0.0008 Tg CH4 (800 000 kg) and 0.2 Tg CO2 (200 000 000 kg). Measurements reveal that more than 50% of the cumulative gross delivery is happening during the fresh period, making the season dynamics extremely important.

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 al

No other region has warmed as rapidly in the past decades as the Arctic. Funded by the British Natural Environment Research Council and the German Federal Ministry of Education and Research, the 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 carbon pools, so that shelf ecosystems are intimately linked to freshwater supplies. Arctic ecosystems also contain permafrost 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 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 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 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 and Kolyma delta region

The Arctic ocean receives 11% of the global river discharge and the Arctic rivers drain large permafrost rich catchments. Where these rivers outflow into the marginal shelf seas of the Arctic ocean the terrestrial dissolved organic matter (tDOM) which they transport has an important role to play in the coastal ecosystem. This tDom is derived from inland permafrost and as it thaws under future climate scenarios there are expected to be changes to both the composition and quantity of riverine tDOM. At the same time there will be changes to the seasonality and magnitude of river discharge, due to increased precipitation and earlier snow melt, and to the light availability, due to reduced seasonal sea ice. To understand the possible impact of these changes on the coastal ecosystem it is important to understand the present role of permafrost derived tDOM and the possible changes to the nearshore circulation. We model the hydrodynamics of the extensive shallow shelf of the Laptev sea, into which drains the Lena river – the 13th largest in the world by discharge. The output from the hydrodynamic model is used to drive the ecosystem model ERSEM which has been adapted to explicitly include a permafrost tDOM input. This coupled model system allows us to investigate both the role of present day tDOM in an Arctic coastal ecosystem and to hypothesise on the impact of increases in future. In particular we attempt to quantify the efficacy of the microbial carbon pump under different tDOM inputs.

The Laptev sea in the Russian Arctic receives organic rich sediments from both erosion of coastal cliffs and from the Lena river, the worlds 13th largest by discharge, which drains a large permafrost rich area of the Asian continent. These sediments differ both in their physical characteristics and in the bioavailability of their carbon. The comparative distribution and residence time of these two sediment types will therefore have an impact on the shelf biogeochemistry. We use the unstructured grid hydrodynamic model FVCOM to model the Laptev sea around the Lena outflow. Output from this model drives a simple model of sediment tracers, with seperate classes for coastal and riverine input. Existing sediment observations are used to calibrate the model setup. This setup is used to study the spatial distribution and residence times of the two sediment sources both under current conditions and under a future scenario reflecting the expected change in freshwater input and ice cover.

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

No other region has warmed as much or as rapidly in the past decades as the Arctic. A new project, CACOON, will investigate 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 also help to better predict changes to the Arctic coastal-marine environment. Arctic rivers 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. 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 CACOON addresses this knowledge gap by investigating the near-shore regions of the two major Arctic rivers, the Lena and Kolyma, which together drain 19% of the pan-Arctic watershed area. CACOON will quantify the effect of changing freshwater export and terrestrial permafrost thaw on the type and fate of river-borne organic matter delivered to Arctic coastal waters, and the resultant changes to ecosystem functioning in the coastal Arctic Ocean. We will achieve this though a combined observational, experimental and modelling study. We will conduct laboratory experiments to parameterize the susceptibility of terrigenous ca