Records of southern and central Baltic Sea eutrophication in δ13C and δ15N of sedimentary organic matter

The Baltic Sea catchment area has seen considerable agricultural and industrial development over the last century, but observational evidence of eutrophication is ambiguous. We examined organic carbon accumulation rates and the stable isotope signature of delta(13)C and delta(15)N in organic matter in four Pb-210-dated short sediment cores from the southern and central Baltic Sea to search for sedimentary records of increased nutrient input and biological production. Organic carbon accumulation rates vary little through time in the cores from the Oder estuary and the Arkona Basin, but increase significantly in the Bornholm Basin and in the Gotland Deep records over the last 40 years. delta(15)N in the sediments show distinct increases over the last 80-50 years in the Oder estuary, Arkona Basin and Bornholm Basin with the greatest increase near the mouth of river Oder. The Gotland Deep record shows increasing delta(15)N values from 1900 to 1975 and decreased delta(15)N values during the last 20 years. The delta(15)N records are interpreted to trace the effects of eutrophication in nearshore and basinal sediments over the last 100 years and the magnitude of delta(15)N increase may be a measure for its extent. The switch to lower delta(15)N values in the Gotland Basin sediment core after initially increasing values coincides with increasing abundance of summer blooms of nitrogen fixing cyanobacteria as observed in monitoring programs. delta(13)C((TOC)) values of organic matter increase in the Oder estuary over the last 80 years and over the last 30 years in the Bornholm Basin and Gotland Basin. The tendency to heavier delta(13)C indicates decreasing isotope fractionation during massive plankton blooms; that this effect is nor well documented in the sediments of the Arkona Basin may be due to enhanced degradation processes in the permanently mixed water column. A comparison of organic carbon concentrations and delta(13)C((TOC)) and delta(15)N from early Litorina stage (around 7,000 years BP) and modem sediments from the Gotland Sea suggests that the trophic state of the Baltic Sea was comparable to that of the modern environment: productivity was nitrate based in spring and based on nitrogen fixation in summer. Much higher organic carbon concentrations and C-13-enrichment in modem sediments are indications of enhanced productivity compared to early Litorina times. (C) 2000 Elsevier Science B.V. All rights reserved.