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 δ13C 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.
Nitrogen (N) retention and transformation in the Szczecin Lagoon, southern Baltic Sea, were studied by means of budget calculations and stable isotope data of dissolved and particulate matter. Two stations, one located at the main outlet of the lagoon (Swina Strait) and the other 100 km to the south, on the Oder River (Widuchowa), were sampled biweekly over the years 2000-2002. The Oder River is one of the five largest rivers draining into the Baltic Sea and the largest one discharging its waters into the western Baltic. According to our data, the Oder River carried approximately 60 kt y(-1) total N, of which 7 kt y(-1) (<12 %) are particulate organic nitrogen and 46 kt y(-1) (77 %) dissolved inorganic nitrogen. Seasonal patterns of particulate nitrogen and nitrate concentrations were similar at Widuchowa and Swina Strait station, but nitrate concentrations in the Swina Strait were much lower, pointing not only to the dilution effect but also to considerable nutrient removal capacity (especially of nitrate) in the lagoon. The loss of nitrate suggests that denitrification is the major N-removal process, whereas primary production was only a minor contributor, due to the very low particle load. Combining budget calculations with stable isotope measurements reveal unique information about nitrogen turnover processes in the lagoon.
Nitrogen (N) inputs from human activities have led to ecological deteriorations in large parts of the coastal oceans along European coastlines, including harmful algae blooms and anoxia.
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 four-fold over the last 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 46 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 (30–70%), whereas in the northern Baltic, i.e., the Gulf of Bothnia, NO3− originates from nitrification in pristine soils (47–100%). 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 31–62% 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.
Abstract. Nitrate input to a river is largely controlled by land use in its catchment. We compared the information carried by the isotopic signatures of nitrate in 12 Baltic rivers, in relation to the vegetation cover, land use, and fertilization of agricultural land of their catchments. We found isotope values in nitrate ranging from −2 to 14‰ for δ15N and 8 to 25‰ for δ18O. The annual variability of riverine nitrate isotope signatures is presented in detail for one Nordic, the Kemijoki, and two southern rivers, the Vistula and Oder. Nordic rivers with relatively pristine vegetation in their catchments show not only low δ15N values and high δ18O-NO3− but also lower annual variability than rivers draining densely populated land. Seasonal signals were found in all the rivers. We used load weighted nitrate isotope data and data from the three major N sources (farmland/sewage, atmospheric deposition and from runoff of pristine soils) to theoretically estimate the shares of nitrate from these sources. The results of an isotope mixing model (IMM-1) agree reasonably well with the same estimates for agricultural land derived from a Global Land Cover (GLC) data base, with a deviation varying from −16% to +26%. The comparison with an emission model (EM) reveals relatively good agreements for intensively used catchments (−18 to +18% deviation). Rather unsatisfactory agreement was found between the IMM-1 and GLC calculations for pristine catchments (−36 to +50% deviation). Advantages and limitations of the tested model are discussed.
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