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).

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). 

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