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dc.contributor.authorRydell, Nicholas J.
dc.date.accessioned2020-05-21T21:54:11Z
dc.date.available2020-05-21T21:54:11Z
dc.date.issued2018
dc.identifier.urihttp://digital.library.wisc.edu/1793/80154
dc.descriptionEXECUTIVE SUMMARY: Eurasian Watermilfoil (EWM; Myriophyllum spicatum) is one of the most prolific aquatic invasive plants in North America. Since the 1950s, the herbicide 2, 4-dichlorophenoxyacetic acid (2, 4-D) has been used to control EWM. Little was known regarding the effect of 2, 4-D treatments on zooplankton and fishes outside of a few laboratory studies. One of these laboratory studies reported a 15.6% reduction in larval Fathead Minnow Pimephales promelas survival at 2, 4-D concentrations of 0.05 parts per million (ppm). This could be a concern because the United States Environmental Protection Agency allows spot treatments with concentrations of 4 ppm and whole-lake treatments of 2 ppm. Increasing demand for whole-lake 2, 4-D treatments to control EWM in Wisconsin lakes warranted additional examination of fish and zooplankton responses to these treatments. The objectives of this study were to determine if whole-lake 2, 4-D herbicide treatments used to control EWM affected: 1) abundance, diversity, and size of zooplankton; 2) feeding, growth and size structure of larval fishes, and 3) abundance, diversity, and survival of fishes at different life history stages. Sampling occurred over three years (2015-2017) on six lakes in northern Wisconsin. No herbicide treatment occurred on any lake in 2015 (pre-treatment) or 2017 (post-treatment). In 2016, whole-lake treatments using the DMA® 4 IVM formulation of 2, 4-D occurred on three lakes between May 24th and June 7th; the remaining three lakes served as reference systems. Sampling took place from May through August of each year and included collection of limnological data, aquatic plant surveys, zooplankton collection, sampling of larval fish using quatrefoil light traps and ichthyoplankton tows, seining, net pen trials and collection of water samples to determine 2, 4-D concentrations. In the laboratory, all crustacean zooplankton were counted and identified to determine density (i.e., number/L), and body length was measured for Daphnia spp., and calanoid and cyclopoid copepods. Zooplankton density and body length data were compared using mixed-effects models by the main effects of lake type (i.e., reference or treatment) and year, along with the interaction of lake type and year. All larval fishes from both gears were identified. Cyprinid and Largemouth Bass Micropterus salmoides peak relative abundance from quatrefoil light traps and Yellow Perch Perca flavescens, Black Crappie Pomoxis nigromaculatus and Bluegill Lepomis macrochirus peak relative abundance from ichthyoplankton tows were compared using mixed-effects models. From ichthyoplankton tows, Yellow Perch, Black Crappie and Bluegill were measured to total length. Yellow Perch and Black Crappie hatch dates indicated that Yellow Perch hatching occurred well before herbicide application, so only Black Crappie diets, foraging success, and mean daily growth rates were analyzed. As a metric of growth, linear regressions of Yellow Perch and Bluegill total length in relation to day of year were compared among lake type-year combinations using analysis of variance. Peak concentrations of 2, 4-D were lower than (0.152 to 0.257 ppm) than the target concentration of 0.3 ppm and degradation of 2, 4-D occurred fastest in Kathan Lake and was slowest in Manson Lake. No EWM was detected in treatment lakes after herbicide treatments in 2016. In 2017, EWM was sampled in Kathan Lake (4% vegetative coverage) and Manson Lake (9.4% vegetative coverage), but was not detected in Silver Lake. No statistically significant responses to the herbicide treatments were detected in any of the zooplankton or larval fish metrics I measured. However, different trends were observed for some zooplankton taxa in treatment lakes during 2017, the year after the herbicide applications occurred. Specifically, Daphnia spp. densities in Kathan and Silver lakes during 2017 were low during May when peak densities had been observed in 2015 and 2016 and were high during mid-summer when low abundances had been observed in the two previous years. This trend was also observed for Bosmina spp. in Kathan Lake. Additionally, cyclopoid copepod densities remained low in Kathan and Manson lakes in 2017 when compared to 2015 and 2016. While these zooplankton trends may reflect delayed responses to the herbicide treatments, the trends were not consistent among treatment lakes and no statistical differences between treatment and reference lakes were detected. No significant differences in larval abundance of Largemouth Bass, cyprinids, Yellow Perch, Black Crappie, and Bluegill were detected between treatment and reference lakes. Peak relative abundance of larval Yellow Perch from ichthyoplankton tows appeared to be lower in treatment lakes in 2017 (the year after herbicide was applied), a trend that was not observed in reference lakes, but the differences between lake types was not statistically significant. Slopes of larval Yellow Perch and Black Crappie total length in relation to day of year were not significantly different among lake types (reference vs. treatment) or years. Larval Black Crappie showed no detectable response to herbicide application in terms of diet, feeding success, or mean daily growth rates. Net pen trials for juvenile Bluegill and Yellow perch indicated no significant change in mortality resulting from herbicide treatments, and no treatment effect on catch-per effort of juvenile Bluegill and Yellow Perch in August seine hauls was detected. My findings suggest that 2, 4-D herbicide treatments had little effect on the metrics I measured. However, the lack of statistically significant responses to 2, 4-D herbicide treatments observed in this evaluation does not necessarily mean that herbicide application has no effects on these or other metrics. Potential effects may not be detectable in a lake setting given the inherent variation in many of the metrics measured and the number of lakes included in my study. Observed declines in Yellow Perch abundance and changes in zooplankton trends for treatment lakes in the year after herbicide application occurred may be a result of changes in aquatic plant communities and not a direct effect of the herbicide. These observations warrant further investigation and this work suggests that additional laboratory assessments might focus on Yellow Perch, along with zooplankton such as Daphnia spp., cyclopoid copepods, and Bosmina spp. Additionally, this assessment did not address the effects of repeated herbicide applications on the same lake over time, which remains an important question, because EWM coverage in some lakes may return to levels where there is public interest in subsequent herbicide applications.en_US
dc.language.isoen_USen_US
dc.publisherUniversity of Wisconsin-Stevens Point, College of Natural Resourcesen_US
dc.titleEffects of 2, 4-D Herbicide Treatments Used to Control Eurasian Watermilfoil on Fish and Zooplankton in Northern Wisconsin Lakesen_US
dc.typeThesisen_US


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