Stable nitrogen and carbon isotopes as indicator of eutrophication of the Oder river (Baltic sea)

For the Pomeranian Bight of the Baltic Sea the influence of riverine POM is clearly reflected in the isotopic (δ15N, δ13C) signature of suspended matter and surface sediments. High δ15N values of 10–14%. characterize recent river borne particulates and surface sediments along the outflow routes and in the upper 10–15 cm of sediment cores from the lagoon. Surface sediments of the Arkona Basin, 100 nautical miles from the river mouth, have δ15N values of 6%. which is assumed to be a mixture of autochthonous and riverine POM. The high δ15N is likely due to isotopic fractionation during partial utilization of DIN in the lagoon. Carbon isotopes show a similar spatial and temporal pattern with a value of − 28.3%. in surface sediments of the lagoon and a rapid increase along the western outflow route towards the Arkona Basin with − 23.8%. in the surface sediments, which is characteristic for marine organic matter. Delta 13C in the lagoon sediments decrease downcore to lower than − 29%., which is typical for terrigenous matter. The Arkona Basin sediments, however, show a slight increase in δ13C with depth. The higher modern δ13C in the lagoon is likely due to CO2 limitation resulting from eutrophication and high primary production in river and lagoon. Changes in both C and N isotopes of sediments from cores dated using 210Pb indicate an increasing nutrient load of the river over the last 50 years. Using calculated mixing rates an input from the Oder river of 60 to 95% of nitrogenous material and 50–60% POC into the Pomeranian Bight and 34% PON and 13% POC into the Arkona Basin is calculated for today.