Aquatic weeds and their utilization 319processed. Because plants grow and die relatively quickly under tropical
conditions, harvesting of these plants from the water body needs to be done
properly to prevent plant decay and impairment of the water quality. Application
of the area doubling time in plant harvesting is given in Examples 7.3 and 7.4
Due to their prolific growth, aquatic weeds can cause many problems, the
major ones being listed below:
- Water loss by evapo-transpiration,
- Clogging of irrigation pumps and hydroelectric schemes,
- Obstruction of water flow (Figure 7.5),
- Reduction of fish yields and prevention of fishing activities,
- Interference with navigation,
- Public health problems, as aquatic weeds can become the habitat for
several vectors, - Retardation of growth of cultivated aquatic macrophyte crops, e.g.,
rice and water chestnut, - Conversion of shallow inland waters to swamps.
The problem of aquatic weed infestation is global but is particularly severe in
the tropics and subtropics where elevated temperatures favor year round or long
growing seasons, respectively. The annual world cost of attempts to control
aquatic weeds is nearly US$2,000 million (Pirie 1978).
Currently the most serious problems associated with aquatic weeds are
caused by water hyacinth, which is now more or less ubiquitous in warm waters.
In the tropical and subtropical southeastern USA, there is a serious water
hyacinth problem. In Florida alone more than 40,000 ha are covered by the
plant, despite a continuous control program costing US$10-15 million annually.
Subsistence-level farmers in the wet lowlands of Bangladesh annually face
disaster when rafts of water hyacinth weighing up to 270 tons/ha are carried
over their rice paddies by floodwaters. The plants remain on the germinating
rice and kill it as the floods recede. In India, large irrigation projects have been
rendered useless by plants that block canals, reducing water flow significantly.
Water hyacinth came originally from South America where it causes few
problems since it is kept in check by periodic flooding and changes in water
level. The plants are flushed out as a water body enlarges due to seasonal
flooding and as the floods subside the aquatic plants are left stranded on dry
land above the receding water level (Mitchell 1976). The absence of natural
enemies in their new environments has often been implicated as a casual factor
in the rampant growth of water hyacinth and other aquatic weeds. Therefore, the
absence of periodic flooding in artificial lakes and irrigation schemes may be the
major contributing factor to the development of aquatic weed problem. This
problem is further exacerbated by eutrophication from human, animal and agro-