Nitrate source identification in the Baltic Sea using its isotopic ratios in combination with a Bayesian isotope mixing model

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.