Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems

Nonindigenous Aquatic Plants

Waterlettuce (Pistia stratiotes)L.

Waterlettuce (Fig. 4) is one of the most widely distributed hydrophytes that occurs on all continents except in Europe and in Antarctica (Giliett el al. 1968; Stoddard 1989). The most commonly accepted pathway of this species into the United States is in ballast water in ships from South America. Waterlettuce was first reported in Florida by John and William Bartram in 1765 (Bartram 1942). This led many to believe waterlettuce was native to North America. The origin of this species is still speculative; however, the absence of coevolved herbivorous insects is the strongest evidence of an exotic origin (Dray et al. 1988). Cordo et al.(1981) suggested that waterlettuce may be native to South America because of the abundance of regionally native insects associated with waterlettuce.

Waterlettuce poses many of the same environmental and economic problems as waterhyacinth, namely, loss of habitat, flood control, and unrestricted navigation (Sharma 1984). Dense mats deplete oxygen in underlying water and in sediments by blocking the air-water interface, by respiration of the roots, and by the decay of dead plants (Attionu 1976).

Waterlettuce reproduces by propagation and by seeds (Schardt and Schmitz 1990). Seed production is important in this species because seeds can remain dormant for months and withstand drought and freezing.

The abundance of waterlettuce is low statewide; its status was reduced from the ninth most abundant plant in public waters in 1982 (1,359 ha) to 41st in 1990 (684 ha; Schardt and Schmitz 1990). Although waterlettuce was found in 128 public water bodies, it is considered under control throughout the state except in some lakes in central Florida. Control is conducted primarily with the herbicide diquat; mechanical harvesters are occasionally used. The search for biological controls of waterlettuce began nearly 30 years ago, and practical application in Florida started in 1987. Distribution of Neohydronomus affinis, a weevil, in Argentina during the middle 1970's began in Florida in April 1987. Aquatic plant managers recently began statewide dispersal of another insect, a moth (Namangana pectinicornis).

Seedlings are a particularly significant source of reinfestation of waterways in which waterlettuce was earlier reduced by control or by extreme cold weather (Schmitz et al 1993).

Waterhyacinth generally displaces waterlettuce (Chadwick and Obeid 1966; Bond and Roberts 1978; Tucker and DeBusk 1981; Sutton 1983; Agami and Reddy 1990). Consequently, large floating islands of waterlettuce in the mid- to late- eighteenth century in Florida (Stuckey and Les 1984) became less prevalent with the spread of waterhyacinth (Schardt and Schmitz 1990). Control of waterhyacinth may be followed by increasing populations of waterlettuce.

Evapotranspiration by waterlettuce may exceed evaporation over open water as much as tenfold (Sharma 1984). However, the question of whether these higher evapotranspiration rates affect regional hydrologic cycles remains unanswered. Like waterhyacinth, waterlettuce populations also can bioaccumulate considerable amounts of heavy metals (Sridahar 1986).

The root system of waterlettuce increases siltation, which changed the benthic substrates, making it unsuitable for nesting sites for fishes (Beumer 1980) or as habitat for many macroinvertebrates (Roback 1974). Increased siltation from waterlettuce mats has never been measured in Florida's flowing waters. The effect of waterlettuce on native plant communities also has been poorly documented. The greatest harm from this species is its destruction of native species by excessive shading.

Waterhyacinth and waterlettuce harbor the amphipod Hyalla azteca (Dray et al. 1988). The importance of these amphipods in the ecosystems has not been studied. A study of the fauna associated with mats of waterlettuce in Florida failed to reveal any that were significant control agents of the plant (Dray et al. 1988).