Spatial subsidies can strongly affect ecosystem dynamics. The small size and high perimeter-to-surface ratio of ponds makes them an excellent system for understanding the effects of variation in cross-ecosystem subsidies. Most previous studies have explored only very low or very high resource subsidy values in ponds. This emphasis on extremes assumes linear relationships and thus fails to capture potential nonlinear effects along a gradient of inputs seen in real systems. To test the effects of subsidies in ponds, we manipulated leaf litter subsidies across seven levels in 1000-L replicated experimental ponds during the summer of 2015. We used a simple pond food web that included southern leopard frog tadpoles (Lithobates sphenocephalus) and spotted salamander larvae (Ambystoma maculatum) as the top consumers. These species are often found in the same ponds and occupy different feeding niches, thus comparing the effect of subsidies between them provides insights into food web dynamics. We measured dissolved N and P; dissolved organic carbon (DOC); dissolved oxygen (DO); phytoplankton and periphyton biomass; and amphibian biomass export. We hypothesized that leaf litter would increase nutrient supply and DOC; decrease algal biomass; and that amphibian biomass export would closely track with primary production.
Our results suggest that leaf litter subsidies can have different effects on abiotic and biotic components of ponds. Increased leaf litter linearly increased phytoplankton biomass and concentrations of dissolved N and P. DOC and periphyton biomass, however, increased exponentially. Further, leaf litter decreased DO linearly. Our results also suggest that frogs and salamanders have different responses to the size of leaf litter subsidies, and that the responses are nonlinear. Frog biomass export increased with increased subsidy, while salamander biomass export increased up to moderate subsidy levels, before decreasing to almost zero at high subsidies. In particular, these data show that a dose-dependent approach is fundamental for understanding how the magnitude of the leaf litter subsidy structures pond food webs.