Most ecosystems experience a complex mixture of pesticides and nutrient inputs. Predicting the effects of these chemicals on natural and managed ecosystems will be impossible unless general patterns are identified. We used outdoor pond mesocosms to test whether community and ecosystem-level responses to mixtures of agrochemicals can be predicted from the effects of the individual chemicals in isolation, or are instead a product of non-additive interactions among chemicals. We employed a randomized block experiment with fertilizer and a representative herbicide, fungicide, and insecticide (atrazine, chlorothalonil, and malathion, respectively), each tested in isolation at 1× and 2× their expected environmental concentration (EEC). We also included all possible pair-wise mixtures of the chemicals at 1× EEC. Specifically, we hypothesized that 1) top-down control of primary productivity by zooplankton would be reduced by the insecticide, exacerbating the eutrophic effects of fertilizer, 2) the combination of insecticide and herbicide would be more disruptive to ecosystem function than either alone because each has strong effects on separate trophic levels, 3) herbicides would reduce the impact of fertilizer by reducing primary productivity, and 4) because insect grazers, zooplankton, photosynthetic organisms, and fungi indirectly and directly rely on organic material, fertilizer would accelerate the recovery of systems impacted by pesticides.

Results/Conclusions

We measured more than thirty community-level parameters, involving phytoplankton, periphyton, aquatic plants, zooplankton, insects, snails, crustaceans, and amphibian larvae and five ecosystem-level parameters including temperature, light, dissolved oxygen, pH, and leaf decomposition rate. Our results showed a complex pattern of additive and non-additive effects of agrochemicals on community components and ecosystem function, but our hypotheses regarding the behavior of agrochemical mixtures were generally supported. For example, phytoplankton was increased by a zooplankton-mediated synergism between malathion and fertilizer, leading to a reduction in light in the water column. Also, fertilizer in general mitigated the negative effects of pesticides on community components and ecosystem function. Chlorothalonil had strong negative effects on survival and reproduction of snails, and, unexpectedly, on zooplankton abundance and survival of amphibians. In contrast, survival and reproduction of the snail Planorbella trivolvis was increased by atrazine at 2× EEC and greatly elevated by an atrazine × fertilizer synergism. Overall, our results indicate that pond communities and ecosystems are strongly influenced by synergisms between agrochemicals, underscoring the need for further research on pollutant mixtures to aid effective management of wetlands.