Inter-Lake Variations in the Isotopic Signatures of Dissolved Inorganic Carbon in Lakes: Within-Lake Processing Versus Watershed Loading

We compared δ ¹³C-DIC in 72 lakes from diverse regions using literature data, as well as new data for 32 lakes in the Northern Highland Lake District of northern WI and the Upper Peninsula of Michigan. We found that geochemical variables (pH, [DIC] and alkalinity) account for a large portion of the inter-lake variation in statistical models. However a process-based model including atmospheric gas exchange, inorganic carbon speciation, and ecosystem metabolism was evaluated for the Northern Highland Lakes. The model provides a reasonable fit to the data compared with the simplest of the statistical models for lakes in which respiration exceeded gross primary production (heterotrophic lakes; 75% of lakes sampled). Lakes for which gross primary production exceeded respiration (autotrophic) were not fit well by the model. The model demonstrates that external inputs of DIC (e.g. groundwater) have relatively little influence on d13C-DIC and therefore, results from the comparisons and models suggest that internal lake processes are important for determining δ ¹³C-DIC in lakes.