Aquatic weeds are plants which hinder or harm man's social or economic activities. Aquatic plants can limit the usefulness of irrigation systems. They can inhibit fishing and the passage of ships and they may harbour insect pests. Aquatic weeds pose a particular problem where there is a concern for water loss because they dramatically increase the rate of transpiration.
The problem is becoming greater with increased trade and development which tend to spread and encourage the growth of weeds. In particular, man-made lakes, reservoirs, canals and irrigation systems, with their different ecology from natural water courses, often provide an ideal environment for the spread of aquatic weed, especially where the water is shallow and rich in nutrients. The concentration of human settlements, the development of sewage systems and the increasing use of fertilizers ensure that large amounts of nutrients are entering water courses, there to stimulate weed growth.
Aquatic weeds are spread very quickly through the medium of the river. Boats distribute weeds up- and downstream. Currents carry them downstream and floods push them into pools and swamps and backwaters. Seeds may be carried by wildlife and the wind. Man-made water systems are particularly susceptible to weed growth, as man-made reservoirs and distributaries are often shallow and clear, as well as rich in nutrients, and weed growth follows. For example, submerged aquatic weeds have cut the flow in one large irrigation canal system in India by 80%. The result is that the reduced flow encourages seepage from the canal and thus contributes to water-logging and salinity. Finally, water cannot be moved to crops, animals, and people in sufficient quantity, on schedule.
The most serious problems are caused by the water hyacinth Eichhornia crassipes which has become ubiquitous in warm waters. For example in Florida, more than 40,000 hectares of water are covered despite multi-million dollar control programmes. In Bangladesh, water hyacinth rafts weighing up to 300t/ha are carried by floodwaters and can destroy rice fields. In the 90,000 hectare lake Laguna de Bay, Philippines, which provides much of the fish supply for Manila, water hyacinths impede access to fish pens and fish cages and frequently destroy fish culture installations when piled up by high winds, especially in typhoons. Another problem weed is the aquatic fern Salvinia sp., which covers about 12,000 ha of swamp and rice fields in Sri Lanka and 600 to 850 km2 (annual fluctuations) of the largest man-made lake in the world, Lake Kariba, Africa (40,000 km2). The edges of the lake are permanent mats of weed which cover hundreds of acres in river estuaries. Harbours are so blocked that ships can scarcely move. Life is uncertain on the lake because the wind and current moves the mats unpredictably. Fishing nets placed at night may become hopelessly entangled and shifted by morning. Fishing camps may be forced to move if the shore is blocked by weed mats to prevent the use of canoes.
Some aquatic weeds have high food potential. The best known example is water spinach Ipomoea aquatica which is widely consumed by Southeast Asian peoples and has a high protein content (19-34% dry matter). The aquatic fern Azolla prinnata is widely used as livestock feed and as a nitrogenous fertilizer in Vietnam and the Peoples Republic of China. Water hyacinth and other aquatic weeds can be made into compost or silage for agricultural use. Aquatic weeds are also used to feed herbivorous fish, particularly the grass carp, in freshwater ponds. Chopped water hyacinth is being investigated as a fishpond fertilizer since it can provide nutrients which stimulate plankton growth and can also act as a matrix for the growth of detrital bacteria: important foods for some warm water cultured fish, particularly tilapias. Similarly some water weeds are used on a small scale as human food. Young flower buds of water hyacinth are eaten in the Philippines and the leaves of Ipomoea peptans are eaten in many parts of the world. It is the fibre in leaves that limits their widespread use as a source of food, but the protein can be mechanically extracted from water weed and is of good nutritional value.