When insecticides enter aquatic systems, they directly influence survival and behavior in sensitive species, but they also impact many more taxa through indirect pathways. For example, numerous studies have found that insecticide exposure causes large cladoceran declines followed by substantial increases in phytoplankton abundance, resulting in adverse cascading effects on the rest of the community. Most experiments detecting such effects, however, are performed in simplified communities, so our understanding of how environmental context influences the effects of insecticides remains poor. Submerged aquatic macrophytes can remove insecticides from the aqueous environment via accumulation in and on plant tissues. Moreover, macrophytes can directly compete with phytoplankton for nutrients and via allelopathy. Collectively, this suggests that macrophytes may mitigate the effects of insecticides on aquatic communities. Despite the important ecological ramifications, however, few empirical studies have examined this possibility. During summer 2009, we performed an experiment in which we examined the impact of insecticides on aquatic communities varying in submerged macrophyte, Elodea canadensis, density. Using outdoor mesocosms, we established simple aquatic communities and manipulated the presence of an insecticide (malathion) and submerged macrophyte density in a complete factorial experiment.
Results/Conclusions
We found a macrophyte by insecticide treatment interaction on cladoceran and phytoplankton abundance. As predicted, in the absence of E. canadensis, malathion decimated cladocerans, resulting in a subsequent planktonic algal bloom. In all treatments containing macrophytes, however, the addition of insecticides had no effect on cladoceran and phytoplankton abundances. These data demonstrate that a submerged macrophyte can mitigate the effects of insecticides in aquatic environments by increasing community stability (resistance to change following a perturbation). These results have important implications for land management and aquatic conservation strategies and will contribute to better models designed to predict the impacts of insecticides on aquatic communities in variable environments.