Central African mangroves
The Central African mangroves flank portions of the Atlantic Ocean coastline of western and central Africa, in suitable low energy marine environments. The most extensive mangrove stands of this ecoregion are found in the Niger River Delta, which supports the largest area of mangrove occurrence in Africa. The mangroves of this region have no endemic species but support some endangered species, such as manatees and pygmy hippopotami in the Niger Delta. Mangroves here, as elsewhere, are important as nursery and feeding areas for marine fishes, and they also trap large amounts of sediment. The oil industry, clearance for salt pans, and overcutting by an increasing human population pose significant threats to these mangrove habitats, although some of these areas are contained within protected zones.
Location and General Description
The Central African mangrove ecoregion is located in western Africa, and encompasses certain mangrove areas along the coastlines of Ghana, Nigeria, Cameroon, Equatorial Guinea, Gabon, Democratic Republic of Congo (DRC), and Angola (to 19°18' S). The structure of the mangrove areas varies considerably, from the lagoon systems found in the western part of this ecoregion to systems modified by complex patterns of sediment deposition at river mouths in the central and southern portions.
Climatic conditions are primarily humid and tropical, but change to more temperate conditions towards Angola. Off the coast of the DRC, mangrove development is inhibited by the presence of the coldwater Benguela Current, but some stands are found where elevated river water temperatures counteract this current. Annual rainfall varies from a mean of 750 millimeters (mm) in Angola to 6000 mm in Cameroon. In Ghana and the western part of Nigeria, mangroves are primarily associated with extensive lagoons. These are enclosed part of the year by sediments, when rainfall is lower and freshwater outflow is not sufficient to counteract ocean swells. In the remainder of the region, mangroves are primarily associated with river mouths, the largest of which is the Niger River Delta, which may discharge up to 21,800 cubic metres per second at peak flow in mid-October. The sediment load flowing from the Niger River has been estimated to be 20 million cubic metres per annum, most of which is captured in the mangrove swamps. Sediment deposition and channel erosion have created a network of river creeks, estuarine swamps and barrier islands. Soils range from recently deposited unconsolidated, soft dark mud containing silt, clay and peaty clay, to transitional swamps, all of which are associated with different types of vegetation.
The key factors that influence these mangrove ecosystems are river floods and the tidal range. Tidal range increases from west to east, reaching a maximum of 2.8 meters in eastern Nigeria. This allows flood tides to penetrate up to 40–45 kilometers into the interior. The large inputs of freshwater create a low-salinity zone offshore where salinity fluctuations range between zero and 0.5 percent during the rainy season, and 30 to 35 percent during the dry season. Farther south in Cameroon, annual rainfall reaches 6000 millimeters, but is highly changeable because of variation in topography and coastal types. These high freshwater inputs, together with a convergence of the Guinean counter-current, the Benguela current, and an equatorial subsurface current, creates a "piling up" of water that results in an increase in mean sea level of 1.2 m and creates an unusual circulation pattern. It also results in the formation of sandbars and the deposition of large amounts of suspended sediment in the mouths of estuaries.
Five species of mangroves in three families are found in this region, including the Red Mangroves, Rhizophora racemosa, R. mangle, and R. harrisonii, Black Mangrove (Avicennia germinans) and White Mangrove (Laguncularia racemosa), as well as an introduced species, Nypa fruticans. Rhizophora racemosa is the primary colonist in the open lagoon systems, whereas Avicennia africana is the primary colonist in closed systems. Vegetation varies depending on whether the soils consist of sandy troughs or muddy hollows. In the back swamps Nypa fruticans is replacing red mangroves because it is a quicker colonizer and has shallow roots which destabilizes river banks. This is occurring rapidly from the western shores of Nigeria to the interior creeks of the Niger Delta. Rhizophora racemosa is dominant in the tidal and more inundated areas of Cameroon, where mangroves are found concentrated in two locations to either side of Mount Cameroon. Farther south in DRC where mangroves are found around lagoons, the dominant species is Rhizophora mucronata. In Angola, large mangrove communities occur at the mouths of the Cuvo, Longa, Cuanza, Dande, and M'Bridge Rivers, though they are not as extensive as the vast mangrove swamps at the mouth of the Zaire River. The dominant trees are Rhizophora racemosa, R. mangle, R. harrisonii and, Avicennia africana, the former two species reaching heights of approximately 30 meters (m).
While these mangroves contain no endemic species, they are known for their diverse pelagic fish communities, including some narrowly distributed species, abundant avifauna, and the presence of some rare mammals and turtles.
