In Florida, control of aquatic nuisance vegetation is funded by the U.S. Army Corps of Engineers for federal navigation; funded jointly by the corps and the Florida Department of Environmental Protection for navigation on other public waters; and funded by the state for flood control on small lakes, canals, and ditches and for drainage and mosquito control (Nelson and Dupes 1988). Some nuisance-plant control is also done by private individuals or organizations. The first large machines for mechanical control of aquatic plants were designed and built for the U.S. Army Corps of Engineers in 1900 (Center et al. 1991). The machines (Kenny) crushed vegetation like waterhyacinths and discharged them back into the water. In 1937, the Kenny was invented that crushed 81 ha of waterhyacinths per month. In the 1940's, saw boats (large, flat boats with cutting devices on the front, which cut weeds below the surface; saw boats were followed by a barge that was equipped with a conveyor belt that lifted the cut vegetation and piled it; the cut vegetation was later taken to shore and unloaded to rot on the dry land) were used to cut submersed plants that were also left to decompose in the water (Center et al. 1991). The selection of a machine for weed control depends on the plant species, type of disposal, availability of funds, and management objectives for the water body. No one system is universally effective (Center et al. 1991).
Removing aquatic plants from the water simultaneously removes nutrients from aquatic ecosystems (Shireman et al. 1979). However, because aquatic plants are composed of 90% to 95% water, the quantity of the removed nutrients is low in relation to the harvested mass of plant material (Center et al. 1991).
Drawdowns alter the composition of aquatic vegetation but do not always produce desirable changes, and the responses of various aquatic plant species to drawdowns vary widely and sometimes unpredictably (Thayer et al. 1990). For instance, submersed aquatic plants respond variably to drawdown, whereas emergent plants readily tolerate them (Center et al. 1991).
Sometimes, various materials such as black plastic and specially manufactured semipermeable benthic barriers can be used to deprive plants of sunlight or to prevent rooted aquatic plants from growing. However, gases that are produced from the decaying vegetation on pond bottoms accumulate under the accumulated,nonpermeable bottom covers and sometimes cause them to float to the surface (Center et al. 1991). Some specially made materials are gas-permeable, but even these eventually become clogged by debris and microorganisms and then also trap gases. However, new materials that are currently tested may be resistant to colonization by microorganisms. Securely anchoring heavy material to the bottom can sometimes hinder plant growth in ornamental ponds or in swimming areas, but the material is normally expensive (Center et al 1991).
During 1944-46, evaluations of a newly discovered herbicide, 2,4-D, as a control agent for waterhyacinth was initiated by the U.S. Department of Agriculture, the Jacksonville District of the U.S. Army Corps of Engineers, and the Everglades Experiment Station of the University of Florida (Joyce 1991). The herbicide 2,4-D proved effective and economical and was not toxic to fishes, cattle, or humans. Various agencies began ground and aerial applications of 2,4-D in 1947, which marked the beginning of the modern age of scientific aquatic-plant management. For the first time in decades, many miles of infested streams were open to navigation, and as time passed, the control operations shifted back into feeder areas (small isolated concentrations of waterhyacinth that periodically, especially in times of flood, release viable plants into areas where the plants had been removed) as the aquatic weeds became less of a threat to navigation and other uses of open water (Joyce 1991).
