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the economic importance of macrophytes

Updated on November 21, 2009

the importance of macrophytes

Studies at Kainji Lake Research Institute have led to the development of a management control model for  Echinochloa stagnima derived  from the information on its colonization characteristics. This leads to the conclusion that seventy five percent of the area occupied by this plant can be harvested annually for livestock fodder. More similar studies are urgently needed. In K aiji Research Rservoir, probably owing to the extensive clearing and burning that were carried out prior to impoundment, the emergent grass Echinochloa stagnina,in association with the floating grasses, Vossia cuspidata an Sacciolepis Africana  and the broad leaved Polygonum Senegalese, constitute a distinct flora that is capable of covering up to 46% of the surface area of the lake. The presence of these macrophytes has been found to be beneficial to the basins’ fisheries and pastoralism. Over 300 species of game birds, waders and other avian migrants use the aquatic vegetation as nesting sites and a source of food. But the power generation authority views the vegetation as deleterious to the lake’s hydrology and depressive to optimum power generation, because of evapo-transpiration from the vegetation. The dam creation resulted in flooding regime of the river Niger downstream being radically altered, the vegetation and other associated  biological communities  consequently reduced  and the socio-economic activities of the rural communities disrupted.

Macropyhtes play a vital role in the lives of  rural communities  in much of the developing world but unfortunately very little effort is being made towards the conservation of these resources. Any conservation efforts in Nigeria should solicit local support through their awareness of the value and function of the flora and fauna and highlighting the dangers of overexploitation. This approach requires a consultative process among assessors, non-governmental organizations, resource users and the scientific community. Mangrove forest serve as both productive and protective forest resources in the coastal regions. They stabilize and maintain the ecosystem by checking coastal erosion, providing vital food links in the form of detritus as well as serving as breeding grounds for aquatic life. In Nigeria, the development  of the mangrove areas through agro- industrial and other activities has led to pollution of the ecosystem with consequent destruction of the bio-communities.

Nuisance aquatic plants and their control

Aquatic develop explosively large populations only when is altered  either physically or through the introduction of pollution. Non- native species may also develop large uncontrollable populations when intentionally or accidentally introduced into areas where they  have no natural enemies to check their growth. In Nigeria the presence and spread of aquatic plants inwater bodies have recently become subjects of great concern. Of particular interest among the aquatic weeds is the water hyacinth (Eichornia crassipes) in the southern areas of Nigeria. E. CRASIPES is a southern American freswshwater aquatic plant which entered Nigeria from Benin. In Nigeria this aquatic weed  clogs up water channels in the creeks, thus preventing fishing activities and transportation by boat and obstructing light penetration (for photosynthesis) into the water below the vegetation mass. Other masrophytes found in the Nigeria of biological, engineering and economic importance ar3e pistia stratiotes, salvinia spp, and mimosa pigra.

 Aquatic plants can also be a nuisance when high infestation of pistia stratiotes  and ceratophyllum demersum harbor snails that transmit schistosomes. Public health study by NIFFR, (formerly KLRI), of patterns of mortality in the rural population of the Kainji Lake basin show that Guinea worm, sleeping sickness and schistosomiasis form the major debilitating ailments. Investigations in other parts of tropical Africa also indicate that floating and submerged macrophytes do hamper navigation and are detrimental to hydroelectric facilities. Turbine and cooling water intakes can become clogged while hydrogen sulphide can corrode metallic turbine parts.

Perharps, it is the tendency of the aquatic plants to a nuisance that has discouraged their study and utilization and encouraged the tendency to eradicate them. However, it is by applying a suitable management regime that the plants may be controlled and kept within safe limits and some even utilized at a sustainable level.

In complex situations such as most multipurpose waterbodies, as is often the case in Nigeria, there is need for careful assessment of the nature, extent and potential for a weed problem before embarking on expensive control measures. In Nigeria, sheeps and goats are used to keep in check grass and weeds along the banks of large drainage channels, while cutting and raking the cut materials is the traditional and still the most usual method of dealing with most forms of nuisance aquatic weeds. It can be be done either with sickles or specialized and powered mechanical cutters. The materials thus cut is removed by hand rake, dragline, or motor driven elevator. Because of the capital outlay and labour intensiveness of this method Kusemiji et al (1987) have recommended that the cut materials be fed to pigs and rabbits. They also observed that aquatic turtles kept in ponds devoured freshwater hyacinth at very fast rate. Competition for nutrients in an efficient nanoplankton-zooplankton- pelagic-fish food chain also exerts some control on floating macrophytes.

Control and use

The utilization of aquatic plants at a sustainable level can pnly succeed if the habitat of these plants (surface water and wetlands) are properly managed. This demands habitat conservation. Surface water and wetlands, and consequently aquatic plants are constantly threatened by a number of factors which include: drainage of wetlands for crop production and for public health reasons (e.g. mosquito control), stream channelization and flood control, housing development, construction of dykes and dams, solid waste disposal,discharge of industrial waste and nutrient loading from domestic sewage and agricultural runoff. These human activities alter the rate the structure of surface waters and wetlands, gives a selective advantage to one, or a few species which develop a large population, ‘crowd’ out other species and lower the total community productivity. These, however are economic activities directed towards the improvement of human life. Arguments for these activities are likely to receive more sympathy than sentimental arguments on the morals of conservation. Therefore to ensure conservation for utilization the functional value of aquatic plants must be demonstrated.

