Nitrogen cycle of the Baltic Sea from an isotopic perspective

Surface sediment data (0-1cm) of the entire Baltic Sea comprised of organic carbon, total nitrogen and respective stable isotope data that have been used for source allocation of nitrogen. Sediments of the shallow near-coastal area of the southern and eastern Baltic Sea have an average delta15N of 7.3 ± 2.1%, interpreted as a characteristic trace of residual anthropogenic nitrogen delivered by rivers and diffuse runoff. In contrast, delta15N values in sediments deposited in the basins of the central Baltic Sea are depleted (average d15N of 3.5 ± 0.6%), indicating a significant contribution by diazotrophic cyanobacteria.