Effect of terrestrial subsidies on structure, functioning, and dynamics of food webs in pond ecosystems

Theory predicts that subsidies are stabilizing, but experimental tests of this prediction are lacking. I focus on the implications of ecological contexts (species identity and food web configuration of recipient communities, subsidy quality, fluctuating inputs, and mechanisms) in mediating the effects of terrestrial subsidies as colored dissolved organic matter (cDOM) on the structure, functioning, and dynamics of pond plankton.;Species identity of consumers showed dissimilar responses to subsidies. In food webs dominated by selected species, many attributes were increased in the presence of subsidies, but their temporal variability was always increased. However, the zooplankton response was smaller than expected. cDOM is considered a low quality subsidy with bacteria mostly respiring the carbon, thus bypassing its pathway from bacteria to zooplankton. When zooplankton were exposed to a more labile carbon subsidy (sucrose), there was even higher destabilization, suggesting that subsidy quality mediates stability. This study thus provides experimental evidence that, contrary to theoretical expectations, subsidies do not stabilize the dynamics of recipient food webs but destabilize them instead. It also suggests that context-dependencies might affect the match or mismatch between theoretical predictions and observations.;I also examined how fluctuating subsidy inputs would affect plankton communities. I expected that plankton receiving pulsed subsidies would exhibit increased temporal variability compared to a continuous addition. Surprisingly, temporal variability of plankton in pulsed treatments was indistinguishable from that in continuous-addition treatments. These results document the prevalence of robust internal dynamics when faced with external forcing (fluctuating subsidy inputs), and suggest an overriding stabilizing mechanism likely involving the interaction of scale dependencies (e.g., temporal scale matching of internal and external dynamics) in mediating responses to fluctuating subsidy inputs.;Given that the effects of cDOM confound a source of carbon (trophic pathway) and protection from UV radiation through color (non-trophic pathway), I addressed the contribution of each individual pathway. The effects of UV were substantial in affecting the mortality of zooplankton. Interestingly, the carbon effect was positive but had a slight negative effect for some species, possibly through some toxicity of cDOM. These results highlight that cDOM subsidies can affect recipient communities through trophic and non-trophic pathways.