Stable isotope signals of eutrophication in Baltic Sea sediments

Increasing delta N-15 and delta C-13 values in coastal Baltic marine sedime nts are evaluated as indicator of changes in the trophic status of the ecos ystem. The influence of eutrophication on the delta N-15 values was found t o be so dominant that it even overprints the usually observed mixing gradie nt from terrestrial (low isotope values) to the marine environment (high is otope values). A distinct gradient in stable nitrogen isotope values from eutrophic coasta l 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 sedi ments: 13 parts per thousand in the Oder Lagoon and the Pomeranian Eight, o ver 9 parts per thousand in the Gulf of Riga (Daugava River), 7 parts per t housand in the inner Gulf of Finland (Neva River), 6.5 parts per thousand i n the Curonian Lagoon (Nemunas River), and 5.7 parts per thousand in the Gd ansk Deep (Vistula River). In the Baltic Sea Proper, significantly lower de lta 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 w ere 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 a nd recent ones of 2.3-10 parts per thousand. As explanations for this incre ase are suggested, elevated nutrient delta N-15 values of waste water in co mbination with fractionation processes like nutrient uptake by phytoplankto n and denitrification and nitrification processes in rivers discharging int o the coastal water. Delta C-13 Values in sediment surfaces off the river e stuaries primarily indicate differences between the inorganic carbon signat ures of the rivers. However, since the delta C-13 values also decrease down core, we contribute this change to increased primary production caused by t he enhanced nutrient load. Since both stable isotope values in sediments (d elta C-13 and delta N-15) correlate downcore, this strongly suggests that t he anthropogenic nutrient loads in the rivers might be the reason for the c hanges of stable isotope values. (C) 2000 Elsevier Science B.V. All rights reserved.