Carbon flows in lacustrine food webs: natural abundance and deliberate tracer stable isotope approaches

Freshwater systems have recently been recognized as an important component of the global carbon cycle. They can act as a carbon sink (via burial) or source (due to heterotrophic processes) depending on the carbon flows within food webs. Heterotrophs in freshwater systems rely on three major sources of organic matter/energy: (1) local primary production, (2) terrestrial inputs from plants and soils and (3) methanotrophic biomass. Unraveling the relative importance of these carbon sources and their fate is crucial for understanding inland organic carbon cycling. Here we propose to combine stable isotope and organic geochemical approaches to elucidate food web functioning in freshwater systems and to trace carbon flows. Natural abundance isotope signatures of carbon, nitrogen and hydrogen will be used to unravel the relative importance of autotrophic and heterotrophic pathways. Organic molecules characteristic for specific organisms or a specific source (e.g. soil carbon) will be used as a complementary tool to trace carbon flows within lakes. Deliberate tracer experiments in which either primary producers or secondary bacterial producers are labeled will be used to directly quantify carbon transfer between organisms.