Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems

Nonindigenous Aquatic Plants

Waterhyacinth (Eichhornia crassipes; Mart.) Solms.

Waterhyacinth (Fig. 8) is commonly believed to have been introduced from its native South America into the United States at the World's Industrial and Cotton Centennial Exposition of 1884-1885 in New Orleans, Louisiana. It may have been cultivated by a New Orleans horticulturist in the 1860's (Tabita and Woods 1962). A visitor of the exposition in New Orleans in 1884 took several of the plants (Penfound and Earle 1948) to his farm a few kilometers north of Palatka (Tabita and Woods 1962). Perhaps intrigued by the showy lavender flower, the plants were placed in a lawn fountain on the banks of the St. Johns River near Palatka. The plants rapidly multiplied and filled the pond, and excess plants were discarded into the St. Johns River (Anonymous. 1896). By 1896, the plants had spread throughout most of the basin, assisted by cattlemen who introduced water hyacinths from basin to basin because they thought the species would make good cattle feed (United States Congress 1957). By 1893, waterhyacinth was already becoming a nuisance by hindering navigation on the St. John's River (Buker 1982). By 1898, the plant population had increased to the point that steamboats and other vessels were unable to reach docks or pass through navigation openings in bridges or narrow points in the river (Anonymous 1896; Joyce 1991). The introduction of waterhyacinth into Florida waters was an economic and ecological disaster soon after it was introduced. By the late 1950's, the waterhyacinth populations occupied more than an estimated 51,000 ha of Florida's waterways (United States Congress 1965). A 1959 inventory revealed more than 51,030 ha of waterhyacinth in Florida lakes and rivers.

This floating invasive species is probably the most prolific plant species in Florida lakes, rivers, and canals. It was reported in 254 public waters in 1990. Reproduction is by budding and by seeds that germinate after periods of drying and reflooding (Parija 1934). Growth rates exceed dry biomass production of any terrestrial, saltwater, or freshwater vascular macrophyte (Wolverton and McDonald 1978). Doubling times of 6-18 days have been reported (Mitchell 1976). Environmental harm from large waterhyacinth populations is degraded water quality and drastic changes in plant and animal communities. Light and oxygen diffusion are severely curtailed by the floating plant (Gopal 1987), and water movement can be reduced by 40-95% (Bogart 1949).

Mechanical controls are used where herbicides are inappropriate but have proved too expensive and too slow to keep pace with waterhyacinth growth on a large scale. Several methods such as log booms and barriers, conveyor belts and grapples, toxic sodium arsenite, crusher-boats, blighted worms, and herbicides have been used to eradicate waterhyacinth. None of these methods is practical on a large scale. When waterhyacinth populations decreased after herbicide applications, alligator weed expanded (Schmitz et al 1993).

A computer simulation model was developed to help control personnel determine the best method of control for this species (Akbay et al. 1988). Four biological control agents for this species have been dispersed in Florida (see insect section for details). Although they may stress waterhyacinth, they do not seem to control or prevent further spread of this rapidly growing pest. The herbicides 2,4-D and diquat are effective controls of waterhyacinth; however, only aggressive management reduced waterhyacinth in public waters from the third most abundant aquatic plant n 1982 to 50th in 1990 (Schardt and Schmitz 1990).

In Florida, evapotranspiration rates are higher of waterhyacinth communities than of adjacent open water and ranged from 3.7 to 6.0 (Timmer and Weldon 1967; Rogers and Davis 1972; Reddy and Tucker 1983). Whether large waterhyacinth infestations can affect regional hydrologic cycles in Florida has not been reported (Schmitz et al. 1993).

Center and Spencer (1981) reported that the leaves of the plant represent 60-70% of waterhyacinth plant biomass, and the leaf turnover rate can range from 60 to 70%/month (Schmitz et al. 1993). The decomposition of the plants, if a large biomass of the plants are killed at once, can use up all the oxygen in the water. The resulting anoxia from such detritus can enhance the release of phosphorus into the water during decomposition. The additional detrital load that is generated by dense waterhyacinth mats is believed to burden these systems with additional nutrient loadings. Consequently, because of the rapid growth rate and high evapotranspiration rate of waterhyacinth, populations of this plant species can be a depository for heavy metals and probably for toxic organic compounds, which may pose some risk for the Florida populations of the West Indian manatee (Trichechus manatus), an endangered, herbivorous aquatic species (Schmitz et al 1993). Further study is needed to determine overall ecological effects of waterhyacinth invasions on the dynamics of plankton communities (Schmitz et al 1993).

The roots at the edges of waterhyacinth mats support many invertebrates (O'Hara 1967; Schram et al. 1987). At least seven fish species are associated with waterhyacinth habitat in Florida (Hansen et al. 1971). In contrast, spawning areas for fishes are reduced by waterhyacinth mats, and their dead masses shade out benthic communities and can nearly block the diffusion of oxygen through the water-atmosphere interface. Low oxygen concentrations underneath waterhyacinth mats can kill fishes (Timmer and Weldon 1967). Drifting mats of waterhyacinth often smother beds of submersed vegetation and overwhelm marginal plants that are important to waterfowl (Tabita and Woods 1962; Chesnut and Barman 1974). The Florida Everglades Kite (Rostrhamus sociabilis) is endangered because, in part, of the invasions of its habitat by waterhyacinth. Sykes (1987) discussed the feeding habits of the kite in Florida. Large expanses of waterhyacinth uproot emergent vegetation and impede the bird's location of the apple snail (Pomacea paludosa), its most important food item (Griffen 1989). In Louisiana, waterhyacinth completely eliminated resident fish populations in small lakes (Gowanloch 1945).

This species, a federally designated noxious weed and a close relative to water hyacinth, was seized at a private residence in Florida after it was learned the species was obtained from a mail-order aquatic-plant nursery in Ohio (Schmitz et al.1993). Information indicating that this species was introduced into the open waters of Florida seemingly does not exist.