Despite considerable interest in research and development of the fisheries of Nigerian waters in recent years, surprisingly little effort has been directed at the associated non-fish resources. Apart from their ecological role, aquatic macrophytes contribute greatly to the economic, scientific, and recreational importance of Nigerian waterbodies. The present tendency to annihilate these resources without adequate knowledge of their ecology, population dynamics and socio-economic importance may spell doom to other dependent aquatic organisms. If present in large quantities, aquatic plants may have adverse effects on navigation, result in water losses through evapo-transpiration, and provide habitat for vectors of water-borne diseases. In a balanced environment they have recreational, aesthetic and medicinal values etc. This chapter highlights recommendations for their utilization, development and management.
Nigerian aquatic plants occurring in most waterbodies are similar to those in other tropical regions. They are usually strictly zoned across the land/water ecotone and generally follow a pattern such as the fringe, seasonally flooded zone, mud zone and permanently flooded (open water) zone (Obot, 1987).
The fringe zone contains vegetation similar to terrestrial savanna. Woody elements in this zone include among others Daniella oliverii, Khava senegalensis, Syzygium guineense, Isoberlina spp. Terminalia spp., Parkia clappertoniana, Vitellaria paradoxum and Ficus spp. The most frequent herbaceous plants are Crotolaria microcarpa, Cassia mimosoides, Digitaria horizontalis and Andropogon spp. The seasonally flooded zone is usually inundated for a couple of months of the year. The woody communities are usually Mimosa pigra and Mitragyna spp. The most frequent herbaceous plants are Ludwigia erecta, Leersia hexandra, Vossia cuspidata and Oryza spp. The mud zone is almost always covered by water but may be exposed for a few weeks. It is mainly colonized by Echinochloa spp. Polygonum senegalense, and Vossia cuspidata. In the permanently water-logged (open water) zone the most outstanding feature is its shallow depth and fast current (in streams). The major portion is usually vegetation-free but adjacent ponds and depressions along the river systems which contain Pistia stratiotes and Nymphaea spp. do get into this zone when floods join the two areas. A comprehensive checklist of the aquatic plants of Nigeria is given in Table 2, while Table 3 shows aquatic plant zonation. Table 4 lists aquatic plants for selected water bodies of Nigeria.
In lentic habitats, plant zonation is similar to that in rivers: emergent and floating species along the shore, which give way to floating leaved and submerged species as depth increases. But the species content here is strongly modified by water level fluctuations. In lentic habitats generally, four broad vegetation zones may be identified. In Lake Kainji, Nigeria, Obot (1986) classified the vegetation into four broad zones. At the upper water margin, only flooded yearly for a few weeks in December - January, the plant community is essentially dominated by terrestrial plants which can tolerate some degree of flooding. Below this is a zone inundated for two months (January - February) and characterized by the woody plant Mimosa pigra and some flood tolerant grasses such as Sorghum arundinaceum. The next zone, dominated by Echinochloa stagnina and other grasses, is inundated for up to eight months and the fourth zone, which stays flooded except in very dry years, is characterized by floating plants.
These descriptions are based on situations in which the vegetation had reached its optimum development. However, one has to take into account the fact that the component plant species, including the dominants have an annual activity rhythm that varies with the species. Generally, rooted floating plants dominate when water levels rise and floating plants are more dominant at seasons when the water level is low.
There is a general tendency for monocotyledons to predominate freshwater vascular plant communities. The size and the hydrology of water bodies usually determines the type and floristic composition of the communities which can develop. The aquatic vascular plants of Nigeria have received very little scientific attention and this is also true of the productivity of wetlands in general. Figures are available for the productivity of only a few aquatic/wetland plants of Nigeria (Table 5). Of these, only Echinochloa stagnina in Kainji Reservoir and Sorghum arundinaceum in the Kainji wetlands, have been studied in any detail.
