Boundaries

This ecoregion is defined by the rivers that drain into West Africa’s Gulf of Guinea (or Bight of Bénin) and lies within the Dahomey Gap, an area of savanna interrupting the Guinean forest zones along the coast of West Africa, stretching from the Cavally River in Côte d’Ivoire to the Cross River in Ghana (Sayer et al. 1992; Hugueny & Lévêque 1994). Primarily covering the southern portions of Bénin and Togo as well as southwest Nigeria, the ecoregion also extends slightly into the southeastern corner of Ghana.

Topography

In this ecoregion, rivers flow down gently sloping plateaus to the flat coastal plain, where they form extensive swampy deltas and semi-continuous lakes and lagoons before reaching the ocean. 

Freshwater habitats

The ecoregion contains a mosaic of rivers, wetlands, coastal lagoons, and lakes with varying degrees of interconnectedness, both with each other and with the Gulf of Guinea. Along the coast, connecting channels between the lagoons are often transient, drying out in the dry season. Seasonal changes in precipitation and water in-flow, and the resulting variable movement of water between coastal lakes and lagoons, produce fluctuating salinity. Many lagoons tend to be fresh to brackish during the wet season, but have elevated salinity concentrations in the dry season. Major lagoons and coastal lakes are the Lagos (500-600 km²) and Lekki (247 km²) Lagoons (Nigeria), Lakes Nokoué (150 km²), Ahémé (85 km²), Porto Novo lagoon (30 km²) and the coastal lagoon (12 km²) (Bénin), and Lakes Togo (46.6 km²) and Vogan (8 km²) (Togo). These waterbodies are often bordered by swampland that provides a link between individual lagoons and lakes, creating a vast wetland system (Hughes & Hughes 1992).

In the western portion of the ecoregion, floodplains occur along most rivers. In the more mountainous central area of Togo, floodplains occur as narrow strips, often only 25-50 m wide. There are numerous permanent swamps in the headwaters of the Okpara River, a tributary of the Ouémé, and strips of inundated forest occur along the Ouémé River (Hughes & Hughes 1992).

Terrestrial habitats

The vegetation of the ecoregion varies from the low, gently sloping plateau to the coastal plain. In the northern half of the ecoregion, open or fragmented deciduous forest dominates the vegetation, whereas it occurs in patches and bands in the south and in dense patches in the southwest (FAO Forest Resource Assessment Programme 1999). Dominant species in these medium-height deciduous and dry stands include Antiaris africana, A. welwitschii, and Ceiba pentandra (FAO Forest Resource Assessment Programme 1999). Moving south, the landscape is dominated by wooded savanna with deciduous woody trees (a mix of Combretum, Terminalia, and Acacia species) with an understory of tall grasses, shrubs, and herbs (White 1983). Further south, this gives way to secondary grassland and secondary wooded grassland, punctuated by riparian forest patches and wetlands. Tall grasslands (up to 3 m) include fire-resistant species of Andropogon, Hyparrhenia, and Pennisetum (Lawson 1986).

The sandy soils of the coastal plain support small stands of mangrove vegetation (dominated by Rhizophora and Avicennia species) around lagoons. The vegetation in the swampy deltas is a mixture of floodplain grasses, reeds, and cattails. Species of Typha and Cyperus dominate the reed-swamps, while Paspalum and Phragmites species characterize grassy floodplains (Hughes & Hughes 1992).

Justification for delineation

The Dahomey Gap most recently formed around 4,000 years BP and has remained since then, but it was also present during previous dry phases associated with glacial maxima of Ice Ages, including at the height of the last glaciation around 18,000 years BP (Lévêque 1997). Given the low levels of endemism in the coastal rivers of this ecoregion and the Nilo-Sudanian nature of the fish fauna, it is hypothesized that many of these rivers desiccated during dry phases and were subsequently recolonized by fish from the Niger River (Lévêque 1997).

Level of taxonomic exploration

Good