Experimental evidence that terrestrial carbon subsidies increase CO 2 flux from lake ecosystems

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Abstract
Subsidies are donor-controlled inputs of nutrients and energy that can affect ecosystem-level processes in a recipient environment. Lake ecosystems receive large inputs of terrestrial carbon (C) in the form of dissolved organic matter (DOM). DOM inputs may energetically subsidize heterotrophic bacteria and determine whether lakes function as sources or sinks of atmospheric CO2. I experimentally tested this hypothesis using a series of mesocosm experiments in New England lakes. In the first experiment, I observed that CO2 flux increased by 160% 4 days following a 1,000 μm C addition in the form of DOM. However, this response was relatively short lived, as there was no effect of DOM enrichment on CO2 flux beyond 8 days. In a second experiment, I demonstrated that peak CO2 flux from mesocosms in two lakes increased linearly over a broad DOM gradient (slope for both lakes=0.02±0.001 mm CO2·m−2 day−1 per μm DOC, mean±SE). Concomitant changes in bacterial productivity and dissolved oxygen strengthen the inference that increasing CO2 flux resulted from the metabolism of DOM. I conducted two additional studies to test whether DOM-correlated attributes were responsible for the observed change in plankton metabolism along the subsidy gradient. First, terrestrial DOM reduced light transmittance, but experimental shading revealed that this was not responsible for the observed patterns of CO2 flux. Second, organically bound nitrogen (N) and phosphorus (P) accompanied DOM inputs, but experimental nutrient additions (without organic C) caused mesocosms to be satuated with CO2. Together, these results suggest that C content of terrestrial DOM may be an important subsidy for freshwater bacteria that can influence whether recipient aquatic ecosystems are sources or sinks of atmospheric CO2.