Anthropogenic nitrogen input traced by means of δ15N values in macroalgae: Results from in-situ incubation experiments

The macroalgae species Fucus vesiculosus (Phaeophyta), Polysiphonia sp., and Ceramium rubrum (Rhodophyta) originally grown at an unpolluted brackish site of the southern Baltic Sea were incubated for 10 and 14 days at 12 stations along a salinity gradient in a highly polluted estuary. We have expected an adaptation of the initially low δ15N values to the higher ones within the incubation period. In addition to the macroalgae the δ15N values of NO3− were measured to evaluate fractionation processes of the source nitrate. Inside the estuary, δ15N-NO3− values were 6.2–9.7‰, indicating anthropogenic nitrogen sources. The red macroalgae adequately reflected the nitrate isotope values in the surrounding waters, whereas for F. vesiculosus the results were not that clear. The reasons were assumed to be higher initial δ15N values of F. vesiculosus and presumably a too slow nitrogen uptake and growth rate. The method of macroalgae incubations seems suitable as a simple monitoring to study the influence of anthropogenic nitrogen loading in an estuarine environment.