The standing crop of Echinochloa stagnina in Kainji Reservoir varies widely with season and the phenological state of the plant, reaching a peak of up to 161,000t ha-1. This peak often coincides with the dry months of the year when terrestrial production is negative (Obot, 1984, 1985), rather than when the reservoir is fullest. This standing crop holds and recycles an estimated 3000t of nitrogen and 400t of phosphorus (Obot, 1984). Studies on Sorghum arundinaceum show that the plant produces up to 446kg dry matter per square metre during its growth cycle which is biphasic, and the leaf-to-stem ratio varies with the phenological state of the plant reaching a very low value (0.18) at maturity (Obot et. al., 1991). Other aquatic and wetland macrophytes are highly productive. Azolla africana for example developed a standing crop of 120g m-2 in Lake Jebba during the filling phase of the lake. Azolla also fixes atmospheric nitrogen. Neptunia oleracea, a free floating nitrogen-fixing legume, produces up to 650g m-2 dry matter annually in the northern extremities of Lake Kainji (Obot, unpublished data).
Water resources development for irrigation and domestic water supply will ultimately increase the extent of surface water and wetlands in Nigeria. The paucity of knowledge concerning the productivity of aquatic and wetland plants emphasises the urgent need to study these plants for their adequate management and utilization.
Aquatic plants offer a variety of products and services which are of considerable benefit (Table 6). Their major uses are discussed below.
Aquatic plants play an important role in the life cycle of fish. Some plants, for example, Lemna pausicostata are eaten directly (Mbagwu & Adeniji, 1988) while other fish species feed on periphytic algae growing on the surfaces of aquatic plants. Aquatic vegetation also provides breeding substrate for a large number of insects and other invertebrates which serve as fish food. Under non-explosive, non-invasional conditions, fishery managers consider floating aquatic plants as beneficial except where they interfere with methods of harvesting fish.
Aquatic vegetation also provides spawning grounds and shelter for fish fry. In Kainji Reservoir aquatic vegetation has been identified as a valuable spawning and breeding ground for a large variety of economically important fish species (Imevbore & Bakare, 1974). However, the ecological relationship and the optimum plant biomass for fish production still needs to be assessed, not only for Kainji, but also for other reservoirs, ponds and lakes of Nigeria. Various species of game birds, waders and other avian migrants commonly sighted on Nigerian lakes, use the aquatic plants as nesting sites and as a source of food. Conservation of the local manatee (Trichechus senegalensis) and the hippopotamus (Hippopotamus amphibius) also depends on the careful management of aquatic plants.
Aquatic plants accumulate large quantities of nitrogen and phosphorus in their tissues. They will therefore improve soil if applied as soil additives. The concept of biofertilizers, the growing of a minor crop that provides nutrients to a major crop, though relatively new to aquatic science, is already becoming popular. Azolla sp., a free floating fern which fixes nitrogen in a symbiotic relationship with the Cyanobacterium Anabaena azollae, is widely used as a biofertilizer for rice crops. Rice-fish polyculture, with Azolla sp. as a biofertilizer, has become popular in Asian rice farming systems. In Nigeria, the local Azolla africana is not yet exploited in this manner. The possibility of utilizing this species as a biofertilizer to fuel high yielding rice-fish polyculture is urgent and important. Also Azolla has been extensively used in China to fuel rice production in waterlogged areas (Maltby, 1986). In a report from the Phillipines, Ratchance (1972) described the successful use of dried water hyacinth as a bedding material for cultivating mushrooms (Volvaria sp). In Nigeria, there is need to assess the profitability of utilizing water hyacinth for the production of biogas, organic fertilizers, pulp and for sewage treatment.
The utilization of aquatic plants for energy centres mainly around its use as fuel for fish smoking and for domestic energy. Modern ideas on the use of aquatic plants to generate biogas are not well articulated but research results from the University of Florida Center for Aquatic Weeds are promising. The potential for and economics of conversion of aquatic plants into biogas for fish smoking and preservation need to be assessed for Nigeria.
Reports from other developing countries have shown that through biomethanation, fermentation and pyrolysis, aquatic plants can be converted into fuels whether in liquid, gaseous or solid forms. For example, in China and India, Eichhornia crassipes is digested directly to produce biogas that serves as a source of inexpensive energy supply to rural communities. In Nigeria stems of Aschynomene crassicaulis, Echinochloa spp. and Cyperus papyrus among others are used as fuel especially for cooking and fish smoking (Kio & Ola-Adams, 1987).
