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Ecoregion Description
506: Niger Delta
Major Habitat Type:
large river deltas
Author:
Ashley Brown and Michele Thieme, WWF-US, Conservation Science Program, Washington, DC, USA
Reviewers:
Christian Lévêque, Centre National de la Recherche Scientifique (CNRS), Paris, France and Emmaneul Obot and Ojei Tunde, Nigerian Conservation Foundation, Lagos, Nigeria
Countries:
Nigeria
Drainages flowing into:
Gulf of Guinea, Atlantic Ocean
Climate:
The climate in the delta is cooler than upstream, though it is still tropical. Temperatures near Port Harcourt in the east of the delta range from an average of 24oC during the coolest month to 26oC during the warmest month. The ecoregion is extremely humid, with the wet season lasting for 10-11 months, usually with only one dry month in December (Hughes & Hughes 1992). At Forcados, in the western part of the delta, rainfall averages 3,800 mm per year, but decreases heading east to Port Harcourt where the annual rainfall is 2,480 mm.
Freshwater habitats:
Over 80% of the delta floods seasonally, with swamps and pools remaining when floodwaters subside (Moffat & Linden 1995). In addition to precipitation, tidal movements and the Niger River flood determine the hydrological regime. The flood begins toward the end of the rainy season in August, peaks in October, and tapers off in December. The yearly rainfall determines some fluctuation in flow but since 1968, after the completion of the Kainji Dam, the opening and closing of the dam sluices is also important. The delta experiences a strong tidal influence with sea water penetrating as far inland as the riverine floodplain of the Nun River (Hughes & Hughes 1992).
The Niger Delta consists of three major sections: the upper riverine floodplain, the lower tidal floodplain, and an outer chain of coastal barrier islands (Hughes & Hughes 1992). The upper riverine floodplain stretches for 168 km from the head of the delta at Onitsha to the lower tidal floodplain. The Niger begins to separate into the Forcados and the Nun Rivers in the upper riverine floodplain, where seasonal and permanent freshwater swamps occur. Seasonal swamp forest, dominated by Anthocleista vogelii, Carapa procera, and Chrysobalanus orbicularis, is inundated during the wet season. Permanent swamp forests, with forest floors that are inundated year-round, include members of the Alstonia, Mitragyna, and Raphia genera (Hughes & Hughes 1992).
Swamp forests of the upper riverine floodplain grade into mangrove forests on the lower tidal floodplain. In addition to the Nun, Forcados and Orashi Rivers, other main channels of the tidal floodplain are the Sombriero, Bonny, Brass, and New Calabar. Mangrove forest covers approximately 5,000 km2 of the delta and species include Rhizophora racemosa (up to 40 m height), R. harrisonii, and R. mangle (Hughes & Hughes 1992; Shumway 1999).
Sediments from the Niger River, sculpted by marine processes, have created the 20 major islands along the coast of the delta. In total, the islands comprise a surface area of 2,000 km2 and several of them reach heights of 4 m above sea level (Hughes & Hughes 1992; Ajao 1994).
The Niger Delta is ancient and has a history of sedimentation spanning tens of millions of years, with three main depositional cycles: Middle Cretaceous, Paleocene, and Eocene to the Holocene. In the Pleistocene, during a period of low sea level, the Niger River cut a deep gulf into the ancient delta. The present Niger Delta was formed during the Holocene since the end of the last Ice Age. Over the last 6,000 years the delta floodplain has advanced across brackish water mangrove swamps and the delta front beach ridges have been built up progressively further seawards. At the present times, two thirds of the sediment delivered to the delta by the Niger is supplied by the Benue (Lowe-McConnell 1985).
Fish Fauna:
Both black and whitewater rivers flow into the highly productive Niger Delta, which supports an extremely rich freshwater fauna and includes the highest concentration of monotypic fish families in the world. Cichlids, citharinids, rivulines and mormyrids dominate the fish fauna. Of 150 freshwater fish within the ecoregion, about twenty are endemic, including several freshwater representatives of marine families. The five monotypic fish families are Denticipidae, Pantodontidae, Phractolaemidae, Hepsetidae, and Gymnarchidae.
Relatively nutrient-rich silt-laden whitewater rivers (e.g., the Niger and Orashi Rivers) and nutrient-poor blackwater rivers (e.g., the Sombreiro and New Calabar Rivers) converge at the delta. Blackwater rivers contain a majority (65%) of forest fish species while whitewater rivers are dominated by savanna species (46.5%) but also contain a number of forest species. Forest species are typically smaller and unlikely to undergo large migrations (Lowe-McConnell 1985). Savanna species are more likely to migrate to and utilize floodplains, as well as to survive the more severe dry season of the savannas.
