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

• Main Authors: F. Korth, B. Deutsch, C. Frey, C. Moros, M. Voss
• Format: Article
• Language: English
• Published: Copernicus Publications 2014-09-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/11/4913/2014/bg-11-4913-2014.pdf
• ISSN: 1726-4170; 1726-4189

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.