Whole-lake Herbicide Treatments for Eurasian Watermilfoil in Four Wisconsin Lakes: Effects on Vegetation and Water Clarity

Abstract

Four pilot whole-lake herbicide treatments for extensive Eurasian watermilfoil (EWM) (Myriophyllum spicatum L.) infestations were conducted in Wisconsin between 1997 and 2001 using fluridone at a range of dosages (6–16 μg/L). Annual post-treatment data (4–7 years) were evaluated to assess (1) effects on exotic plants; (2) changes to native plant communities; and (3) effects on water clarity. Temporal shifts in treatment lakes were compared against natural fluctuations in untreated reference lakes. In conjunction with aggressive follow-up spot treatments with 2,4-D or manual removal, fluridone treatments provided between 1 and 4 years of substantial EWM relief, with the exotic ultimately re-establishing at pre-treatment levels or greater in 3 of the 4 lakes. Native plant communities shifted in all 4 lakes following fluridone treatment. The large decreases, outside the range seen in untreated lakes (first quartile of the reference lake distribution) for all treatment lakes containing EWM, Elodea canadensis, Ceratophyllum demersum, and Najas flexilis, strongly suggest a direct effect of the fluridone treatment. We observed large increases, outside the range seen in untreated lakes (fourth quartile of the reference distribution), for 1 of 2 treatment lakes with Potamogeton crispus, and 1 of 2 treatments with Chara spp. Secchi depth decreased significantly in 2 of the 3 lakes for which data were available. Future applications should consider, among other criteria, the dominant natives in the plant community, their sensitivity to fluridone, and potential impacts associated with decreased water clarity.

Key Words:

• aquatic plant control
• fluridone
• herbicide
• invasive species
• lake management
• macrophytes
• Myriophyllum spicatum
• water quality