Abstract. To test the hypothesis whether high molecular weight dissolved organic matter (HMW-DOM) in a high latitude marginal sea is dominated by terrestrial derived matter, 10 stations were sampled along the salinity gradient of the central and northern Baltic Sea and were analyzed for concentrations of dissolved organic carbon as well as δ 13 C values of HMW-DOM. Different end-member-mixing models were applied to quantify the influence of terrestrial DOM and to test for conservative versus non-conservative behavior of the terrestrial DOM in the different Baltic Sea basins. The share of terrestrial DOM to the total HMW-DOM was calculated for each station, ranging from 43 to 83 %. This shows the high influence of terrestrial DOM inputs for the Baltic Sea ecosystem. The data also suggest that terrestrial DOM reaching the open Baltic Sea is not subject to substantial removal anymore. However compared to riverine DOM concentrations, our results indicate that substantial amounts of HMW-DOM (> 50 %) seem to be removed near the coastline during estuarine mixing. A budget approach yielded residence times for terrestrial DOM of 2.8, 3.0, and 4.5 yr for the Bothnian Bay, the Bothnian Sea and the Baltic Proper.

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Abstract. Nitrate (NO3−) is the major nutrient responsible for coastal eutrophication worldwide and its production is related to intensive food production and fossil-fuel combustion. In the Baltic Sea NO3− inputs have increased 4-fold over recent decades and now remain constantly high. NO3− source identification is therefore an important consideration in environmental management strategies. In this study focusing on the Baltic Sea, we used a method to estimate the proportional contributions of NO3− from atmospheric deposition, N2 fixation, and runoff from pristine soils as well as from agricultural land. Our approach combines data on the dual isotopes of NO3− (δ15N-NO3− and δ18O-NO3−) in winter surface waters with a Bayesian isotope mixing model (Stable Isotope Analysis in R, SIAR). Based on data gathered from 47 sampling locations over the entire Baltic Sea, the majority of the NO3− in the southern Baltic was shown to derive from runoff from agricultural land (33–100%), whereas in the northern Baltic, i.e. the Gulf of Bothnia, NO3− originates from nitrification in pristine soils (34–100%). Atmospheric deposition accounts for only a small percentage of NO3− levels in the Baltic Sea, except for contributions from northern rivers, where the levels of atmospheric NO3− are higher. An additional important source in the central Baltic Sea is N2 fixation by diazotrophs, which contributes 49–65% of the overall NO3− pool at this site. The results obtained with this method are in good agreement with source estimates based upon δ15N values in sediments and a three-dimensional ecosystem model, ERGOM. We suggest that this approach can be easily modified to determine NO3− sources in other marginal seas or larger near-coastal areas where NO3− is abundant in winter surface waters when fractionation processes are minor.

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