Composition and origin of organic matter in surface sediments of Lake Sarbsko: A highly eutrophic and shallow coastal lake (northern Poland)

► First data on methanogenic potential of coastal lakes on the Polish coast. ► Composition of lake phytoplankton vs. planktonic biomarkers in sediments. ► Organic matter composition and preservation in oxygenated polymictic lake. ► Degradation of organic matter vs. composition of lake water. We present an organic geochemical study of surface sediments of Lake Sarbsko, a shallow coastal lake on the middle Polish Baltic coast. The aim was to provide evidence concerning the origin of the organic matter (OM) and its compositional diversity in surface deposits of this very productive, highly dynamic water body. The content and composition of the OM in the bottom sediments were investigated at 11 sampling stations throughout the lake basin. OM sources were assigned on the basis of bulk indicators [total organic carbon (TOC), total nitrogen (TN), δ 13C TOC and δ 15N and extractable OM yield], biomarker composition of extractable OM and compound-specific C isotope signatures. The source characterization of autochthonous compounds was verified via phytoplankton analysis. The distribution of gaseous hydrocarbons in the sediments, as well as temporal changes in lake water pH, the concentration of DIC (dissolved inorganic carbon) and δ 13C DIC were used to trace OM decomposition. The sedimentary OM is composed mainly of well preserved phytoplankton compounds and shows minor spatial variability in composition. However, the presence of CH 4 and CO 2 in the bottom deposits provides evidence for microbial degradation of sedimentary OM. The transformation of organic compounds in surface, bottom and pore waters via oxidative processes influences carbonate equilibrium in the lake and seasonally favours precipitation or dissolution of CaCO 3. The data enhance our understanding of the relationships between the composition of sedimentary OM and environmental conditions within coastal ecosystems and shed light on the reliability of OM proxies for environmental reconstruction of coastal lakes.