Records of southern and central Baltic Sea eutrophication in δ 13 C and δ 15 N of sedimentary organic matter

Abstract

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 δ13C and δ15N in organic matter in four 210Pb-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. δ15N 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 δ15N values from 1900 to 1975 and decreased δ15N values during the last 20 years. The δ15N records are interpreted to trace the effects of eutrophication in nearshore and basinal sediments over the last 100 years and the magnitude of δ15N increase may be a measure for its extent. The switch to lower δ15N 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. δ13C(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 δ13C indicates decreasing isotope fractionation during massive plankton blooms; that this effect is not 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 δ13C(TOC) and δ15N from early Litorina stage (around 7,000 years BP) and modern 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 13C-enrichment in modern sediments are indications of enhanced productivity compared to early Litorina times.