Controls of δ 13 C‐DIC in lakes: Geochemistry, lake metabolism, and morphometry

We investigated δ 13 carbon (C)‐dissolved inorganic carbon (DIC) values in 72 lakes from diverse regions using literature data as well as new measurements for 32 lakes. δ 13 C‐DIC varied broadly among lakes from ~31 to +2.6‰. This variation of surface‐water δ 13 C‐DIC among lakes is greater than the seasonal variation within most lakes. Several statistical models account for a large portion of the interlake variation and indicate that geochemical (e.g., DIC, pH, alkalinity) and morphometric (area) variables are important, whereas biological (e.g., gross primary productivity [GPP], respiration [R], chlorophyll a) variables are generally not significant. A process‐based model including gas exchange with the atmosphere, inorganic carbon speciation, and ecosystem metabolism was also constructed. The model provides a reasonable fit to the data for lakes, in which respiration exceeded GPP (heterotrophic lakes; 75% of lakes sampled). Lakes for which GPP exceeded respiration (autotrophic) were not fit well by the process‐based model. The data and models indicate that metabolism creates substantial variation in δ 13 C‐DIC around the potential δ 13 C‐DIC that is set by geochemical factors of the watershed.