Ecosystem carbon cycles: Whole -lake fluxes estimated with multiple isotope

Inorganic 13C additions to lakes were used to: (1) determine the atmospheric flux of CO2 isotopes in high pH lakes, (2) examine models of photosynthetic fractionation in planktonic algae, (3) estimate the relative contribution of algal and terrestrial sources of dissolved organic carbon (DOC), and (4) validate a combined bioenergetics/carbon model for Chaoborus.;Increased algal productivity generally is considered to increase the delta 13C of dissolved inorganic carbon (DIC). However, increased productivity in Peter Lake caused conditions of low dissolved CO2, high pH, and chemically enhanced diffusion of atmospheric CO2 into the lake. Isotopic fractionation of CO2 during the process of chemically enhanced diffusion created very negative delta13C-DIC (about -20‰).;An accurate estimate of algal delta13C, either by measurement or models, is necessary to evaluate the delta13C of other organic carbon pools. Algal delta13C is dependent on the source delta13C-DIC and photosynthetic fractionation. Models of photosynthetic fractionation did not accurately predict algal delta 13C in these study lakes. In addition, algal delta13C may not be equivalent to particulate organic carbon delta13C because of the presence of non-algal material, such as terrestrial detritus.;I used the distinct algal delta13C to estimate the contribution of algal carbon to the dissolved organic carbon pool. The DOC isotope signature increased in all four whole-lake experiments due to the addition of 13C labeled algal DOC. Algae contributed between 5--40% to the dissolved organic carbon pool in the four lakes studied. Percentage of algal contribution appears to be positively related to levels of primary productivity, and inversely related to water color, a potential indicator of terrestrial carbon loading.;I constructed a bioenergetics model of Chaoborus spp. coupled with a carbon isotope mass balance model to predict dynamics in Chaoborus isotope ratios. The model reasonably reproduced the observed Chaoborus delta13C dynamics. This model framework can be applied to organismal isotope studies where diet isotope signatures are dynamic or the consumer is not in equilibrium with the diet isotopic composition.