Fish and Zooplankton Community Responses to the Cessation of Long-Term Invasive Eurasian Watermilfoil (<i>Myriophyllum spicatum</i>) Chemical Treatments in a North-Temperate, USA Lake

2,4-Dichlorophenoxyacetic acid (2,4-D) is commonly used to control invasive aquatic macrophytes, including Eurasian watermilfoil (<i>Myriophyllum spicatum</i>) (EWM). Potential influences of 2,4-D on non-target organisms are poorly understood; however, research has suggested the possibility of lethal effects on certain fish species. Lake Ellwood, Wisconsin was treated with 2,4-D to control EWM annually during 2003–2012. Fish surveys following treatment revealed natural recruitment failures of several regionally thriving species, including largemouth bass (<i>Micropterus salmoides</i>) and bluegill (<i>Lepomis macrochirus</i>). We hypothesized that these species had been negatively influenced by the chemical treatments. We monitored the post-chemical treatment fish community and aquatic ecosystem for responses during 2013–2019. Similar data were collected from Cosgrove (EWM absent) and Seidel lakes (EWM present, no chemical treatment) as reference systems. Limnological and submersed aquatic vegetation conditions did not change on Lake Ellwood post-chemical treatment. Total zooplankton density increased immediately post-chemical treatment on Lake Ellwood and then stabilized, whereas total zooplankton density did not change on the reference lakes over time. Analyses indicated immediate increases in recruitment post-chemical treatment for largemouth bass, smallmouth bass (<i>Micropterus dolomieu</i>), bluegill, and yellow perch (<i>Perca flavescens</i>). Back calculation of year class strength confirmed failures occurred during treatment for northern pike (<i>Esox lucius</i>), largemouth bass, bluegill, and black crappie (<i>Pomoxis nigromaculatus</i>). Our results provide evidence to suggest long-term 2,4-D treatments may negatively influence fish through lethal and sublethal mechanisms. Thus, there remains a critical need for directed research on whole-lake herbicide treatment side-effects.