Niger Delta Ecosystems: the ERA Handbook/The Natural Brackish-water Alluvial Equatorial Monsoon (BAM) Ecozone
7 THE NATURAL BRACKISH-WATER ALLUVIAL EQUATORIAL MONSOON ECOZONE
- Mangrove Forests and their Distribution
- Plant Species Composition of the Nigerian Mangroves
- The Dynamics of Mangrove Ecology
- Food Chains of the BAM Ecosystem
- Animal Communities of the BAM Ecozone
- The Brackish-water/Freshwater Ecotone
This ecozone is often referred to as 'the Mangroves'. However, as with the other ecozones of the Delta, we need a term that can apply to both the natural ecozone and its present-day situation as a human ecosystem. As mangrove forests have in many instances been damaged or cleared, we will again use the ERA terminology and refer to this ecozone as the Brackish-water Alluvial Tropical Monsoon, or BAM.
The ecozone extends from the Benin Estuary to the Sanaga Estuary in Cameroon, and in the original and natural ecosystem mangrove forest covered more than 95% of the area, the remaining portions being ecotones between Brackish and Fresh-water ecosystems.
7.2 MANGROVE FORESTS AND THEIR DISTRIBUTION
Human influence has been far less dramatic here than in other parts of the Niger Delta and much of the ecozone remains mangrove forest.
#The following conditions are necessary for the natural development of mangrove forests:
- tropical or sub-tropical tidal waters
- water-logged alluvial deposits
- a brackish water regime, lying between the high and low tide levels
- protection from the battering of oceanic waves
Mangrove forests therefore developed along the more sheltered parts of the African coasts, where freshwater from lagoons and rivers mixes with seawater.
#Three key points should be made regarding their ecology:
- Mangroves tolerate, rather than prefer, brackish water. There are only a few mangrove species, and they are not all closely related; what they have in common is the ability to tolerate brackish water. It must be stressed that mangroves do not require brackish conditions; they actually prefer lower salt concentrations, but compete badly with other species. However their ability to thrive in brackish, waterlogged conditions is unique amongst the tropical plants, and gives them the competitive edge to overrun these coastal areas.
- Mangroves are pioneers. Mangrove forests represent pioneer communities, colonising new alluvial deposits and helping to form new land. Once in place, mangrove roots trap alluvial deposits and organic matter; this material, in addition to the biomass of the mangrove trees themselves, eventually creates a rich organic soil rising above the high tide mark. From then on, freshwater plant species can begin to invade the ecosystem and the weaker mangrove species are displaced.
- Mangroves will not develop on bare sand. They need at least a layer of silty or sandy clay. Because alluvial mud and the subsequent mangrove soils develop in waterlogged anaerobic conditions in a brackish-water regime, they are acid sulphate soils or 'sulphaquepts' (see 4.5.5). They range from the younger Cat Clays, recently deposited alluvial muds with little organic matter, to the peaty Chicoco soils that have a high organic matter content as a result of the mat of roots that have developed.
#The Niger Delta Contains the largest discrete mangrove forest in Africa
Mangrove forests do occur on the East coast of Africa and in the Red Sea, but the most extensive African mangrove forests extend from Lake Nokoue, in the Benin Republic, to the Gabon Estuary. Along the coasts of Benin Republic and Southwest Nigeria, between the Imo and Calabar/Cross River Estuaries and in the Cameroon Estuary, they have been greatly disturbed.
The Niger Delta mangrove forest is itself disturbed as a result of urban and industrial development around Warri, in the Forcados Estuary and particularly around Port Harcourt and along the Bonny River, where invasion by Nypa Palm is a problem. However it remains the largest discrete mangrove forest in Africa, covering about 5000 kms.
In terms of species composition the mangrove forests of the West African coast, including Nigeria, are the same as those on the tropical American shores on the other side of the Atlantic, thousands of miles to the West. Yet those of East Africa are more similar to the Asiatic mangrove forests. This is because the cold waters off the southern tips of Africa and South America limit the drift of mangrove seeds in or out of the Atlantic (while they are able to move between the Indian and Pacific oceans more comfortably).
7.3 PLANT SPECIES COMPOSITION OF THE NIGERIAN MANGROVES
West African coastal mangrove forests, from Senegal through Nigeria to Angola, all have the same plant species composition. Three species of Red mangrove live in the deeper water: Rhizophora racemosa, R. harrisonii and R. mangle. The White mangrove species Avicennia germinans and A. nitida colonise more shallow waters, together with two of the Black mangrove family, Conocarpus erectus and Laguncularia racemosa.
However, the Nigerian mangrove forests are dominated by the Rhizophora genus, mainly the stilt-rooted R. racemosa (in association with Avicennia nitida), and the shrub mangrove, Laguncularia racemosa. Rhizophora tends to be the pioneer, while Avicennia grows on firmer established ground.
