The energetic importance of terrestrial carbon in lake ecosystems

Over the past decade, ecologists have begun to appreciate how subsidies---matter and energy transported from outside the boundaries of an ecosystem---influence population dynamics, community structure, and ecosystem processes. Terrestrial ecosystems export large quantities of dissolved organic matter (DOM) to nearby aquatic ecosystems. This DOM surpasses the amount of living biomass and thus constitutes a source of potential energy for inland waterbodies.; The energetic importance of terrestrial DOM in lakes is ultimately determined by aquatic microbial metabolism. I exposed bacteria to leachates from different soils to test the hypothesis that the source and supply of terrestrial DOM are strong determinants of aquatic microbial metabolism. Overall, metabolism increased with DOM concentrations, but productivity was differentially affected by DOM source. Phosphorus content, which may be linked to the identity of terrestrial vegetation, was indicative of DOM quality for freshwater bacteria.; Inputs of DOM to lakes are variable through time and linked to hydrological events. I addressed how variable DOM supply influences bacterial productivity (BP) using field experiments and simulation models. Time-integrated BP was 2--5X higher in pulsed versus pressed treatments even though both received the same amount of DOM. Simulation models suggest that these differences may be linked a combination of factors, including mortality and rapid uptake labile DOM.; Inputs of terrestrial DOM may modify lake ecosystem processes. CO 2 flux and BP increased linearly over an experimental DOM gradient, but were not influenced by DOM-correlated attributes (i.e., shading and nutrients). These results suggest that the carbon content of terrestrial DOM may be an important subsidy, which can influence whether recipient aquatic ecosystems function as sources or sinks of atmospheric CO2.; Using naturally-occurring stable isotope ratios, I evaluated how terrestrial DOM influences carbon cycling in a comparative study of northeastern U.S. lakes. Epilimnetic CO2 concentrations increased with dissolved organic carbon (DOC), indicating that terrestrial-derived DOM was assimilated by lake biota. However, isotope evidence suggests that the assimilated terrestrial-carbon was used rather inefficiently. Nevertheless, phytoplankton did not comprise the entire diet of crustacean zooplankton, suggesting that DOM may serve as an energetic subsidy for higher trophic levels in lake food webs.