Half Moon Lake, a shallow oxbow of the Chippewa River located in Eau Claire, Wisconsin, is an important recreational resource for swimming, fishing, and boating and provides critical aquatic habitat for waterfowl, songbirds, fish, and native submerged and emergent vegetation. In recent decades, the lake has exhibited dense invasive curly-leaf pondweed growth that covers nearly the entire lake, recent invasion by Eurasian watermilfoil, and severe cyanobacterial blooms. The lake was classified as hypereutrophic (trophic state index > 70). Mean summer total phosphorus concentration can exceed 100 μg/L, resulting in poor water transparency and low light penetration which limits native submersed macrophyte growth. Anoxia develops in the bottom water of the lake throughout much of the summer, which exacerbates phosphorus flux from sediments and greatly increases the potential for algal uptake and growth.
The overall management goal for Half Moon Lake is to improve water clarity and light penetration to promote the re-establishment of healthy native aquatic plant communities. Strategies to accomplish this goal are twofold: 1) reduce infestations of canopy-forming curly-leaf pondweed and Eurasian watermilfoil and 2) control internal phosphorus loading from sediment to limit algal productivity and improve the light condition for native plant growth. First, motor boat activity has been restricted on the lake to reduce phosphorus resuspension and internal loading. Second, annual early spring herbicide treatments have been conducted during the years 2009-2013 to selectively target curly-leaf pondweed with minimal impact to native plants. Similar selective control of Eurasian watermilfoil was also accomplished in 2009. Finally, phosphorus release from sediments was managed in June 2011, via application of buffered alum-aluminate to drive algal productivity toward phosphorus-limited growth and increase light penetration for native macrophyte communities.
To assess and evaluate native macrophyte community response to these management measures, extensive limnological monitoring has been conducted by faculty and students at the Center for Limnological Research and Rehabilitation. Macrophyte biomass is monitored in June and August using point-intercept sampling techniques to evaluate the effectiveness of the herbicide treatments in controlling curly-leaf pondweed and Eurasian watermilfoil and to quantify native macrophyte community response to changes in water quality and management of the targeted invasive species. These surveys were also part of an early detection monitoring program that found a pioneer population of Eurasian watermilfoil in the southwestern embayment of the lake. Germinated turion frequency of occurrence is quantified in early April of each year to determine areas in the lake that need further herbicide treatment and to evaluate the viable turion seed bank in the sediment. In addition, in situ (i.e., temperature, dissolved oxygen, pH, conductivity) measurements, light penetration, phosphorus, chlorophyll, and algal abundance are monitored at six stations in the lake to assess improvement in underwater light habitat for comparison with the native macrophyte community response.