Only a few other plant types are associated with these trees. Most commonly found are the Fern Acrosticum aureum, Bamboo, the Raffia Palm Raphia vinifera (the raffia palm from which wine is tapped), the purple branched climbing shrub Hibiscus tiliaceus (?) and the exotic Nipa Palm (Nypha fructicans), introduced around 1919 from Malaya.
Around towns and industrial installations in the Niger Delta, the relatively useless Nipa Palm is the tree of the future.
7.3.1 RHIZOPHORA RACEMOSA
Rhizophora racemosa is the predominant species; an arched tangle of R. racemosa stilt roots gives the Niger Delta mangrove forest its characteristic appearance.
An ordinary root system would be unable to support a large tree on such swampy ground. R. racemosa roots do not penetrate the soil to any great depth; instead, they divide into a mass of thick rootlets immediately below the surface. The tree therefore stands upon a system of arches entirely supported by a thick, felt-like raft of its own making.
Larger trees also put out rubbery adventitious roots which drop down over the water.
Mature R. racemosa can reach 2.4 metres in circumference and 45 metres in height above the stilt roots, but the more usual maximum height is 20 to 30 metres. Mature individuals can grow to this size if they are the pioneers on fresh deposits of soft mud on the riverine or lagoon-side margins of the sub-ecozone. However, later plants, developing from seedlings that have started out on mature mangrove sites, may not grow above 4 metres, as competition and impoverished soils prevent them from fully developing their root systems. It is therefore usual to find a vigorous outer fringe of mangrove forest where new silt is being deposited, with a lower shrubby tangle of smaller trees on the older soil inland.
The seeds of R. racemosa develop singly and are remarkable because they germinate and develop into a seedling, or propagule (see fig. II), while still hanging on the parent tree. The propagule is a complete young plant, with a strong stem about 30 cm in length and a hard, pointed and heavy root tip. When the propagule drops from the parent tree, the remaining fruit protects the growing point and allows the plantlet to float until the root tip catches on mud. If conditions are right, it will then sprout roots; the fruit detaches and leaf growth starts. The plant will survive if it is sufficiently well anchored to avoid being dislodged by currents or wave action, and upon depth (it must not be entirely submerged by the tide for long periods).
Growing in the most shallow inner zone of the mangrove forest, Avicennia species are also characterised by root type. Their stubby and vertical pneumatophore or 'breathing' roots grow upwards from submerged lateral roots; they rise above the reduced mud and water to obtain oxygen. Mature stands of Avicennia trees are widely spaced and straight, rising above a carpet of pneumatophores. Avicennia typically grow on the small tidal brackish-water creeks that penetrate the sand barrier islands.
Avicennia seeds also differ from those of Rhizophora; they develop in bunches and resemble soft dicotyledonous seeds. However they do also germinate on the parent tree, forming the first pair of leaves and a small pre-root before dropping and floating away until taking root in the next available mud patch. Although they are less hardy than the seedlings of Rhizophora, more of them are produced so that sufficient numbers will survive.
7.4 THE DYNAMICS OF MANGROVE ECOLOGY
Mangrove forests are dynamic over space and time, developing in continual succession. They serve as transitory communities, colonising and stabilising new ground before being replaced by more permanent vegetation. A typical mangrove forest could be described as follows:
- On the sides of the main rivers and lagoons, where fresh silt is being deposited, a belt of tall Rhizophora racemosa trees grow on the young 'Cat Clay' soils. Their adventitious roots drop downwards from stems hanging over the open water.
- On the more established new ground behind these tall trees is a tangle of much smaller R. racemosa. A Chicoco soil is developing, and adventitious roots are noticeably absent.
- Further inland, the trees become more tangled and shrublike; R. mangle individuals begin to appear.
- Where the soil has risen to such a degree that the long Rhizophora propagules are unable to float across it during the shallow tidal inundation, there tend to be only a few sparsely spaced shrubby trees. Avicennia are found here, because their small pre-germinated seeds are more successful in the shallow water and there is room for them to spread out their lateral roots.
- Finally the soil is raised well above the waterline and is well aerated by the burrows of land crabs. Concurrent with these soil changes, the mangrove vegetation is replaced by dry-land communities.
It takes about a hundred years for fresh alluvial mud to be converted into fresh-water land by mangroves: this is also the average life span of a successful Rhizophora racemosa tree.
However, the dynamics of the Niger Delta landscape itself may interrupt this simple progression. Rivers move across the delta plain and erosion patterns change; young alluvial deposits with tall Rhizophora racemosa may be eroded away by a shifting river course before they have a chance to develop Chicoco soils; a widening estuary (as at Sangana) may erode a spit and remove the protection it offered from the Atlantic breakers to a young mangrove forest developing on the edge of the lagoon behind.
Generally the sea is the final sink for organic and inorganic materials being washed off the land. In open rivers and estuaries, mangrove forests lose dissolved nutrients, detritus and semi-decomposed litter to the sea; in lagoons and more protected areas, nutrients are washed out with the tide.
