The water hyacinth Eichornia crassipes is the world's worst aquatic weed. A native of South America, it has been spread all over the world. It constitutes a serious hindrance to river traffic and affects fishing and irrigation. Large masses of it, floating or immersed, accumulating in hydro-electric installations, behind dams and in reservoirs, may create various problems by clogging grids and speeding up silting processes.
Eichhornia crassipes occurs in many streams, lakes, reservoirs and swamps of the tropics. It is in the Nile and Zaire rivers, Lake Victoria, the delta of the Mississippi, and is found all along the coast of southeastern USA, and is distributed all across South Asia. In parts of the world this weed has always been a major problem. But Eichornia crassipes is still spreading to new areas; when the plants enter a stream they are carried up the river by boats. The wind, floods and currents push them into back-waters and pools and swamps. River people use water hyacinth plants as pads in canoes, and to plug holes in their charcoal sacks as they are transported from the bush. The seeds may be carried in mud on the legs and fur of wildlife. Finally, humans have introduced the plant around the world of their attractive flowers.
The first infestation was seen in the Zaire river in 1952 and by 1955 it covered 1,500 kilometres. Using herbicides, several thousand kilometres of the river were cleared by 1957. But after all this effort, it was estimated that 150 tonnes per hour were still passing Kinshasa down near the sea.
The first infestation in the Nile was noted in 1958. The government of Sudan created a Water Hyacinth Section of the Plant Protection Department which took charge of control measures. Previously, the weed was contained above the Jebel Aulia dam near Khartoum. But seedlings have now been found far downstream. It can be commonly seen floating in the Nile in Cairo. As it was found that to eradicate the weed in Sudan is impossible, current activities concentrate on its conversion into animal feed, fertilizer, and for energy production.
Chemical and mechanical means for controlling water hyacinth have proved either ineffective, too expensive, or environmentally hazardous. In the USA, Australia, Sudan and some other countries, substantial reduction in water hyacinth production has resulted from introducing biological control using the weevil Neochetina eichorniae. A programme for the mass culture and release of these insects in the Sudan was commenced in 1979, and they are now successfully established and dispersing in the White Nile. More biocontrol agents are being tested.
Infestation of Lake Victoria, the second largest freshwater lake in the world, has been triggered by tree clearance in the water catchments. This has caused excessive soil erosion and accumulation of nutrients in the waters of the lake. Additional nutrients are contributed by agricultural activities, such as sugar cane farms, upstream of the lake. Release of Neochetina eichorniae into Lake Victoria was reported to have reduced the population of the weed by 60% in two years.
The main threats to water quality in Africa include eutrophication, pollution and the proliferation of invasive aquatic plants such as the water hyacinth Eichhornia crassipes and Salvinia molesta weeds. The water hyacinth has seriously affected most water bodies in the region, including Lake Victoria, the Nile River and Lake Chivero. As no effective means of controlling this weed has yet been found, the water hyacinth will continue to disrupt water transport, water supplies to urban areas, the fishing industry, power generation and the livelihoods of many local communities.
Eichornia crassipes stops ships, and villagers on rivers who need the protein from fish to supplement their grain diets cannot reach their fishing grounds. Hydro-electric schemes and irrigation pumps are affected. Bridges are pushed over, and in the Far East the floods cause great islands of the weed to go crashing through the fences placed in rivers for fish culture. Insects which are vectors of human and animal diseases are harboured in the weeds, and the dangers from snakes and crocodiles are increased. Fishing is reduced because there is little light and oxygen under the thick weed mats.
Recently, benefits of water hyacinth have been recognized. It can be put to use in a freshwater environment to remove chemical pollutants from waste water, and can be used for feed, fertilizer or biogas production. In specially built treatment ponds, organically rich waste water is efficiently purified by this plant and the water exits clear, odourless and essentially unpolluted. One hectare of water hyacinth has the potential to extract about 4 tonnes of nitrogen and 1 tonne of phosphorous each year. Water hyacinth can also be used as mulch to prevent water losses from evaporation. Other uses, such as manure, compost, and addition to animal feeds have been applied with varying success.