In Nigeria, conflicts arise between resource developers and conservationists due to lack of standardization, inadequate legislation, non-enforcement of legal provisions where they exist, plain ignorance  or disregard for other users, as well as myopic planning of projects. For example, apart from the creation of ecological imbalance, the Kainji Lake project submerged old Bussa and neighbouring communities, displacing about 44,000 people while Tiga Dam displaced 12,000, bakolori 14,000 and Goronye 20,000. Moreover, Daddy (1987) inn his review, confirmed that while other, politically morepowerful inhabitants displaced by Kainji Dam were residentially and professionally taken care of, the nomads were not considered, thus the subsequent increased value of land resulted in constant conflicts between land users, which in some cases resulted in deaths. For instance, with ever increasing efforts to restrict cattle movement into the Kainji Reserved Areas, more arrests are made each year because the traditional routes and grazing areas of the nomads have been taken up by the development of the hydro electric project. Thus the herders are always prepared to take the risk and infiltrate into the reserved areas for the survival of their animals. If the developers had taken into consideration the inerets of the nomads by establishment of grazing reserves similar to those for arale crop farmers (through the establishment of River Basin Authority), the competition fo common resources would have been reduced.

The construction of dams deprives the downstream floodplain of the annual flooding and that forces farmers onto marginal lands, and in some cases cause nomads to graze their cattle on smaller areas of floodplain pasture, leading in turn to overgrazing, rising livestock mortality and emigration of many herding communities into surrounding arid rangelands where the degradation continues. Under these circumstances conservation activities are usually limited. To solve this problem, there is need to precisely analyze and  identify impacts of these man indused hydrological perturbations and address them accordingly.

In Nigeria aquatic macrophytes serve as important sources of energy for terrestrial trophic levels, especially livestock in the dry season.

In the savanna and semi-arid zones of Nigeria, livestock fodder is grossly inadequate during the long dry season as terrestrial grasses dry up and are usually burnt. Livestock thus lose form and weight. Cattle farmers therefore often retreat into river floodplains and lake basins to feed their stock with aquatic plants and other wetland macrophytes apart from this, aquatic plants act as a sink for nutrients (from agricultural runoffs) that would otherwise pollute the water. However, the presence of macrophytes in rivers, lakes and reservoirs is usually viewed as undesirable due primarily to some negative effects on fishing, transportation and recreation, water storage and engineering operations of dams and canals and for public health reasons.

In storage and engineering operations aquatic macfrophytes cause water loss through evapo transpiration. Data from Kainji Reservoir, Nigeria (obot, 1987) show that the relationship between evapo transpiration and the area of lake surface covered by macrophytes is of the exponential form. This implies that there is a critical area above which the presence of an additional area of macrophytes will not have a significant effect on water loss.

The presence of macrophytes may have health implications. Floating and submerged macrophytes provide both habitat and food for variety of disease vectors. The presence of pistia stratiotes, for example, encourages the breeding of mosquito vectors of yellow fever and malaria. Eichornia crassipes and ceratophyllum demersum provide good habitat and grazing material for Bilharzia carrying snails. With such varied interests, conflicts are bound to arise in the conservation and management of aquatic vascular plants

From ourm present knowledge of aquatic macrophytes, it is the floating plants that often become a nuisance especially when the habitat is altered either through engineering construction or nutrient loading. When the habitat is altered through engineering construction, the classical succession pattern of aquatic plants is from a floating macrophyte phase, through a submerged macrophyte phase to a stable draw-down flora. It is the floating macrophyte phase that needs to be controlled. This may be achieved by the manipulation of the morphometric and hydrological features of the water body.

To optimize the production of aquatic macrophytes as livestock fodder and wild life habitat, ideally the water levels of man made lakes should me manipulated so as to enhance the growth of rooted emergent macrophytes and repress the growth of floating and submerged aquatic macrophytes. This is hardly ever done as reservoirs are constructed with other priorities in mind. Lakes with a wide littoral zone and a large gradual drawdown are probably most suitable for the development of macrophytes for livestock fodder. The growth of rooted emergent macrophytes  could therefore be controlled through regulated harvesting. It is  pertinent to summarise  the Kainji  lake experience in this regard.

The control management strategy in Lake Kainji is based on the knowledge that the aquatic macrophyte community requires the 10m annual drawdownof the lake for successful establishment and growth. The area of the lake surface covered by the plant community in a given year has been shown to depend on the minimum and maximum water levels of the previous year (Morton &Obot, 1984). This strategy, which was derived by a mathematical modeling approach, assumes that all harvested plants will be killed. Plants cut below water surface usually die  and rot away. However, the result of  experimental harvesting (Obot, 1985) showed that when a component of the vegetation such as Echinochloa stagnina is harvested above water, the above water nnodes produce new tillers  and tiller density increases with each harvest. Harvesting above water thus provide an option of harvesting the same area more than once thus greatly increasing the total harvest. It may however, seem that this option wil not control Echinochloa stagnina. Tillers produced during continuous harvesting are not likely to  contribute  to regeneration by tillering due to loss of vigour, nor colonization through seed dispersal since the tillers produced under frequent harvesting may not flower and set seed (Obot, 1985). Regeneration will largely be derived from unharvested material. Therefore the harvest of 75% of area covered, for control, applies to harvesting above water as well as harvesting below water.  


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