However the mangrove forests of the BAM are one of the few terrestrial ecosystems (salt flats in temperate zones are another) that also take matter up from the sea. The tide brings in mineral salts, especially phosphate and nitrates, together with detritus and other sediments that become trapped in the mangrove roots and incorporated in the soil-making process. These are vital nutrients as the mangrove soils become dry land.
7.5 FOOD CHAINS OF THE BAM ECOSYSTEM
BAM ecosystems have very low biodiversity and biomass compared to the adjacent FAM ecosystems. For example, in the Okoroba-Nembe district there is mile after mile of Rhizophora racemosa and nothing else. But despite this, mangrove forests provide a refuge and breeding ground for large populations of aquatic animals which depend on them for part or all of their life cycles.
Even the most developed BAM ecosystems still have a relatively low biomass of about 150 tonnes of dry matter per hectare. However the turnover or productivity of organic matter is high—up to 15 tonnes/ha per year under favourable conditions. Half of this falls as leaves and dead wood, to be decomposed by fungi and bacteria.
The chemical compounds and detritus products of this decomposition provide nutrients for the plants and animals within the mangrove forest. However, via tidal inundation they also provide the nutrients to start food chains in associated rivers and estuaries.
Detritus-consuming animals include species of nematodes, polychaete worms, molluscs and crustaceans. They feed on the detritus itself and produce faeces; these are in turn consumed by microorganisms, which are themselves another source of food for the larger species. This is only the beginning of a complex but wonderfully productive aquatic system of interrelationships, including the following examples:
- Small fin-fish feed both directly on the detritus and upon the polychaete worms. Their faeces are in turn a substrate for more microorganisms.
- These small fin-fish are food for larger carnivorous and omnivorous fish, which are in turn food for even larger fish and for birds.
- Large zooplankton feed on the larvae of crabs and other arthropods.
- These zooplankton themselves support fish and prawns.
In this sense the mangroves of the Niger Delta are one of the most productive ecosystems in Africa, and are the beginning of a food chain that supports all the rich fishing grounds of the Bight of Guinea.
At the top end of this food chain, the dominant animals of the BAM ecozone are crustaceans, molluscs and fish. Compared with the neighbouring inshore and freshwater ecosystems, higher animal species diversity is low; comparatively few animals are fulltime residents of the mangrove forest. However, within the ecozone, greatest diversity is found in protected areas where there is strong ebb and flow of tides; here are also found high populations of crabs and mudskippers.
7.6 ANIMAL COMMUNITIES OF THE BAM ECOZONE
The animal groups are distributed as follows:
Oysters (Crassostrea gasar) are found on stilt roots of Rhizophora. Periwinkles (Pachymelania aurita) and other species) are found on the floor of more protected areas.
Land crabs burrow into the mangrove soil itself, and during high tides take refuge in Rhizophora stilt roots, stems and branches, or on adjacent dry land. Shrimps and prawns spend part of their juvenile phase feeding on mangrove detritus.
There are few vertebrates associated with the ecozone.
Being either fresh-water species that can tolerate low levels of salinity, or salt-water species that can move freely between salt and fresh-water (see Chapter 11 for a discussion of fish as a resource). However the mudskipper, Periophthalmus spp., is endemic to the BAM ecozone.
Are absent in the ecozone because they cannot tolerate brackish or salt water.
Although there is little evidence, reptiles are said to survive in the mangroves. Reptiles from adjacent ecosystems presumably visit the mangrove forest for short periods; these may include monitor lizards, the dwarf crocodile, marine turtles and snakes.
Similarly, few birds spend their entire life in the ecozone. Some nest in the BAM, such as the Hammerkop; many others visit the ecozone to scavenge and to feed on insects or fish. Some of the most common are heron, egret, kingfishers, sunbirds, weaverbirds, swallows, African Grey Parrots and Fishing Owls.
There are no fully resident mammals, although visitors include the Mona monkey (Cercopithecus mona), Squirrels (Epixerus spp.) and Bats. There is some evidence of Otter (Aonyx spp.) and the Manatee Trichechus senegalensis).
As mammals, humankind is also only a temporary resident, building small hunting, fishing and gathering island camps using chicoco soil, river sand and shells.
7.7 THE BRACKISH WATER/FRESHWATER ECOTONE
As explained above, mangrove forest serves as a purely transitory community colonising new ground, stabilising it and then being replaced by other more permanent vegetation. While mangrove species are uniquely able to tolerate brackish-water regimes, in freshwater they do not compete well with other plants and are quickly overtaken.
This means that the ecotone between brackish and freshwater systems is very narrow. It tends to be dominated by shallow rooting freshwater plants, or the deeperrooting plants that can tolerate very low salinity (such as the raffia and pandanus palms, and the paspalum grass Paspalum vaginatum). Mangrove trees themselves are rare in the ecotone, although Avicennia is sometimes seen.
Animal species of the ecotone are dominated by the adjacent freshwater communities (see Chapter 5). However, burrowing crabs are common.
Freshwater river margins may become brackish at the peak of the dry-season; in these instances they will hold similar plant communities to the brackish water/freshwater ecotone.