Aquatic plants offer a wide range of materials that could serve the needs of innovative industry. The materials could be used for construction, matting, bedding and pulp/paper.
The mature silky inflorescences of the spike of Typha australis for example, are used in stuffing pillows and mattresses and the fragrant dry tuber of Cyperus maculatus is sold in Northern Nigeria as perfume. Another source of perfume is the underground stems of Cyperus articulatus, whose leaves are burnt over the fire as a mosquito repellant, and aerial stems are used in the weaving of colourful mats commonly sold in the north of Nigeria. Vossia cuspidata, Cyperus papyrus and Eichhornia crassipes have economic potential for pulp, paper and fibre. These may be investigated as possible sources of secondary income to wetland dwelling human communities.
The potential for aquatic plants in water treatment has also been investigated but no attempt has yet been made to quantify this potential in Nigeria. Some aquatic plants are very sensitive to pollutants and could be used as biomonitoring agents. This possibility needs further investigation.
In other developing countries mangroves are being exploited and shipped for pulp and particle board. In Nigeria, there is as yet no major exploitation except for fuel and poles. The inflorescence of the nypa palm (Nypa fruticans) yields palm wine and sugar and the foliage is extensively used for thatching. Raphia vinifera is used as raw material for brushes, brooms and mats, and by retting it in running water, then beating and combing it yields piassava. It is also used locally to make roofing poles and bedsheets, cards, fishing tackle and snares for game. According to Kio & Ola-Adams (1990) strips of the young, unopened fronds and mid ribs of Phoenix rectinata are used for weaving sleeping mats, sieves and bags. Laguncularia racemosa yields timber, tannin and dyeing materials.
The most widely known aquatic plant used as food is the floating rice (Oriza sativa). Less known but also collected for food in local communities are two wild rice varieties Oriza longistaminata (in the northern states) and Oriza punctata (in the floodplains of the Cross River). The importance and potential yield of these have not yet been determined and documented for Nigeria.
Other aquatic plants offer various food items. For example, the seeds of burgu (Echinochloa stagnina) are collected for food in Monai on the western shore of Lake Kainji. The seed of Echinochloa stagnina is also an important food item for fishermen in the Inner Delta of the Niger River and a prized sugary syrup is obtained from the stalks of the grass. Kio & Ola-Adams (1987) reported that the rhizome, floral receptacle and fruits of Nymphaea lotus (water lily) are either eaten raw or cooked for food and Ludwigia stolonifera is used as an ingredient of soup in the Yelwa area of Kebbi State (Obot and Ayeni, 1987).
In Nigeria, livestock production systems are predominantly nomadic and traditional. The animals depend on the natural forage legumes and grasses for subsistence but these are usually unavailable in the dry season. Thus the animals move from their arid home range to the wetter southern parts where vegetation remains green and suitable for grazing. In a survey of aquatic environments Imevbore (1971) identified 52 macrophytes but only 14 of these were found utilizable for livestock production (Obot. 1984). Some aquatic plants can be processed as animal feeds. Even water hyacinth can be used in limited quantities in a mixture with other feeds by cattle, sheep, goats and other ruminants. Aquatic macrophytes used as fodder include Vossia cuspidata, Leersia hexandra, Bracharia mutica, Echinochloa pyramidalis, Sorghum arundinaceum, Paspalum virginatum and Echinochloa stagnina. About 7–9% of Lake Kainji has been reported to be covered at various times by plants.
The mangrove palm (Nypa fruticans) of the saltwater swamps, yields alcohol, sugar and vinegar (Maltby, 1986). This species, which was introduced to Nigeria between 1906 and 1912 (Hutchinson & Dalziel, 1963), and is presently displacing the native mangrove flora of the Niger Delta, is not exploited for these products in Nigeria.
