Hydrilla (Hydrilla verticillata)(L.F.) Presl.
Hydrilla (Hydrilla verticillata)(L.F.) Presl.
By the mid-1960's, severe problems with this species throughout the state were reported. Hydrilla has been the reported most abundant submersed aquatic plant in Florida's public waters since 1983. In 1990, it was found in 187 lakes and rivers or 23,085 ha. Hydrilla is in more than 40% of Florida's public waters and appears in more waters each year. Nineteen new infestations were identified at boat ramps in 1990. This invasive plant grows in water that can be from several inches to 10.7 meters deep. It does not sexually reproduce in Florida where only the female plant occurs. However, a monoecious male was recently introduced into the Potomac River. The introduction of such a male into Florida would create genetic diversification and a more aggressive hydrilla population, especially in the colder waters of northern Florida (Schmitz et al. 1993; Conant et al. 1984). Blackburn et al. (1969) compared hydrilla with similarly appearing species such as American elodea (Elodea canadensis). Langeland (1990a) discussed the life history and general problems with hydrilla in Florida.
The most important vector of dispersal to new waters seems to be the transportation of fragments by boat trailers. A single node is sufficient to propagate an entire plant (Haller 1978). Once established, boat traffic continues to enhance dispersal by fragmenting plants. Detached stem fragments readily develop into new plants that attach themselves to the hydrosoil by unbranched adventitious roots (Cook and Luond 1982). Germination usually occurs in spring and summer but also year-round in southern Florida waterways (Sutton and Portier 1985). Other forms of reproduction include the formation of axillary buds and subterranean tubers. Tubers are important to managers because they penetrate the substrate by several centimeters (Joyce et al. 1980). Hydrilla is resistant to control with conventional methods because tubers may resprout long after control operations are terminated (Bruner and Batterson 1984).
Because hydrilla grows rapidly and is competitive, populations usually exceed beneficial levels and adversely affect aquatic systems. Dense hydrilla mats form at the water's surface and accelerate the filling of water bodies, cause wide fluctuations in the amount of dissolved oxygen, pH, and temperature (Bowes et al. 1979), reduce plant and animal diversity (Barnett and Schneider 1974), and stunt sport-fish populations (Colle and Shireman 1980). Water flow in flood-control canals and rivers may also be restricted. Access to a water body may become limited and preclude water recreation and associated income for local businesses (Colle et al. 1987). Colle et al. (1987) discussed the influence of hydrilla on harvestable sport-fish populations, angler use, and angler expenditures at Orange Lake, Florida, in relation to the cost of controlling the plant. Another study of the effects of hydrilla in lakes Harris and Griffin in Lake County, Florida, revealed the costs and loss of activities on a lake that is overpopulated with this species (Milon et al.1986). The researchers discussed the effect of this plant on the economy of the area.
Hydrilla may be controlled variously. The current recommendation is control with the triploid grass carp (Ctenopharyngodon idella). Leslie et al.(1987) summarized the problems with their use to control vegetation and its impacts on the ecology of the water body.
Mechanical controls are slow, short term, and expensive and, because they fragment plants, increase the spread and establishment of the species and frequently cause algal blooms. They are not recommended except for small areas (Center et al. 1991; Shireman and Haller 1980, Canfield et al. 1983, Leslie et al. 1987). Several herbicides including diquat, endothal, and copper provide acceptable small-scale control. Slow, partial eradication must be used on serious infestations because mass destruction of the dense vegetation for a short time may deplete the dissolved oxygen in the water and kill fishes. Fluridone may provide large-scale control of hydrilla with reasonable selectivity and long-term control.
The release in Florida of four host-specific insects that feed on hydrilla were approved by the U.S. Department of Agriculture, but the effectiveness of the agents is as yet undetermined (Bennett and Buckingham 1991; G. E. Buckingham, Research Entomologist, Biological Control Laboratory, U.S. Department of Agriculture, Gainesville, Florida, personal communication). Sterile triploid grass carp provide excellent control but are not specific to hydrilla (Clugston and Shireman 1987, Thayer et al. 1990) and are inappropriate for most rivers and natural lakes where submersed native vegetation is a valuable component of the system (Center et al. 1991). The cost of control in Florida approached $50 million in federal and state funds during the 1980's (Schmitz et al. 1991).