Description of endemic fishes:
Among the monotypic families, the endemic denticle herring (Denticeps clupeoides, family Denticipidae) and the hingemouth (Phractolaemus ansorgii, family Phractolaemidae) have the most limited distributions. The hingemouth possesses a completely alveolated swim bladder that functions as lungs and permits the species to survive in unoxygenated waters (Beadle 1981). Pantodon buchholzi, also from a monotypic family (Pantodontidae), is capable of aerial respiration with its swim bladder and also can leap out of the water for short distances and glide (FishBase 2001).
Other noteworthy fishes:
The vulnerable, near-endemic freshwater stingray (Dasyatis garouaensis) and the endangered thorny freshwater stingray (Urogymnus ukpam) live in the delta (IUCN 2002).
Other noteworthy aquatic biotic elements:
Sixty percent of Nigeria\'s mangrove forests (the third largest in the world and the largest in Africa) are found within the Niger Delta (Moffat & Linden 1995). The mangrove forest and freshwater swamp forests in the delta provide habitat for aquatic mammals, molluscs, and herpetofauna. The mangrove forests of the tidal floodplain are also important for numerous waterbirds, including species of heron (Ardeidae), ibis (Threskiornithidae) and pelican (Pelecanidae). Eleven species of frogs, a variety of monitor lizards, and three turtle species also reside in the delta. Crocodiles include the slender-snouted (Crocodylus cataphractus), Nile (C. niloticus), and the vulnerable dwarf crocodile (Osteolaemus tetraspis) (IUCN 2002).
Aquatic mammals present in the delta include the hippopotamus (Hippopotamus amphibius) and the vulnerable pygmy hippopotamus (Hexaprotodon liberiensis) (Moffat & Linden 1995). The vulnerable West African manatee(Trichechus senegalensis), semi-aquatic sitatunga (Tragelaphus spekei), marsh mongoose (Atilax Paludinosus), and the spotted-necked otter (Lutra maculicollis) also inhabit the ecoregion (Happold 1987; Sayer et al. 1992; Ajao 1994).
Evolutionary phenomena:
The Niger Delta is globally outstanding for its higher taxonomic endemism including five monotypic fish families (Denticipidae, Pantodontidae, Phractolaemidae, Hepsetidae, and Gymnarchidae).
Justification for delineation:
This ecoregion is delineated based on the extent of the Niger Delta and is distinguished by a rich freshwater fauna and one endemic fish family (Denticipidae). The Benin River and the Imo River mouths delimit the western and eastern boundaries of the ecoregion and the city of Onitsha sits at its apex. Between the Benin and Imo rivers the delta stretches for about 500 km along the coast (Ajao 1994).
Level of taxonomic exploration:
Good
References/sources:
Ajao, E. A. (1994). "Coastal aquatic ecosystems, conservation and management strategies in Nigeria" Southern Africa Journal of Aquatic Sciences 20(1-2) 3-22.
Beadle, L. C. (1981). "The inland waters of tropical Africa" England: Longman Group Limited.
FishBase (2001) "Search FishBase" <http://www.fishbase.org/search.cfm>(2001)
Happold, D. C. D. (1987). "The mammals of Nigeria" Oxford, UK: Clarendon Press.
Hughes, R. H.,Hughes, J. S. (1992). "A directory of African wetlands" Gland, Switzerland, Nairobi, Kenya, and Cambridge, UK: IUCN, UNEP, and WCMC.
Lowe-McConnell, R. H. (1985)"The biology of the river systems with particular reference to the fishes" In Grove, A.T. (Ed.). The Niger and its neighbors. (pp. 101-140) The Netherlands: A. A. Balkema.
Moffat, David,Linden, Olof (1995). "Perception and reality: Assessing priorities for sustainable development in the Niger River Delta" Ambio 24(7-8) 527-538.
Sayer, J. A., Harcourt, C. S., et al. (1992) The conservation atlas of tropical forests: Africa. London, UK: IUCN.
Shumway, C. A. (1999) Forgotten waters: Freshwater and marine ecosystems in Africa. Strategies for biodiversity conservation and sustainable development. Alexandria, VA, USA: Global Printing, Inc..
