Strategies for the management, development and conservation of aquatic plant resources in Nigeria are still largely inadequate. This is because basic studies on these plants are few. For instance, of the 52 macrophytes documented for Lake Kainji since 1971 only 5 among the 15 found utilizable by livestock (Obot, 1984) have been studied in detail. This dearth of information on the various macrophytes, despite their importance, compounds the issue of evolving sound management, development and conservation strategies. Studies at Kainji Lake Research Institute have led to the development of a management control model for Echinochloa stagnina (see p.17) derived from 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 Kainji Reservoir, 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 and Sacciolepis africana and the broad-leaved Polygonum senegalense, constitute a distinctive flora that is capable of covering up to 46% of the surface area of the lake (Morton & Obot, 1984). The presence of these macrophytes has been found to be beneficial to the basins' fisheries and pastoralism. Over 300 species of gamebirds, waders and other avian migrants use the aquatic vegetation as nesting sites and a source of food (Imevbore & Bakare, 1974; Ayeni, 1983). But the power generation authority (NEPA) views the vegetation as deleterious to the lake's hydrology and depressive to optimum power generation, because of evapotranspiration from the vegetation (Obot & Ayeni, 1987). The dam creation resulted in the 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 distrupted.
Macrophytes play a vital role in the lives of rural communities in much of the developing world but unfortunately very little effort is being directed towards the conservation of these resources. For example, conservation efforts in Zambian floodplains have done little to overcome the problems faced by the local communities who depend on the aquatic and other macrophytes. According to Dugan (1990) the two national parks on the Kafue's floodplain have reduced the available pasture for cattle, and placed restrictions on hunting of lechwe, an aquatic antelope. Consequently, the local people generally oppose the parks while illegal grazing, fishing and hunting have become prevalent. 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 forests 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 (Oddo, 1983). In Nigeria, the development of the mangrove areas through agro-industrial and other activities has led to pollution of the system with consequent destruction of the bio-communities.
Aquatic plants develop explosively large populations only when the environment is altered either physically or through the introduction of pollutants. 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 in waterbodies have recently become subjects of great concern. Of particular interest among the aquatic weeds is the water hyacinth (Eichhornia crassipes) in the southern areas of Nigeria (Ogunye, 1988). E. crassipes is a South American freshwater 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 macrophytes found in Nigeria of biological, engineering and economic importance are Pistia stratiotes, Salvinia spp. and Mimosa pigra (Ogunye, 1988).
Aquatic plants can also be a nuisance when high infestations of Pistia stratiotes and Ceratophyllum demersum harbour the snails transmitting schistosomes. Public health studies by NIFFR (formerly KLRI) of patterns of mortality in the rural population of the Kainji Lake basin (involving 4,652 people in 1,092 households) show that Guinea worm, sleeping sickness and schistosomiasis form the major debilitating ailments (Adekolu-John, 1982). Investigations in other parts of tropical Africa also indicate that floating and submerged macrophytes do hamper navigation and are detrimental to hydro-electric facilities. Turbine and cooling water intakes can become clogged while hydrogen sulphide can corrode metallic turbine parts. Some plants such as Salvinia cucullata, Azolla pinnata and Lemna spp. are known to pass through intake screens and clog pumps and other equipment. Island forming and floating macrophytes such as Cyperus papyrus, Salvinia molesta, Eichhornia crassipes and Pistia stratiotes have also been shown to interfere with the navigability of waterbodies (Goldman, 1978). The presence of these aquatic plants, though not presently at nuisance levels, calls for detailed studies of their nuisance potential in Nigeria.
Perhaps it is the tendency of aquatic plants to become 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 man-made lakes, for example, some aquatic macrophytes will tend to grow explosively as a result of habitat modifications. Since floating macrophytes tend to grow predominantly in reservoirs with a small drawdown, and that rooted macrophytes are associated with lakes in which the water level fluctuates significantly during its annual hydrological cycle, man-made lakes could be intentionally managed to limit the growth of floating plants. Where explosive growth is the result of pollution, the macrophytes will be brought under control by removing or modifying the source of pollution.
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, sheep 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 done either with sickles or specialised and powered mechanical cutters (Ogunye, 1988). The material thus cut is removed by hand rake, dragline or motor-driven elevator. Because of the capital outlay and labour intensiveness of this method Kusemiju 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 a very fast rate. Competition for nutrients in an efficient nanoplankton-zooplankton-pelagic-fish food chain also exerts some control on floating macrophytes.
