Lake management (muck removal) and hurricane impacts to the trophic state of Lake Tohopekaliga, Florida

Lake Tohopekaliga is a large (surface area 9,800 ha) and shallow (mean depth 2.1 m) natural lake in central Florida. Cultural eutrophication and lake water level stabilization led to accelerated growth of invasive native and non-native aquatic macrophytes, resulting in the buildup of thick deposits of organic matter along the shoreline. Those shoreline areas were often devoid of oxygen, and the muck buildup filled feeding grounds for wading birds and spawning fish. Muck build up has also reduced aesthetics and boat access. To remove organic accumulation, the lake water level was dropped and heavy equipment was used to scrape the plants and dead organic materials from the underlying sand substrates from more than 1,420 ha of the littoral zone. Most of this material was heaped into large piles in shallow parts of the lake to form 29 artificial islands with basal areas from 0.4 to 3.3 ha each. Our study was designed to determine: (1) amount of nutrients stored in islands relative to annual inflows, (2) nutrient release to the lake from the islands, and (3) changes in lake trophic state due to the muck scraping and construction of the islands. The lake enhancement project was completed in late summer 2004, and the average thickness of organic materials in the scrapped areas was reduced from 46 cm to 1.6 cm, improving access and aesthetics tremendously. The islands stored several times the annual inflow of total phosphorus (TP, 3.1 times) and Total Nitrogen (TN, 6.5 times) and thus could potentially affect the lake's trophic state by leaching nutrients. Our study of water quality in the vicinity of the islands indicates that the islands had no statistically significant impact on the water chemistry of the lake through leaching of nutrients. In the 2 years following the muck removal, substantial increases in average TP (39%), chlorophyll (56%), and color (53%) and a decline in dissolved oxygen (-10%) were found in open water stations. An unintended complication to our experimental design was the occurrence of 3 major hurricanes with high winds and heavy rainfalls that passed over the Lake Tohopekaliga area immediately following the muck removal project. To account for the effects of hurricane activity we examined monthly TP, TN, chlorophyll, Secchi depth data, and quarterly color values measured for 55 relatively small (median surface area 33 ha), nearby lakes. Our sample of 55 nearby lakes showed significant increases in TP (8.2%), TN (4.1%), chlorophyll (20.1%), and water color (23.8%), and decreases in Secchi depth (-8.2%) coinciding with the passage of the hurricanes. Additionally, data from a larger control lake (Kissimmee, surface area 19,800 ha) located 10 km south of Lake Tohopekaliga showed a much larger increase in total phosphorus (66%). Therefore, some or possibly all of the differences we measured before and after scraping could have been the result of low quality water (high nutrients and organic color) flushed into the lake following the heavy rains (93 cm in August and September of 2004) accompanying the storms. The effects of muck removal cannot be completely separated from those of hurricanes because they both occurred at the same time. However, aquatic plant (Florida Department of Environmental Protection) and water chemistry (Florida Fish and Wildlife Conservation Commission) data collected after this project was completed show that submersed aquatic macrophytes in Lake Tohopekaliga have returned and total phosphorus and chlorophyll concentrations are down to levels measured prior to muck scraping and hurricane impacts. Thus, the changes in water chemistry caused by muck removal and/or the hurricanes were relatively short lived (approximately 2 years).