Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 3 4 2006 10.5194/bg-3-663-2006 http://www.biogeosciences.net/3/663/2006/ http://www.biogeosciences.net/3/663/2006/bg-3-663-2006.html http://www.biogeosciences.net/3/663/2006/bg-3-663-2006.pdf 663 676 2006-12-20 Source identification of nitrate by means of isotopic tracers in the Baltic Sea catchments M. Voss maren.voss@io-warnemuende.de B. Deutsch R. Elmgren C. Humborg P. Kuuppo M. Pastuszak C. Rolff U. Schulte Baltic Sea Research Institute, Seestr. 15, 18119 Rostock, Germany Stockholm University, Department of Systems Ecology, 10691 Stockholm, Sweden Stockholm University, Department of Applied Environmental Science, 10691 Stockholm, Sweden Finnish Environment Institute, P.O. Box 140, 00251 Helsinki, Finland Sea Fisheries Institute, Kołłątaja 1, 81-332 Gdynia, Poland Ruhr Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany 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. 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