Carbon cycling in lacustrine food webs, an in situ 13C-labeling experiment

Terrestrial organic carbon in freshwater systems is traditionally assumed to be aged and thus refractory, yet evidence indicates that in lakes it may fuel parts of the food web [1] by enhancing secondary production [1,2]. So far, which secondary producers are involved and relative contributions of in situ produced and allochthonous organic matter to secondary productivity remain poorly quantified. Two Swiss lakes were studied for their food web structure and carbon flows. In Lake Lucerne, a pre-alpine oligotrophic lake with a limited terrestrial input, DIC and DOC concentrations and δ13C analyses on inflowing streams, lake water and outflowing streams were used to deduce whether this lake is a net carbon source or sink. In Rotsee, a small eutrophic lake with a major methane-based productivity, an in situ, light and dark 13C-labeling experiment was carried out. DOC is the main coupling between autotrophs and heterotrophic bacteria and forms an important step in the food web. By DIC and DOC labeling it becomes possible to trace carbon flows within the food web. POM is often used as a representative of phytoplankton carbon and was analyzed for bulk carbon isotopic composition. To differentiate between algal and bacterial classes, POM was also analyzed for biomarker presence and abundance and subsequently for compound-specific δ13C to infer label incorporation. References [1] Cole et al. (2006) Ecology Letters 9: 558-568 [2] Pace et al. (2004) Nature 427: 240-243