Factors controlling d¹³C values of sedimentary carbon in hypertrophic Baldeggersee, Switzerland, and implications for interpreting isotope excursions in lake sedimentary records
Teranes, Jane L., and Stefano M. Bernasconi
Limnol. Oceanogr., 50(3), 2005, 914–922

We use stable carbon isotope values (¹³C) 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 (¹³C_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 (¹³C_CaCO3). Baldeggersee ¹³C_org values are influenced by both variations in the relative inputs of eukaryotic biomass, which becomes enriched in ¹³C with increasing primary productivity, and the contribution of microbial biomass produced in the expanding anoxic bottom waters, which is typically very depleted in ¹³C. We also examined the fractionation within the organic matter–CO₂–CaCO₃ system, calculated as _{total organic carbon(TOC)} = 10³{[(¹³C_CaCO3 + 1,000)/(¹³C_org + 1,000)] - 1}. Thresholds within the measured _TOC values represent Baldeggersee CaCO3 trophic status more accurately than ¹³C_org or ¹³C_CaCO3 values alone. In alkaline lakes with endogenic carbonate precipitates, _TOC 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.