Stable isotope signals of eutrophication in Baltic Sea sediments

Abstract

Increasing δ 15N and δ 13C values in coastal Baltic marine sediments are evaluated as indicator of changes in the trophic status of the ecosystem. The influence of eutrophication on the δ 15N values was found to be so dominant that it even overprints the usually observed mixing gradient from terrestrial (low isotope values) to the marine environment (high isotope values). A distinct gradient in stable nitrogen isotope values from eutrophic coastal areas to open more oligotrophic waters in the central Baltic Sea and Gulf of Bothnia was found. Our data show high δ 15N values in surface sediments: 13‰ in the Oder Lagoon and the Pomeranian Bight, over 9‰ in the Gulf of Riga (Daugava River), 7‰ in the inner Gulf of Finland (Neva River), 6.5‰ in the Curonian Lagoon (Nemunas River), and 5.7‰ in the Gdansk Deep (Vistula River). In the Baltic Sea Proper, significantly lower δ 15N values of 3–5‰ are found. A decrease in δ 15N values with depth/age of the sediment was indicated in some cores that were analyzed down to 15–40 cm depth in 1-cm steps. There is a great overall difference between pre-industrial δ 15N values in coastal sediments and recent ones of 2.3–10‰. As explanations for this increase are suggested, elevated nutrient δ 15N values of waste water in combination with fractionation processes like nutrient uptake by phytoplankton and denitrification and nitrification processes in rivers discharging into the coastal water. Delta 13C values in sediment surfaces off the river estuaries primarily indicate differences between the inorganic carbon signatures of the rivers. However, since the δ 13C values also decrease downcore, we contribute this change to increased primary production caused by the enhanced nutrient load. Since both stable isotope values in sediments ( δ 13C and δ 15N) correlate downcore, this strongly suggests that the anthropogenic nutrient loads in the rivers might be the reason for the changes of stable isotope values.