Articles | Volume 11, issue 17
Research article
15 Sep 2014
Research article |  | 15 Sep 2014

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

F. Korth, B. Deutsch, C. Frey, C. Moros, and M. Voss

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 NO315N-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.

Final-revised paper