[1] Climate change is expected to have a strong effect on the Eastern Siberian Arctic Shelf (ESAS) region, which includes 40% of the Arctic shelves and comprises the Laptev and East Siberian seas. The largest organic carbon pool, the dissolved organic carbon (DOC), may change significantly due to changes in both riverine inputs and transformation rates; however, the present DOC inventories and transformation patterns are poorly understood. Using samples from the International Siberian Shelf Study 2008, this study examines for the first time DOC removal in Arctic shelf waters with residence times that range from months to years. Removals of up to 10%-20% were found in the Lena River estuary, consistent with earlier studies in this area, where surface waters were shown to have a residence time of approximately 2 months. In contrast, the DOC concentrations showed a strong nonconservative pattern in areas with freshwater residence times of several years. The average losses of DOC were estimated to be 30%-50% during mixing along the shelf, corresponding to a first-order removal rate constant of 0.3 yr −1 . These data provide the first observational evidence for losses of DOC in the Arctic shelf seas, and the calculated DOC deficit reflects DOC losses that are higher than recent model estimates for the region. Overall, a large proportion of riverine DOC is removed from the surface waters across the Arctic shelves. Such significant losses must be included in models of the carbon cycle for the Arctic Ocean, especially since the breakdown of terrestrial DOC to CO 2 in Arctic shelf seas may constitute a positive feedback mechanism for Arctic climate warming. These data also provide a baseline for considering the effects of future changes in carbon fluxes, as the vast northern carbon-rich permafrost areas draining into the Arctic are affected by global warming. Citation: Alling, V., et al. (2010), Nonconservative behavior of dissolved organic carbon across the Laptev and East Siberian seas, Global Biogeochem. Cycles, 24, GB4033,

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The summer heat wave in 2018 led to the highest recorded water temperatures since 1926 -up to 21 • C -in bottom coastal waters of the Baltic Sea, with implications for the respiration patterns in these shallow coastal systems. We applied cavity ringdown spectrometer measurements to continuously monitor carbon dioxide (CO 2 ) and methane (CH 4 ) surface-water concentrations, covering the coastal archipelagos of Sweden and Finland and the open and deeper parts of the Northern Baltic Proper. This allowed us to (i) follow an upwelling event near the Swedish coast leading to elevated CO 2 and moderate CH 4 outgassing, and (ii) to estimate CH 4 sources and fluxes along the coast by investigating water column inventories and air-sea fluxes during a storm and an associated downwelling event. At the end of the heat wave, before the storm event, we found elevated CO 2 (1583 µatm) and CH 4 (70 nmol/L) concentrations. During the storm, a massive CO 2 sea-air flux of up to 274 mmol m −2 d −1 was observed. While water-column CO 2 concentrations were depleted during several hours of the storm, CH 4 concentrations remained elevated. Overall, we found a positive relationship between CO 2 and CH 4 wind-driven sea-air fluxes, however, the highest CH 4 fluxes were observed at low winds whereas highest CO 2 fluxes were during peak winds, suggesting different sources and processes controlling their fluxes besides wind. We applied a box-model approach to estimate the CH 4 supply needed to sustain these elevated CH 4 concentrations and the results suggest a large source flux of CH 4 to the water column of 2.5 mmol m −2 d −1 . These results are qualitatively supported by acoustic observations of vigorous and widespread outgassing from the sediments, with flares that could be traced throughout the water column penetrating the pycnocline and reaching the sea surface. The results suggest that the heat wave triggered CO 2 and CH 4 fluxes in the coastal zones that are comparable with maximum emission rates found in other hot spots, such as boreal and arctic lakes and wetlands. Further, the results suggest that heat waves are as important for CO 2 and CH 4 sea-air fluxes as the ice break up in spring.

We are using the coupled models in a decision support system, Nest, to evaluate the response of the marine ecosystem to changes in external loads through various management options. The models address all the seven major marine basins and the entire drainage basin of the Baltic Sea. A series of future scenarios have been developed, in close collaboration with the Helsinki Commission, to see the possible effects of improved wastewater treatment and manure handling, phosphorus-free detergents, and less intensive land use and live stocks. Improved wastewater treatment and the use of phosphorus-free detergents in the entire region would drastically decrease phosphorus loads and improve the marine environment, particularly the occurrence of cyanobacterial blooms. However, the Baltic Sea will remain eutrophic, and to reduce other effects, a substantial reduction of nitrogen emissions must be implemented. This can only be obtained in these scenarios by drastically changing land use. In a final scenario, we have turned 50% of all agricultural lands into grasslands, together with efficient wastewater treatments and a ban of phosphorus in detergents. This scenario will substantially reduce primary production and the extension of hypoxic bottoms, increase water transparency in the most eutrophied basins, and virtually eliminate extensive cyanobacterial blooms.