Effects of Exotic Floating Vegetation Control on Native Aquatic Plants in Lake Okeechobee

Late in 2018, several Florida lake users voiced concerns about herbicide use and their impact on fish, wildlife, and aquatic ecosystems. In response, FWC paused all aquatic herbicide treatments and hosted public meetings to gather input from stakeholders regarding the aquatic plant management program. Throughout 2019, FWC incorporated this input into aquatic plant management planning, with the goal of balancing public recommendations within our legislative mandate to control nuisance exotic vegetation (e.g., water hyacinth Eichhornia crassipes and water lettuce Pistia stratiodes). One change that was implemented included the designation of a no-treatment zone of approximately 500 acres encompassing Bird Island on Lake Okeechobee. During 2020 and throughout the study, no herbicide treatments were conducted in the designated area, which was last treated in July 2019. Researchers established study sites at Bird Island and additional study sites within a treatment zone at Second Point, near King’s Bar. Quarterly sampling was completed from March 2020 through March 2022 to document percent coverage of vegetation and stem density of bulrush, which can be used to indicate expansion, reduction, or no change in vegetation communities. Our study objective was to compare the effects of controlled and uncontrolled exotic floating plants (water hyacinth and water lettuce) on bulrush stem density and coverage. Results were intended to assist researchers and stakeholders in understanding the effects of exotic vegetation and our herbicide program on native plant communities that provide habitat benefits for fish and wildlife.

Research revealed that bulrush stem density declined by an average of 56% in all study plots combined. The observed reduction of bulrush stem density was greater in plots with abundant floating plants present (97% reduction, e.g., Figure 1) compared to plots with little or no floating plants (13% reduction, e.g., Figure 2). Similarly, percent coverage of bulrush declined by 43% overall, including a 92% decrease in areas with abundant floating plants and a 12% decrease in areas with little or no floating plants. When comparing the treatment zones, there were no statistically significant differences in bulrush stem density or coverage at the beginning of the study. By study’s end, we found that bulrush stem density was nearly twice as high at Second Point (treatment zone) than at Bird Island (non-treatment zone). Bulrush coverage at Second Point was 8% higher than that at Bird Island at the end of the study period. By March 2022, there was significantly higher bulrush stem density and coverage at Second Point, which was the location that included herbicide treatments. Our results support continued efforts to control exotic floating plants for the purpose of protecting native aquatic plant communities that provide habitat benefits for fish and wildlife.