General Impact
E. crassipes is one of the world's worst weeds (Holm et al. 1977, in Room and Fernando 1992). People have spread it to most tropical and subtropical regions in the world where it forms thick mats that cover rice paddies, clog irrigation channels, impede navigation, halt fishing, sweep away buildings during floods and foster breeding by disease-transmitting mosquitoes (Carter 1950, Chow et al. 1955, Williams 1956, Kotalawala 1976, in Room and Fernando 1992). Doubling in biomass every 6 to 18 days, the exact time being dependent on location and time of year (Lindsey and Hirt 1999, in Williams Undated), this weed rapidly invades water-ways and has caused problems for people around the globe. Populations living along Lake Victoria in Africa have been negatively affected by the weed which clogged water ways, resulted in the closure of a hydroelectric plant at Jinga and increased cases of vector borne diseases (Williams Undated). In Papua New Guinea water hyacinth disrupted water transport by canoes, dinghies and larger vessels, obstructing people's access to schools, health centres, government services, food gardens, fishing grounds and local markets (Julien and Orapa 2001, in Plant Protection Services 2006).
Invasive plant theory predicts that a release from environmental constraints due to altered hydrology can often lead to a successful invasion (Galatowitsch et al. 1999, in Toft 2000). In other words: disrupted or modified environments that have been altered by humans pave the way for invasive species' establishment. Disruptions of wetland ecosystems involving irrigation canals, hydroelectric projects and construction of artificial lakes have made areas particularly susceptible to invasion by water hyacinth (Barret 1989, in Toft 2000). Dams are thought to have exuberated the effects of water hyacinth in the Sacramento/San Joaquin Delta in California, where the weed was present in 1947 but did not begin to hinder boat traffic until the 1980s (Toft 2000).
Environmental problems associated with the water hyacinth are exuberated in warm areas where the weed grows throughout the year and develops into dense large, free-floating, monospecific islands or mats which compete with other aquatic species for light, nutrients and oxygen (Gopal 1987, in Batcher Undated; FDEP Undated; Toft 2000). These mats shade out native submersed plant species and uproot native emergent species (FDEP Undated). They reduce dissolved oxygen levels and light, significantly altering ecosystems and plant and animal communities. Low oxygen levels harms native fish populations (FDEP Undated) and fish spawning areas may be reduced, as well as critical waterfowl habitat degraded (Schmitz et al. 1993, in Batcher Undated). Mats also deposit large amounts of organic matter which increases the organic content of sediments and greatly accelerates succession patterns, allowing emergent and riparian vegetation to colonise (Penfound and Earle 1948, Trivedy et al. 1978, Gopal 1987, Woods 1997, in Toft 2000).
E. crassipes has a detrimental impact on water use by humans. In drainage canals it reduces the flow, which can result in flooding and damage to canal banks and structures. In irrigation canals it impedes flow and clogs intakes of pumps used for irrigation. Water flow patterns have been disrupted in utility cooling reservoirs. Water hyacinth interferes with navigation of both recreational and commercial craft, negatively impacting fisherman, sports-fisherman, water-skiers and swimmers in recreational waters. Limitations on water use can reduce real estate values and tourism (Batcher Undated). Economic losses may be the result of attempts to control the weed. Manual removal of the weed in China alone cost an estimated 100 million RMB yuan (US$12m) each year but was neither economic nor effective (Jianqing et al. 2001).
Location Specific Impacts:Guam 
Ecosystem change: Eichhornia crassipes can quickly cover bodies of water, creating shade and restricting oxygen exchange at the surface. Florida (USA) (United States (USA))
Ecosystem change: Pennywort is a common native that functionally occupies similar habitats as hyacinth. Differences between pennywort and hyacinth habitats highlight the role of water hyacinth as an invasive "ecosystem engineer". Research specific to hyacinth and pennywort (Hydrocotyle umbellata) in Florida has shown that overall dry biomass of hyacinth is 259% greater than pennywort (Reddy 1984, in Toft 2000). Other research in Florida on hyacinth and a different species of pennywort (Hydrocotyle ranunculodies) has shown that overall dry biomass of hyacinth is 161% greater, and the maximum root length of hyacinth is 164% greater (Jantrarotai 1990, in Toft 2000). The roots of hyacinth can be important habitat for epiphytic macroinvertebrates (aquatic invertebrates living on macrophytes) which illustrates just one of the changes brought about by the invasion of native habitats by water hyacinth (Hutchinson 1967, Schramm et al. 1987, in Toft 2000), especially amphipods (Schramm et al. 1987, Bailey et al. 1993, Bryan 1993, in Toft 2000).
Physical disturbance: Boat traffic on several rivers was halted; hundreds of lakes and ponds were covered from shore to shore with up to 200 tons of water hyacinths per acre. Texas (United States (USA))
Modification of natural benthic communities: Eichhornia crassipes has the lowest dissolved oxygen levels as compared to milfoil, hydrilla, pondweed, and a native mix of submersed plants in Texas, and was the only plant to have averages below 5 mg/L (Madsen 1997, in Toft 2000). 5 mg/L represents the level at which fishes start to experience oxygen stress (Madsen 1997, in Toft 2000).
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