The Siberian shelf seas cover large shallow areas that receive substantial amounts of river discharge. The river runoff contributes nutrients that promote marine primary production, but also dissolved and particulate organic matter. The coastal regions are built up of organic matter in permafrost that thaws and result in coastal erosion and addition of organic matter to the sea. Hence there are multiple sources of organic matter that through microbial decomposition result in high partial pressures of CO2 in the shelf seas. By evaluating data collected from the Laptev and East Siberian Seas in the summer of 2008 we compute an excess of DIC equal to 10 · 1012 g C that is expected to be outgassed to the atmosphere and suggest that this excess mainly is caused by terrestrial organic matter decomposition.

[1] The upper halocline of the Arctic Ocean has a distinct chemical signature with high nutrient concentrations as well as low oxygen and pH values. This signature is formed in the Chukchi and East Siberian seas, by a combination of mineralization of organic matter and release of decay products to the sea ice brine enriched bottom water. Salinity and total alkalinity data show that the fraction of sea ice brine in the nutrient-enriched upper halocline water in the central Arctic Ocean is up to 4%. In the East Siberian Sea the bottom waters with exceptional high nutrient concentration and low pH have typically between 5 and 10% of sea ice brine as computed from salinity and oxygen-18 values. On the continental slope, over bottom depths of 150-200 m, the brine contribution was 6% at the nutrient maximum depth (50-100 m). At the same location as well as over the deeper basin the silicate maximum was found over a wider salinity range than traditionally found in the Canada Basin, in agreement with earlier observations east of the Chukchi Plateau. A detailed evaluation of the chemical and the temperature-salinity properties suggests at least two different areas for the formation of the nutrient-rich halocline within the East Siberian Sea. This has not been observed before 2004 and it could be a sign of a changing marine climate in the East Siberian Sea, caused by more open water in the summer season followed by more sea ice formation and brine production in the fall/winter.

Abstract. Shelf seas are among the most active biogeochemical marine environments and the East Siberian Sea is a prime example. This sea is supplied by seawater from both the Atlantic and Pacific Oceans and has a substantial input of river runoff. All of these waters contribute chemical constituents, dissolved and particulate, but of different signatures. Sea ice formation during the winter season and melting in the summer has a major impact on physical as well as biogeochemical conditions. The internal circulation and water mass distribution is significantly influenced by the atmospheric pressure field. The western region is dominated by input of river runoff from the Laptev Sea and an extensive input of terrestrial organic matter. The microbial decay of this organic matter produces carbon dioxide (CO 2 ) that oversaturates all waters from the surface to bottom relative to atmospheric level, even when primary production, inferred from low surface water nutrients, has occurred. The eastern surface waters were under-saturated with respect to CO 2 illustrating the dominance of marine primary production. The drawdown of dissolved inorganic carbon equals a primary production of ∼0.8 ± 2 mol C m −2 , which when multiplied by half the area of the East Siberian Sea, ∼500 000 km 2 , results in an annual primary production of 0.4 (± 1) × 10 12 mol C or ∼4 (± 10) × 10 12 gC. Microbial decay occurs through much of the water column, but dominates at the sediment interface where the majority of organic matter ends up, thus more of the decay products are recycled to the bottom water. High nutrient concentrations and fugacity of CO 2 and low oxygen and pH were observed in the bottom waters. Another signature of organic matter decomposition, methane (CH 4 ), wasCorrespondence to: L. G. Anderson (leifand@chem.gu.se) observed in very high but variable concentrations. This is due to its seabed sources of glacial origin or modern production from ancient organic matter, becoming available due to sub-sea permafrost thaw and formation of so-called taliks. The decay of organic matter to CO 2 as well as oxidation of CH 4 to CO 2 contribute to a natural ocean acidification making the saturation state of calcium carbonate low, resulting in under-saturation of all the bottom waters with respect to aragonite and large areas of under-saturation down to 50 % with respect to calcite. Hence, conditions for calcifying organisms are very unfavorable.

Abstract. Over the past couple of decades it has become apparent that air-land-sea interactions in the Arctic have a substantial impact on the composition of the overlying atmosphere (ACIA, 2004). The Arctic Ocean is small (only ∼4 % of the total World Ocean), but it is surrounded by offshore and onshore permafrost which is thawing at increasing rates under warming conditions, releasing carbon dioxide (CO 2 ) into the water and atmosphere. The Arctic Ocean shelf where the most intensive biogeochemical processes have occurred occupies 1/3 of the ocean. The East Siberian Sea (ESS) shelf is the shallowest and widest shelf among the Arctic seas, and the least studied. The objective of this study was to highlight the importance of different factors that impact the carbon system (CS) as well as the CO 2 flux dynamics in the ESS. CS variables were measured in the ESS in September 2003September and, 2004 and in late August-September 2008. It was shown that the western part of the ESS represents a river-and coastal-erosion-dominated heterotrophic ocean margin that is a source for atmospheric CO 2 . The eastern part of the ESS is a Pacific-water-dominated autotrophic area, which acts as a sink for atmospheric CO 2 .Our results indicate that the year-to-year dynamics of the partial pressure of CO 2 in the surface water as well as the air-sea flux of CO 2 varies substantially. In one year the ESS shelf was mainly heterotrophic and served as a moderate summertime source of CO 2 (year 2004). In another year gross primary production exceeded community respiration in a relatively large part of the ESS and the ESS shelf was only a weak source of CO 2 into the atmosphere (year 2008). It was shown that many factors impact the CS and Correspondence to: I. I. Pipko (irina@poi.dvo.ru) CO 2 flux dynamics (such as river runoff, coastal erosion, primary production/respiration, etc.), but they were mainly determined by the interplay and distribution of water masses that are basically influenced by the atmospheric circulation. In this contribution the air-sea CO 2 fluxes were evaluated in the ESS based on measured CS characteristics, and summertime fluxes were estimated. It was shown that the total ESS shelf is a net source of CO 2 for the atmosphere in a range of 0.4 × 10 12 to 2.3 × 10 12 g C.

Aiming to inform both marine management and the public, coupled environmental-climate scenario simulations for the future Baltic Sea are analyzed. The projections are performed under two greenhouse gas concentration scenarios (medium and high-end) and three nutrient load scenarios spanning the range of plausible socio-economic pathways. Assuming an optimistic scenario with perfect implementation of the Baltic Sea Action Plan (BSAP), the projections suggest that the achievement of Good Environmental Status will take at least a few more decades. However, for the perception of the attractiveness of beach recreational sites, extreme events such as tropical nights, record-breaking sea surface temperature (SST), and cyanobacteria blooms may be more important than mean ecosystem indicators. Our projections suggest that the incidence of record-breaking summer SSTs will increase significantly. Under the BSAP, recordbreaking cyanobacteria blooms will no longer occur in the future, but may reappear at the end of the century in a business-as-usual nutrient load scenario. Keywords Climate change Á Coastal seas Á Cyanobacteria Á Extremes Á Numerical modeling Á Sea surface temperature Electronic supplementary material The online version of this article (