Effects of a Low-dose Fluridone Treatment on Submersed Aquatic Vegetation in a Eutrophic Minnesota Lake Dominated by Eurasian Watermilfoil and Coontail

Whole-lake, low-dose applications of fluridone herbicide are often used to control infestations of Eurasian watermilfoil (Myriophyllum spicatum L.). Studies in mesotrophic lakes have documented temporary reductions in Eurasian water-milfoil with little effect on native macrophytes and water quality. Less is known regarding the effects of low-dose fluridone (l-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(lH)-pyridinone) applications in eutrophic lakes. Therefore, we studied the effects of a low-dose fluridone treatment (approx. 6 ppb) on macrophytes and water clarity in a eutrophic Minnesota lake dominated by Eurasian watermilfoil and coontail (Ceratophyllum demersum L.). We estimated the frequency of occurrence of macrophyte species in the treatment lake and one untreated reference lake one year before, the year of, and for two years following treatment. We also mapped vegetation biovolume (percent of water column occupied by vegetation) with hydroacoustics just prior to and for two years following the fluridone treatment. Shortly after the treatment, the frequency of Eurasian watermilfoil declined to zero and remained at zero through the last survey. Nevertheless, Eurasian watermilfoil returned at 12% frequency during surveys one year after our study was completed (three years post-treatment). Mean percent biovolume also declined dramatically following treatment and remained low through the last survey. Secchi transparencies in Schutz Lake declined modestly following treatment. Plant frequency and percent biovolume varied over a smaller range in the reference lake. We conclude low-dose fluridone applications in infested, species-depauperate eutrophic lakes can kill large amounts of submersed vegetation that in turn, may negatively affect water clarity and recovery of native plants.