Food web theory predicts the responses of aquatic communities to agrochemical mixtures

Background/Question/Methods

Aquatic ecosystems are exposed to mixtures of chemical contaminants more often than a single contaminant, but we lack a theoretical framework to predict the effects of mixtures on aquatic communities.  We propose that by integrating information on the functional groups that are generally sensitive to particular chemical classes (i.e., direct effects), their reproductive rates (i.e., recovery from the chemical), and the strong interactions those groups have with other species in the food web (i.e., indirect effects), food web theory can predict the effects of mixtures on freshwater communities. To test this hypothesis, we conducted an outdoor mesocosm experiment exploring the response of twenty freshwater pond taxa to the estimated environmental concentration of a fertilizer, herbicide, insecticide, and fungicide in isolation and in all pairwise mixtures.  We predicted that fertilizer should mitigate the negative effects of the biocides by either reducing direct toxicity or facilitating recovery of impacted populations. We also predicted that community responses to mixtures of biocides would generally be additive and worse than effects of each biocide in isolation, with the exception of herbicides and insecticides, which should mitigate one another because herbicides should counteract algal blooms that would be expected by insecticides killing herbivorous zooplankton and insect populations.

Results/Conclusions

Our results generally supported our predictions.  Fertilizer mitigated the effects of fungicide and insecticide, resulting in communities more similar to the uncontaminated control relative to either biocide alone. Communities in fungicide-insecticide and fungicide-herbicide mixtures were characteristic of the additive effects of each agrochemical in isolation and were further from the controls than either biocide alone. In contrast, the herbicide-insecticide mixture exhibited aquatic communities intermediate to either biocide alone.  These results suggest that community ecology theory has great promise to predict the effects of agrochemical mixtures, as well as offer recommendations to agriculture on which types of agrochemicals to apply together and separately to reduce their impacts on aquatic communities.