By Chris Anderson
East Lake Tohopekaliga (ELT) is a 4843-ha mesotrophic lake located in the Kissimmee Chain of Lakes in Osceola County. In the 1960s, water control structures and canals were constructed in the Kissimmee Chain for flood control. These structures stabilized water levels in ELT and the other lakes in the chain, reducing the magnitude of seasonal water level fluctuations. The water level stabilization eventually led to increased accumulation of organic material and excessive growth of invasive aquatic plants that contributed to an accelerated rate of lake succession in ELT. The vast monocultures of invasive aquatic vegetation (e.g., torpedograss Panicum repens and cattails Typha sp.) growing within the littoral zone created and trapped additional organic material and ultimately resulted in the formation and expansion of floating tussocks. Extremely dense vegetation and tussocks resulted in degraded fish and wildlife habitat and limited recreational access for homeowners, anglers and boaters.
A lake drawdown and subsequent habitat restoration is set to begin on October 1, 2019. It will be a collaborative effort by FWC’s Aquatic Habitat Restoration/Enhancement subsection (habitat restoration), the South Florida Water Management District (water level regulation schedule changes), and the United States Army Corps of Engineers (overseeing drawdown). Once ELT has been dewatered, FWC biologists will utilize a combination of herbicide treatments and prescribed burning as well as mechanical removal of woody vegetation, tussocks and organic sediment to restore the littoral zone habitat (Figure 1). Herbicide treatments and prescribed burning will be used to control/remove monotypic stands of invasive vegetation (e.g., torpedograss and cattails) along the northern and western shores. Mechanical removal of woody vegetation (e.g., willows Salix sp. and water primrose Ludwigia spp.), tussocks and organic sediments will be completed along the eastern shore. Two areas along the southern shore will not receive any treatment and will be considered control areas. Once the habitat restoration is complete, ELT will be refilled via precipitation during the summer of 2020.
To evaluate the effects of the restoration, fisheries biologists are assessing potential changes in fish community composition, water quality, and habitat structure/composition in shallow (< 2 feet deep) littoral habitats pre- and post-restoration. Researchers are deploying mini-fyke nets (MFNs) and dissolved oxygen (DO) logging sondes to sample the fish communities and DO regimes, respectively, in each treatment/control area (18 sites per area, 90 sites per year). Qualitative assessments of aquatic vegetation and quantitative assessments of organic sediment depth are also completed at each sampling site. Pre-restoration sampling will continue until the restoration begins in 2019 and post-restoration sampling will be conducted for at least two years after ELT has refilled.
In 2016, the first year of pre-restoration sampling, a total of 4,847 fish comprising 26 species were captured across all successfully sampled sites (n = 88). Habitat data from 2016 indicated that the eastern shore (Mechanical Removal and Scraping area; Figure 1) had significantly deeper organic sediments and significantly lower aquatic plant density than the other four treatment/control areas of ELT. The second year of pre-restoration sampling was scheduled for 2017 but was cancelled because precipitation from Hurricane Irma resulted in water levels that were too high to effectively sample the littoral zone. Results from this study will help researchers and managers understand how habitat restorations influence shallow water fish communities, water quality, and habitat structure/composition. Understanding the effects that different restoration actions have on those parameters will provide managers with an idea of how future restorative efforts may influence the ecology of littoral habitats in lakes.