East Siberian Sea, an Arctic region of very high biogeochemical activity

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<sub>2</sub>) 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<sub>2</sub> illustrating the dominance of marine primary production. The drawdown of dissolved inorganic carbon equals a primary production of ~0.8 ± 2 mol C m<sup>−2</sup>, which when multiplied by half the area of the East Siberian Sea, ~500 000 km<sup>2</sup>, results in an annual primary production of 0.4 (± 1) × 10<sup>12</sup> mol C or ~4 (± 10) × 10<sup>12</sup> 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<sub>2</sub> and low oxygen and pH were observed in the bottom waters. Another signature of organic matter decomposition, methane (CH<sub>4</sub>), was 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<sub>2</sub> as well as oxidation of CH<sub>4</sub> to CO<sub>2</sub> 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

Extent and Causes of Chesapeake Bay Warming

Coastal environments such as the Chesapeake Bay have long been impacted by eutrophication stressors resulting from human activities, and these impacts are now being compounded by global warming trends. However, there are few studies documenting long-term estuarine temperature change and the relative contributions of rivers, the atmosphere, and the ocean. In this study, Chesapeake Bay warming, since 1985, is quantified using a combination of cruise observations and model outputs, and the relative contributions to that warming are estimated via numerical sensitivity experiments with a watershed–estuarine modeling system. Throughout the Bay’s main stem, similar warming rates are found at the surface and bottom between the late 1980s and late 2010s (0.02 +/- 0.02C/year, mean +/- 1 standard error), with elevated summer rates (0.04 +/- 0.01C/year) and lower rates of winter warming (0.01 +/- 0.01C/year). Most (~85%) of this estuarine warming is driven by atmospheric effects. The secondary influence of ocean warming increases with proximity to the Bay mouth, where it accounts for more than half of summer warming in bottom waters. Sea level rise has slightly reduced summer warming, and the influence of riverine warming has been limited to the heads of tidal tributaries. Future rates of warming in Chesapeake Bay will depend not only on global atmospheric trends, but also on regional circulation patterns in mid-Atlantic waters, which are currently warming faster than the atmosphere. Supporting model data available at: https://doi.org/10.25773/c774-a36

Fluxes and transformation of carbon in the Siberian shelf seas under changing environment

The Arctic is especially vulnerable to the increased air temperature caused by emissions of greenhouse gases to the atmosphere, carbon dioxide being one of them. In this thesis, both fieldwork and modelling of the East Siberian Arctic Shelf (the Laptev Sea and the East Siberian Sea) have been carried out to investigate the carbon system in this region. Fieldwork in 2008 revealed two distinct hydrological regimes in the East Siberian Sea, one in the western area and one in the eastern area. The western area is dominated by freshwater from rivers flowing into the East Siberian Sea but also from the Lena River plume coming from the Laptev Sea. Nearly all waters in this area are supersaturated with respect to carbon dioxide compared to the atmosphere due to mineralization of substantial amounts of terrestrial organic matter coming from thawing permafrost and coastal erosion. This excess carbon dioxide may be a potential source to the atmosphere and thus increase the atmospheric greenhouse gas content, a positive feedback mechanism. The eastern area is dominated by inflow of Pacific-derived waters that are clear, salty and nutrient rich and therefore favour primary productivity. Phytoplankton consumes carbon dioxide that lowers its partial pressure (pCO2) making it undersaturated compared to the atmosphere and the Eastern East Siberian Sea becomes a sink for atmospheric carbon dioxide. In addition, the Laptev Sea had supersaturated pCO2 equal to an excess of dissolved inorganic carbon of around ~5 1012 gC, which is in the same order as for the Western East Siberian Sea. This excess is also a potential source of carbon dioxide to the atmosphere that could enhance climate change. Modelling work with a one-dimensional, time dependent coupled physical biogeochemical model confirms this conclusion for the late summer when the pCO2 in the seawater increases due to mineralization and water mixing. Model simulations for the Laptev Sea were utilized to investigate the net annual sea-air flux caused by different forcings; doubled atmospheric partial pressure of carbon dioxide; 4oC air temperature increase; doubling the concentration in the runoff of dissolved organic carbon or nutrients; increasing the river discharge by 25 %; increasing the wind speed by 10 % or a combination of these forcings. The result revealed decreasing uptake of carbon dioxide when changing the river properties except for the increase of nutrients when the uptake of carbon dioxide increased. The uptake also increased with the changed forcings in air temperature, wind speed and atmospheric partial pressure of carbon dioxide, separately and in combination

A model sensitivity study for the sea-air exchange of methane in the Laptev Sea, Arctic Ocean

The ocean's sinks and sources determine the concentration of methane in the water column and by that regulating the emission of methane to the atmosphere. In this study, we investigate how sensitive the sea-air exchange of methane is to increasing/decreasing sinks and sources as well as changes of different drivers with a time-dependent biogeochemical budget model for one of the shallow shelf sea in the Siberian Arctic, the Laptev Sea. The applied changes are: increased air temperature, river discharge, wind, atmospheric methane, concentration of nutrients in the river runoff or flux of methane from the sediment. Furthermore, simulations are performed to examine how the large range in observations for methane concentration in the Lena River as well as the rate of oxidation affects the net sea-air exchange. In addition, a simulation with five of these changes applied together was carried out to simulate expected climate change at the end of this century. The result indicates that none of the simulations changed the seawater to becoming a net sink for atmospheric methane and all simulations except three increased the outgassing to the atmosphere. The three exceptions were: doubling the atmospheric methane, decreasing the rivers' concentration of methane and increasing the oxidation rate where the latter is one of the key mechanisms controlling emission of methane to the atmosphere