Isotopic signatures of eelgrass (Zostera marina L.) as bioindicator of anthropogenic nutrient input in the western Baltic Sea

•Anthropogenic nitrogen (N) inputs are a global problem, but difficult to quantify.•We tested the use of eelgrass δ15N as proxy of such inputs in the Baltic Sea.•The method revealed distinct spatial patterns in sewage N across a eutrophic bay.•Traditional eutrophication measures corroborated the results from δ15N values.•Eelgrass δ15N ratios have high potential as proxy of sewage-derived N in the Baltic.

Eutrophication is a global environmental problem. Better management of this threat requires more accurate assessments of anthropogenic nitrogen (N) inputs to coastal systems than can be obtained with traditional measures. Recently, primary producer N isotopic signatures have emerged as useful proxy of such inputs. Here, we demonstrated for the first time the applicability of this method using the widespread eelgrass (Zostera marina) in the highly eutrophic Baltic Sea. Spatial availability of sewage N across a bay with one major sewage outflow predicted by eelgrass δ15N was high near and downstream of the outflow compared to upstream, but returned to upstream levels within 4 km downstream from the outfall. General conclusions were corroborated by traditional eutrophication measures, but in contrast to these measures were fully quantitative. Eelgrass N isotope ratios therefore show high potential for coastal screens of eutrophication in the Baltic Sea, and in other areas with eelgrass meadows.

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