The effects of many pesticides on aquatic ecosystems remain poorly understood, especially in naturalistic communities in which organisms are connected by a complex array of direct and indirect interactions. Moreover, multiple stressors can interact, and the addition of apex predators, such as fish, may introduce additional ecosystem changes that exacerbate or mitigate pesticide effects. Despite being both common and environmentally persistent, the effects of the herbicide metolachlor on realistic aquatic communities have received insufficient research attention. We tested the effects of metolachlor on pond mesocosms at three concentrations (0, 20, and 80 ppb), along with the presence vs. absence of fish (black crappie, Pomoxis nigromaculatus) to determine the independent and combined effects of these two environmental changes. We found both metolachlor and fish altered the pond mesocosms, but their effects did not interact. Metolachlor reduced phytoplankton as expected, but had nonlinear effects on dissolved oxygen. Metolachlor also altered tadpole behavior, making them less prone to hiding. Fish presence increased periphyton and decreased snail counts, as well as changing the behavior of tadpoles, reducing their hiding behavior. This work demonstrates previously undocumented effects of metolachlor in the presence and absence of fish and suggests future avenues of investigation.
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