Factors controlling δ13C values of sedimentary carbon in hypertrophic Baldeggersee, Switzerland, and implications for interpreting isotope excursions in lake sedimentary records

We use stable carbon isotope values (delta C-13) in sedimented organic matter and carbonate as proxy indicators of productivity changes in a highly eutrophic to hypertrophic lake. Stable isotope data from a seasonally sampled sediment core recovered from Baldeggersee, Switzerland were compared with direct and inferred total phosphorus (P) concentrations. Carbon isotope values of sedimentary organic matter (delta C-13(org)) in both seasonal varves and over the past 100 yr are not linearly related to total P concentrations, nor do they mirror observed trends in carbonate isotope values (delta C-13(CaCO3)). Baldeggersee delta C-13(org) values are influenced by both variations in the relative inputs of eukaryotic biomass, which becomes enriched in C-13 with increasing primary productivity, and the contribution of microbial biomass produced in the expanding anoxic bottom waters, which is typically very depleted in C-13. We also examined the fractionation within the organic matter-CO2-CaCO3 system, calculated as epsilon(total organic carbor(TOC)) = 10(3){[(delta C-13(CaCO3) + 1,000)/(delta C-13(org) + 1,000)] - 1}. Thresholds within the measured epsilon(TOC) values represent Baldeggersee trophic status more accurately than delta C-13(org) or delta C-13(CaCO3) values alone. In alkaline lakes with endogenic carbonate precipitates, STOC values can facilitate accurate interpretation of values and trends in organic carbon isotope data, and can help to better assess trophic history and lake remediation efforts in lake systems that have been heavily affected by cultural eutrophication.