The utilization of aquatic plants at a sustainable level can only 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 structure of surface waters and wetlands, give 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 a 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 (Dore, 1983). 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 Goronyo 20,000. Moreover, Daddy (1987) in his review, confirmed that while other, politically more powerful, 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 interests of the nomads by establishment of grazing reserves similar to those for arable crop farmers (through the establishment of the River Basin Authority) the competition for 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 causes 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 (Dugan, 1989). Under these circumstances conservation activities are usually limited. To resolve this problem, there is need to precisely analyse and identify impacts of these man induced hydrological pertubations and address them accordingly.
The conflicts highlighted above are resolvable when conservation efforts are linked to the overall development of the available resources. To achieve this, however, the conservation community will need to broaden its perspective and support those development measures which contribute to the sustainable utilization of Nigerian aquatic resources, thus helping to maintain these ecosystems for both people and other bio-communities that depend on them.
Freshwater littorals are among the most productive regions on earth (Wetzel, 1975). Aquatic macrophytes in this zone play an important role in the energy cycle of the associated water body being an important source of energy for other trophic levels in the aquatic system. 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 run-off) 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 macrophytes cause water loss through evapotranspiration. Data from Kainji Reservoir, Nigeria, (Obot, 1987) show that the relationship between evapotranspiration 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 a variety of disease vectors. The presence of Pistia stratiotes, for example, encourages the breeding of mosquito vectors of yellow fever and malaria. Eichhornia 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.
A compromise strategy can however be found. From our present knowledge of aquatic macrophytes, it is the floating plants that often become a nuisance especially when the habitat is altered either through engineering constructions or nutrient loading. When the habitat is altered through engineering constructions, the classical successional 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 (Obot, 1988).
In order to determine those morphometric and hydrologic characters which could be manipulated with best results, a comparison of these features and the macrophyte flora of other well studied African man-made lakes is necessary. Of the well known African man-made lakes, Lake Kariba (Zambia), Lake Volta (Ghana), Lake Cabora-Bassa (Mozambique), Lake Nasser-Nubia (Egypt), Lake Kossou (Côte d'Ivoire), Lakes Kainji and Jebba in Nigeria and the Kafue Gorge Reservoir in Zambia, only for Volta, Kariba, Kainji and Jebba is there detailed documentation of the macrophyte flora and its succession. These are therefore considered further. The low inflow to volume ratio of Volta and Kariba encouraged the growth of floating macrophytes due, probably, to the high load of available nutrients in such slow filling reservoirs. The 10m drawdown of Lake Kainji, compared with the drawdown magnitude of 3m for Volta and 4m for Kariba, suggests that a greater percentage of surface area of Lake Kainji, than of Volta and Kariba favours the establishment of rooted emergent species. Currently, the vegetation of Lake Kainji is dominated by Echinochloa stagnina which is an important source of dry season livestock fodder in the basin.
To optimize the production of aquatic macrophytes as livestock fodder and wildlife habitat, ideally the water levels of man-made lakes should be 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 drawdown of the lake for successful establishment and growth. The area of 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). The management is for a harvest of livestock fodder from 75% of the area covered per year (Morton & Obot, 1984). This strategy, which was derived by a mathematical modelling approach, assumes that all harvested plants will be killed. Plants cut below the water surface usually die and rot away. However, the results of experimental harvesting (Obot, 1985) showed that when a component of the vegetation such as Echinochloa stagnina is harvested above water, the above-water nodes produce new tillers and tiller density increases with each harvest. Harvesting above water thus provides an option of harvesting the same area more than once thus greatly increasing the total harvest. It may however, seem that this option will not control Echinochloa stagnina. Tillers produced during continous 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.
The rapid rate of loss of wetlands, with their associated aquatic macrophytes, sustained through the development of water systems for agro-industrial and urban development, without adequate knowledge of their values calls for urgent attention. A good knowledge of the biology, properties and potential uses of Nigerian aquatic plant resources will assist in their better management, protection and conservation. In Nigeria, urgent attention to the following is needed:
the preparation of field manuals for the identification of aquatic plants;
a comprehensive collection of aquatic plants for herbaria in order to enhance information on distribution and for the mapping of species;
a national survey of all Nigerian waterbodies including the dynamics and seasonality of vegetation and phenology;
data on biology, biomass and production and detailed studies on macrophytes in various aquatic ecosystems;
the socio-cultural importance and the role of aquatic macrophytes in nutrient cycling and fish production;
the ecology and management of Nigerian macrophytes;
the efficiency of the current aquatic weed control.