Schmitz et al.(1993) summarized the literature on the significant effects of heavy populations of aquatic weeds on fishes and their habitats. Because of its ability to adapt to low light levels, hydrilla can displace native aquatic vegetation in Florida lakes and rivers (Bowes et al. 1977). In August 1987, approximately 1,618 ha of the 11,332-ha lake Istokpoga in south-central Florida was covered with hydrilla (Schardt and Schmitz 1990). By December 1988, this plant species covered nearly 8,000 ha (Schmitz et al. 1993).
Excessive growth of hydrilla in many lakes has been attributed to increased eutrophication (Canfield et al. 1983). Canfield et al. (1983) reported that the effects of hydrilla on lake-water chemistry, water clarity, and planktonic chlorophyll are related to the percentage of the lake's volume that is infested with hydrilla and to macrophyte standing crop. The lake's pH, conductivity, or total nitrogen concentrations did not change with changes in hydrilla levels. The magnitude of any change in the water quality of a lake depends on the abundance of aquatic macrophytes and the lakes's trophic status (Confield et al. 1983). Large reductions in macrophyte coverage, therefore, may cause unacceptable changes in phytoplankton biomass and in water clarity in some but not all lakes. The major reason for the changes hydrilla imparts on a lake's ecosystem is due to the dense shade the plant creates (Schmitz et al. 1993). Heavy infestations of hydrilla in Florida lakes have decreased zooplankton abundance and increased the number of species (Schmitz and Osborne 1984; Richard et al. 1985; Schmitz et al. 1993). Case histories comparisons of the use of grass carp in a number of Florida lakes were recently made Colle and Shireman 1995; Leslie et al. 1995; Hestand, Thompson, and Mallison 1995; Mallison, Hestand, and Thompson 1995; Van Dyke 1995; Jaggers 1995; Eggeman 1995). The results depended on many factors including degree of infestation, size of lake, and type of vegetation.
Hydrilla begins to harm fish populations when its coverage eliminates open-water feeding and spawning areas. The almost total coverage by hydrilla may significantly reduce bluegill (Lepomis macrochirus), redear (Lepomis microlophus), and black crappie (Pomoxis nigromaculatus) fisheries (Shireman et al. 1983). Populations of these species become skewed to smaller individuals because of insufficient predation (Colle et al. 1985). As the vegetation increases forage, smaller game fishes gain more cover and protection from predators and increase in numbers. The smaller game species are too small and provide no recreational fishing. On the other hand, Colle and Shireman (1980) believed that the complete elimination of hydrilla and other macrophytes may be detrimental to sport-fish communities. Hydrilla cover in excess of 30% resulted in low condition (fatness) of the larger (>250 mm TL) largemouth bass (Micropterus salmoides). Until cover exceeded 50%, smaller bass had high condition factors.
Many factors determine the response of a water body to attempted reductions of nuisance vegetation with grass carp). In Florida, the removal of nuisance vegetation from lakes has been attempted, for example, from Deer Point Lake (Van Dyke et al. 1984), from the Lake Conway Chain (Lazor 1983), and from several smaller lakes such as Lake Baldwin (Shireman and Maceina 1981) and Lake Wales (Shireman 1976). Leslie et al.(1987) summarized the history and management of the use of grass carp to control nuisance aquatic plants.
Montegut et al.(1976) found that waterfowl heavily fed on hydrilla in Lake Wales. Johnson and Montalbano (1987) also found that hydrilla is important habitat for ducks (Anatinae), coots (Fulica americana), and common moorhens (Gallinula chloropus) and supports the highest species density. Because of the extensive loss and degradation of wetlands in Florida since the turn of the century (Fernald and Patton 1984), managers noted increased use of hydrilla-infested habitats by waterfowl. Aquatic vegetation is a major food item for waterfowl (Hardin et al. 1984; Kerwin and Webb 1971). Chamberlain (1960) and Johnson (1987) discussed Florida's waterfowl populations and their habitats and management.