A variety of aquatic plants are also used in curative therapy in traditional communities. A good number of these ethnobotanic materials may thus yield compounds which could be of use as modern drugs and pharmaceuticals. For example, Kio & Ola-Adams (1987), report that Polygonum senegalense is pounded with native hydrated sodium carbonate and rubbed on the limbs for rheumatic and other swellings. This concoction is also applied to syphilitic sores. Althernanthera nodiflora, on the other hand, is used for simple stomach disorders and Pistia stratiotes is used for ulcerative conditions of the mouth and tongue. Obot and Ayeni (1987) also report that Pistia stratiotes is used as part of a concoction for the treatment of 'flu. The emergent nitrogen-fixing legume Neptunia oleracea is used in the treatment of yellow fever and Guinea worm infection.
The use of these ethnobotanic materials is community and culture specific. There is a need for a complete co-ordinated survey and documentation of these materials for subsequent development and utilization. Bubayero (1986) confirmed that between 75 and 80% of the Nigerian populace patronise the traditional healers that make use of a variety of plants including aquatic macrophytes (Table 6). Many of these aquatic medicinal plants yield exceptionally promising compounds for use in modern drugs and pharmaceutical industries. Polygonum senegalense, Nymphaea lotus stems and roots are traditionally used in eruptive fevers and for urethral discharges. The stems and roots are also regarded as emollient and diuretic while the decoction of the flower is narcotic and sedative. In some parts of the country Heliotropium indicum is used for treatment of fever in children and also as a vermifuge and eye-lotion. In Ghana, it is commonly mixed with clay to arrest abortion. The juice of Ethulia conyzoides can be squeezed into the eyes for headache; the root when mixed with red pepper treats constipation, and the leaves are given in food to prevent abortion. Cyperus articulatus is used to treat cough and when mixed with grains of paradise (Aframomum melegueta) would cure headache if applied to the forehead. Cyperus articulatus when dried and pulverised, is used as a fumigant and can be mixed with scented resins for the clothing, and as air-fresheners in rooms.
The bark of mangroves is extensively used in many parts of West Africa for diarrhoea or dysentery especially in children, to check haemorrhage, for sore throat and also for urethral infection (Dalziel, 1937). Kio & Ola-Adams (1990) also confirmed that, in Cameroon, mangrove bark is used for leprosy and craw-craw, when pulverised and rubbed into the scarified skin and it is also boiled for use as a lotion.
Mobility, through nomadism and transhumance, is a practical means of avoiding the over-use of land resources in one location. In the sahelian zone it is practised by the Fulanis who, currently, produce up to 80% of the animal protein utilized in the country. However, due to physical land development the grazing land available to nomadic livestock is being progressively reduced. This has raised critical and urgent ecological problems arising from the over use of environmental resources, (fodder, water and pasture) in the sahelian areas. This results in the frequent migration of nomadic cattle to the savanna areas where the habitat is further impoverished through the lopping of tree branches, by the nomadic Fulani, to feed their herds. This process encourages desertification. Moreover, conflicts often result between Fulanis and the resident farmers either because the Fulanis refuse to pay ‘jangali’ or they arrive too early and graze unharvested crops.
There are aquatic plant species of great potential in horticulture and recreation. Of greater potential are some members of the Family Orchidaceae such as Eulophia caricifolia, Eulophia horsfallii and Eulophia angolensis. Smaller aquatic plants such as Najas sp. may also be used in the aquarium as ornamentals and as agents of aeration.
The presence of aquatic plants has favoured the possibilities of most Nigerian water-bodies being developed into recreation centres that will include sport, hunting, fishing, birdwatching, nature photography etc. There is an urgent need for the Nigerian Government to develop this sector to the standard obtained in Kenya. The annual income from visitors to Kenya's Amboseli National Park was estimated as US $1.3 million in 1979, where the associated aquatic systems are one of its principal attractions (MENR, 1981). In 1980, 5.3 million Americans spent US $638 million on hunting macrophyte-dependent water fowl and migratory birds in the USA (US Dept. of the interior and Dept. of Commerce, 1982). The most developed, and in fact the first National Park in Nigeria, is the Kainji Lake National Park. However, most emphasis is placed on the terrestrial wildlife, the Kainji Lake hydroelectric dam complex and historic sites (Ibeun and Nehir, 1989) without due attention to the variety of the associated aquatic fauna and flora.
