Stable isotope signals of eutrophication in Baltic Sea sediments

Increasing delta N-15 and delta C-13 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 delta N-15 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 delta N-15 values in surface sediments: 13 parts per thousand in the Oder Lagoon and the Pomeranian Eight, over 9 parts per thousand in the Gulf of Riga (Daugava River), 7 parts per thousand in the inner Gulf of Finland (Neva River), 6.5 parts per thousand in the Curonian Lagoon (Nemunas River), and 5.7 parts per thousand in the Gdansk Deep (Vistula River). In the Baltic Sea Proper, significantly lower delta N-15 values of 3-5 parts per thousand are found. A decrease in delta N-15 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 delta N-15 values in coastal sediments and recent ones of 2.3-10 parts per thousand. As explanations for this increase are suggested, elevated nutrient delta N-15 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 C-13 Values in sediment surfaces off the river estuaries primarily indicate differences between the inorganic carbon signatures of the rivers. However, since the delta C-13 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 (delta C-13 and delta N-15) correlate downcore, this strongly suggests that the anthropogenic nutrient loads in the rivers might be the reason for the changes of stable isotope values. (C) 2000 Elsevier Science B.V. All rights reserved.