Zooplankton play a critical role as an intermediate step in the food chain between primary producers and higher trophic levels. As such, zooplankton communities can be regulated by bottom-up effects, such as light and nutrients, or by top-down effects, like consumers. Larval amphibians have the potential to shift within-pond plankton communities, but this has rarely been studied. In ponds lacking fish, salamanders become a dominant consumer of zooplankton, and tadpoles can serve as predators or competitors. We experimentally determined if zooplankton biomass and community structure are related to effects of amphibians, the light/nutrient balance, or both. In summer 2014, we conducted an experiment in 1000 L replicated experimental ponds. We manipulated light supply (low/high), nutrients (low/high), southern leopard frog tadpoles (Lithobates sphenocephalus; presence/absence) and larval spotted salamanders (Ambystoma maculatum; presence/absence). We hypothesized that salamander presence and nutrients would have the strongest effects on zooplankton.
Our results do not support our hypothesis. We found no significant effect of salamanders on zooplankton, and nutrients were a significant predictor for nauplii only, where high nutrient conditions increased nauplii biomass. Instead, tadpoles had a strong negative effect on overall zooplankton biomass; overall biomass was 65% higher in treatments without tadpoles. Tadpoles also had a strong negative effect on Chydoridae. Light was the only important factor in predicting Daphnia and Bosmina biomass, but in opposite directions. Low light increased Daphnia and decreased Bosmina biomass. Our research indicates that both ambient conditions and tadpoles are important regulators of zooplankton biomass and communities. Little is known about the ecological roles of pond-breeding amphibians, thus our data add to the understanding of the potential effects of worldwide amphibian declines.