The Impact of Endothall on the Aquatic Plant Community of Kress Lake, Washington

CA dense mat-forming population of Eurasian watermilfoil(Myriophyllum spicatumL.) was interfering with fishing andrecreation in a small western Washington lake. A low concentration(1.5 mg/L active ingredient) of the herbicide endothallformulated as Aquathol® K was used in 2000 to attemptto selectively control the Eurasian watermilfoil. Aquatic plantbiomass and frequency data were collected before treatment,ten weeks after treatment and during the growing season for3 additional years. Macrophyte data were analyzed to assessthe herbicide’s impacts on Eurasian watermilfoil as well asthe rest of the aquatic plant community. Results showed a significantdecrease in Eurasian watermilfoil biomass and frequency10 weeks after treatment. The Eurasian watermilfoilcontinued to be present, but at a significantly reduced levelthrough the remainder of the study (3 years after treatment).Of the native plant species, large-leaf pondweed (PotamogetonamplifoliusTucker.) frequency and biomass was significantlyreduced after treatment. Common elodea (Elodea canadensisRich.), muskgrass (Charasp. Vallaint.) and bladderwort (Utriculariasp. L.) all increased significantly after treatment.(PDF has 6 pages.)