Ecoregion Description

517: Bight Drainages

Major Habitat Type:

tropical and subtropical coastal rivers

Author:

Ashley Brown and Robin Abell, WWF-US, Conservation Science Program, Washington, DC, USA

Reviewers:

Christian Lévêque, Centre National de la Recherche Scientifique (CNRS), France and Philippe Lalèyè, Universite Nationale du Bénin, Faculté des Sciences

Countries:

Benin; Ghana; Nigeria; Togo

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.

Drainages flowing into:

Gulf of Guinea, Atlantic Ocean

Main rivers or other water bodies:

The larger rivers of the ecoregion are the Mono (Togo and Bénin), the Ouémé (Bénin), and the Ogun-Oshun (Nigeria). The Mono River drains the southern and central parts of the Chaine du Togo mountain range and forms the border between Togo and Bénin (Hughes & Hughes 1992). The Ouémé River (510 km long) drains most of Bénin’s low (200 to 300 m asl) southern plateau. The Ogun and Oshun rivers drain a low plateau in the south-west corner of Nigeria and flow into the Lagos and Lekki system of lagoons along the coast.

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. 

Climate:

The region experiences a distinctly bi-modal rainfall pattern, with most annual precipitation falling between March and July and between September and November. Mean annual rainfall decreases from west to east, from 850 mm per year in the Mono River delta to 1,750 mm per year in the Ogun-Oshun basin (Hughes & Hughes 1992). Temperatures are relatively constant year round, with little difference between daily maxima and minima (34ºC and 22ºC) (Sayer et al. 1992).

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).

Fish Fauna:

The rivers and coastal lagoons of the Bight Coastal ecoregion host an aquatic fauna rich in fish and molluscs but low in endemism. The ecoregion contains only 6 endemic fish out of 153 fish species. Dominant fish families include Aplocheilidae, Cyprinidae, Cichlidae, and Mormyridae. 

The system of coastal lagoons and lakes within this ecoregion are a unique habitat type. The salinity and volume of water within these lakes and lagoons varies throughout the year with the wet and dry seasons. The fish species composition of Lake Nokoue may vary with salinity, although the most common species (including Ethmalosa fimbriata, Hemichromis fasciatus, Sarotherodon melanotheron and Tilapia guineensis) are present year-round (Hughes & Hughes 1992; Laë 1992). 

In the Upper Ogun River, research suggests that the fish community exhibits seasonal variation in species composition (Adebisi 1988). At the onset of flooding, an increased relative abundance of piscivores has been observed, whereas omnivores were more dominant as floodwaters receded, and herbivores and insectivores dominated during periods of low water level (Adebisi 1988).

Based on studies in the Ouémé River, Welcomme (1979) suggested a general classification of fish species in the river and its floodplains; researchers have subsequently applied this classification to river systems around the world. The riverine fish communities are loosely categorized as ‘white fish’, ‘black fish’, and ‘grey fish’. White fish, such as Cyprinidae and Mormyridae, depend on main river channels for breeding, and often migrate extensively to breed at the onset of floods (Lévêque 1997). Black fish, such as Anabantidae, Channidae, and Clariidae, live in the floodplain or marshy river fringes, and often have adaptations to resist harsh environmental conditions such as anoxic waters. Black fish limit movements to lateral migrations. Grey fish, such as Cichlidae, Citharinidae, and Mochokidae, live in fringing vegetation, backwaters, and the edges of floodplain lakes during the wet season and inhabit the main river channel during the dry season. With their lateral migrations into the floodplains to breed and feed, grey fish in have more flexible behavior than either the black or white fish, and they adapt to changing hydrological conditions easily (Lévêque 1997).

Other noteworthy aquatic biotic elements:

About thirty species of molluscs, with 3 endemics, are present in Bight Coastal. The ecoregion is also rich in aquatic frogs, with 49 species and 6 endemics, which are primarily associated with forests and wet savanna-woodlands.

Several large mammal and reptile species live in the coastal streams of this ecoregion. Mammals present include the vulnerable West African manatee (Trichechus senegalensis) and the hippopotamus (Hippopotamus amphibius) (IUCN 2002). The Nile crocodile (Crocodylus niloticus), the slender snouted crocodile (C. cataphractus), and the dwarf crocodile (Osteolaemus tetraspis) also live in the rivers of this ecoregion.

A rich and prolific avifauna lives on the Mono River (Hughes & Hughes 1992). Along the upper Mono, the Ramsar Reserve de faune de Togodo, composed of ponds and swamps ideal for wading birds, is a stopover location for migratory birds (Wetlands International 2002). In Bénin, ou Dendi in the northeast of the ecoregion supports large overwintering populations of comb duck (Sarkidiornis melanotos)and white-faced whistling duck (Dendrocygna viduata) (Scott & Rose 1996). 

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

References/sources:

Adebisi, A. A. (1988). "Changes in the structural and functional components of the fish community of a seasonal river" Archiv fur Hydrobiologie 113(3) 457-464.

Food and Agriculture, Organization (2001) "Global forest resources assessment 2000. FAO Forestry Paper 140". Rome, Italy. FAO.

Hughes, R. H.,Hughes, J. S. (1992). "A directory of African wetlands" Gland, Switzerland, Nairobi, Kenya, and Cambridge, UK: IUCN, UNEP, and WCMC.

Hugueny, B.,Lévêque, C. (1994). "Freshwater fish zoogeography in West Africa: Faunal similarities between river basins" Environmental Biology of Fishes 39 365-380.

Laë, R. (1992) "Les pêcheries artisanales lagunaires ouest-africaines : échantillonnage et dynamique de la ressource et de l'exploitation, Collection Etudes et thèses". Paris: ORSTOM.

Lawson, G. W. (1986). "Plant ecology in West Africa: Systems and processes" Chichester, UK: John Wiley & Sons.

Lévêque, C. (1997) Biodiversity dynamics and conservation: The freshwater fish of tropical Africa. Cambridge, UK: Cambridge University Press.

Sayer, J. A., Harcourt, C. S., et al. (1992) The conservation atlas of tropical forests: Africa. London, UK: IUCN.

Scott, D. A.,Rose, P. M. (1996). "Atlas of Anatidae populations in Africa and Western Eurasia. Wetlands International Publication 41" Wageningen, T he Netherlands: Wetlands International.

Welcomme, Robin L. (1979)"Fisheries ecology of floodplain rivers" In London, U. K. and New York, NY: Longman.

Wetlands, International (2002) "Ramsar Sites Database: A directory of wetlands of international importance" <http://www.wetlands.org/RDB/Ramsar_Dir/_COUNTRIES.htm>(2003)

White, F. (1983) "The vegetation of Africa, a descriptive memoir to accompany the UNESCO/AETFAT/UNSO vegetation map of Africa, Natural Resources Research 20: 1-356". Paris, France: UNESCO.