A biogeochemical study of sediments from the eutrophic Lake Luganoand the oligotrophic Lake Brienz, Switzerland

摘要

The biomarker composition and stable isotope (C, 0) ratio values of organic matter (OM) and carbonatefrom sediment cores from the oligotrophic Lake Brienz and the eutrophic Lake Lugano (both in Switzer-land) are compared, in order to obtain information about OM sources and transformation processes.Eutrophic conditions at Lake Lugano are reflected in elevated total organic carbon (TOC) content andhydrogen index (HI) values, as well as higher lipid concentrations. Parallel down core trends in 613C val-ues of TOC and calcite in the Lake Lugano sediments reflect bioproductivity cycles. Variations in 8180 val-ues of calcite are consistent with changes in mean summer temperature over the time interval covered bythe core. In contrast, such a correlation does not exist for Lake Brienz and there the stable isotope com-position of calcite reflects its allochthonous origin. In the sediments of both lakes, fatty acid (FA) distri-butions and the composition of n-alkanols and n-alkanes indicate highly variable proportions ofautochthonous OM sources (algae, zooplankton, bacteria) and OM from land plants. Enhanced in situmicrobial synthesis during sediment deposition in Lake Lugano is suggested by the higher TOC-norma-lised concentrations of branched chain FAs (C15-C17), hopanoic acids and triterpenoid alcohols (i.e. tetra-hymanol, diplopterol). Variations in the concentrations of cholesterol are related to contributions fromzooplankton and/or green algae, while sitosterol concentrations reflect the input of vascular plants. Peri-ods of increased input of OM from diatoms are evidenced by high 24-methylcholesta-5,22-dien-313-ol(either epibrassicasterol or brassicasterol) and/or highly branched isoprenoid (HBI) alkenes concentra-tions. High relative concentrations of diplopterol in Lake Lugano sediments are consistent with the pre-dominance of cyanobacteria commonly observed in eutrophic lakes. The presence of archeol andhydroxyarcheol in very low concentrations in the Lugano sediments argues for the activity of methano-gens and/or anaerobic methanotrophs. Differences in OM degradation processes are reflected in higher chlorin index values in the Brienz sed-iments but higher saturated vs. unsaturated n-FAs in the core from Lugano. Higher concentrations ofbranched chain FAs and 16:1w7 n-FA, as well as enhanced 18:1w7/18:1 co9 n-FA, are consistent withenhanced bacterial biomass in the Lugano water column or sediments. The preservation of phytol seemsto be enhanced in sediments with a high relative contribution of land plant OM. Major factors affectingOM accumulation in the lakes are differences in OM sources (i.e. terrestrial OM vs. autochthonous pro-duction), extent of bacterial activity and most likely oxygen availability in the water column.