Safe and submerged: How ecological effects of aquatic plants mitigate insecticide impacts in freshwater communities

A major goal in ecology and toxicology is to better predict the environmental impacts of anthropogenic contaminants. A key step towards accomplishing this goal is to understand how ecological interactions can influence both the direct and indirect impacts of contaminants in nature. While many of the factors that exacerbate contaminant impacts have been well studied, ecological factors that can mitigate these effects are relatively poorly understood. In this dissertation, I examine the mitigating influence that submerged plants, a common feature of aquatic ecosystems, have on the impacts of the widely used insecticide malathion in freshwater communities. In chapters one and two, I test the degree to which different realistic submerged plant densities and different plant species, respectively, influence malathion's toxicity to the ecologically important zooplankton species, Daphnia magna. I show that each increase in plant density reduced both the amount and duration of malathion's toxicity, and that the ability to mitigate malathion's toxicity is a generalizable phenomenon across submerged plant species. In chapter three, I demonstrate that the mechanism traditionally thought to play the largest role in mitigating insecticide toxicity, sorption to plant tissues, plays virtually no role in the mitigation of malathion. Instead, I present the first evidence that increased water pH caused by plant photosynthesis is the primary mechanism driving the mitigating effects of plants on this insecticide. Finally, in chapter four I test whether plants can mitigate malathion's direct and indirect effects at larger spatial scales and in more ecologically complex communities. I show that in the absence of plants, realistic malathion exposures decimate sensitive cladoceran zooplankton, initiating trophic cascades that result in sustained phytoplankton blooms. However, in the presence of submerged plants, even at low densities, malathion had no effect on community structure. My research provides the first evidence that submerged plants are capable of mitigating the toxicity of a widely used insecticide at multiple spatial scales and levels of biological organization. My findings can help improve toxicological models designed to predict insecticide effects in aquatic environments and mitigation strategies (e.g., best management practices) for reducing the environmental impacts of insecticides.