The mangroves provide habitat to the threatened West African Manatee (Trichechus senegalensis, VU), the African Softshell Turtle (Trionyx triunguis), and in the Niger Delta, isolated populations of Pygmy Hippopotamus (Hexaprotodon liberiensis heslopi, VU). The near-endemic Sclater's Monkey (Cercopithecus sclateri) may also use the brackish portion of the mangroves. Hippopotamus (Hippopotamus amphibius), Nile crocodiles (Crocodylus niloticus) and a variety of amphibians are also found in the rivers, lakes and floodplains surrounding the ecoregion. The mangrove forests are also home to the talapoin monkey (Miopithecus talapoin). Summer visitors include at least five species of marine turtle, Leatherback Sea Turtle (Dermochelys coricea, EN), Loggerhead Sea Turtle (Caretta caretta, EN), Olive Ridley Turtle (Lepidochelys olivacea, EN), Hawksbill Sea Turtle (Eretomychelys imbricata, CR), and Green Turtle (Chelonia mydas, EN).
Coastal mangroves and wetlands are primarily important for large concentrations of birds that use the areas during migration, although some wetland species also breed here. Several of the coastal wetland sites are internationally important for migratory wetland birds. The importance of the coastal lagoons of Ghana for wetland birds is summarized by Piersma and Ntiamoa-Baidu, and for the Niger Delta in Hughes and Hughes. Jones also provides a general summary of the biodiversity values of the Gulf of Guinea.
The mangroves are also important to species found primarily in adjacent habitats, but who may also depend on mangroves for parts of their life cycle. The Niger Delta provides spawning/nursery areas for the fisheries in the Gulf of Guinea. A high diversity is found in the pelagic fish community, with 48 species in 38 families. Pelagic families and species associated with them include Clupeidae (Ethmalosafimbriata, Pellonulaleonensis, Ilishaafricana, Sardinella maderensis), Belonnidae (Ablennes hians, Strongylura senegalensis), Megalopidae (Megalops atlanticus), Hemiramphidae (Hyporhamphuspicarti), Elopidae (Elops lacerta, E. senegalensis), and Albulidae (Albula vulpes).
Estimates of mangrove area provided by Spalding et al. range between 16,673 and 17,176 square kilometers, of which more than two-thirds are found in Nigeria. Delineations on maps of mangrove areas suggest even larger extents, but estimates are problematic because the mangroves are interspersed with swamp forests.
The most important remaining blocks of habitat are found in the Niger River Delta in Nigeria, to the east of the mouth of the Cross River in Nigeria and Cameroon, around Doula in Cameroon, and the Muni Estuary and Como River in Gabon. Smaller areas of habitat are also found in Ghana, in the Conkouati lagoons of Congo, at the mouth of the Congo River in the Democratic Republic of Congo, and in Angola. The Niger Delta has been growing for millions of years and is still in the process of expanding into the Gulf of Guinea. The delta mangroves mark the transition between swamp forest habitats to pioneer communities on the coast and can extend up to 40–45 km wide.
It is thought that 3,165 km2, or 10.68%, of the Central African mangroves , are in some form of Protected Area. These include the Douala-Edea Faunal Reserve in Cameroon (1600 km2) and the Anlo-Keta Lagoon Complex and Songor Lagoon in Ghana. Draft management recommendations have been prepared for the Ghanaian coastal zone, including mangrove areas.
Types and Severity of Threats
Fragmentation itself does not greatly affect mangrove biodiversity, as mangroves are naturally fragmented, and are able to disperse over long distances. Of greater concern is the total amount of mangrove area lost to urbanization, industrialization, and agriculture, as well as impacts from timber and petroleum exploitation. Timber is primarily used for fuelwood and poles for housing construction. Impacts from petroleum exploitation include coastal subsidence that may aggravate the effects of sea-level rise, as well as infrastructure development and oil spills that have led to large mortalities of invertebrates and fishes. Exporting oil from coastal areas is an economically important activity in Nigeria, Gabon, and Cameroon that can lead to oil spills. In Nigeria, during the past 30 years, seismic lines have been placed in the Niger Delta mangrove forests. Other threats include the practice of gas flaring, the use of poison and dynamite for fishing, canalization, discharge of sewage and other water pollutants, siltation, sand mining, erosion, construction of embankments, and growing population pressure in the coastal zone.
Justification of Ecoregion Delineation
The Central African mangroves range from Ghana east of the Dahomey Gap, through the Niger Delta (the largest concentration of mangroves in Africa) south to the mouth of the Congo River, with outlying patches in Angola. This ecoregion generally follows that part of the African coastline that is affected (at least occasionally) by the cold water Benguela Current.
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