Effects of 2, 4-D Herbicide Treatments Used to Control Eurasian Watermilfoil on Fish and Zooplankton in Northern Wisconsin Lakes
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Rydell, Nicholas J.
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University of Wisconsin-Stevens Point, College of Natural Resources
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EXECUTIVE 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.