Publication Date:
2014-04-24
Description:
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.
Print ISSN:
1810-6277
Electronic ISSN:
1810-6285
Topics:
Biology
,
Geosciences
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