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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

516: Volta

Major Habitat Type:

tropical and subtropical floodplain rivers and wetland complexes

Author:

Ashley Brown and Michele Thieme, WWF-US, Conservation Science Program, Washington, DC, USA

Countries:

Benin; Burkina Faso; Ghana; Ivory Coast; Mali; Togo

Boundaries:

The Volta River basin covers parts of six countries: small portions are in Mali, Côte D’Ivoire, Togo, and Benin, and the majority of the basin falls within the borders of Burkina Faso and Ghana. 

Drainages flowing into:

Gulf of Guinea, Atlantic Ocean

Main rivers or other water bodies:

The Volta is one of West Africa’s largest rivers, draining an area of 390,000 km2 (Petr 1986). It is fed by the Black Volta, the White Volta, the Red Volta, and the Oti Rivers, which together drain the plateau in the north, the Atakora Mountains in the east, and several highland areas in the west. 

Climate:

The north of the ecoregion is dry with intermittent streams. Rainfall increases from about 500 mm in northern Burkina Faso to about 1,200 mm in the region of Lake Volta, decreasing again in the lower Volta to about 1,000 mm near the delta. Peak rainfall is in August, although the south of the basin experiences a longer wet season (May- October) than the north (June-September). The average rainfall for the entire basin is about 800 mm/year with the highest (1,500 mm/year) occurring in the southwestern uplands. 

Freshwater habitats:

Four major tributaries contribute their seasonal floodwaters to the Volta River. The intermittent Red and White Volta Rivers, which are dry from January until the onset of rains in May, originate in Burkina Faso, meandering across the low elevation (250 m) plateau that slopes slightly southwards. These two rivers merge before joining the Volta. The Black Volta, a permanent river, arises in the highlands southwest of Koudougou, flowing northwards until its junction with the Sourou River. During flood season, the Black Volta’s waters enter the Sourou and form a large swampy area called the Mare aux Hippos. Extensive floodplains are present along both the upper Black Volta and the Sourou. These two rivers then join and flow southwards into the mainstem Volta. 

The Oti River, which flows from the east, is the Volta’s largest tributary, providing 30-40% of its annual flow. A major tributary of the Oti, in turn, is the Pendjari River, which arises from the Atakora Mountains in Benin, forms the border of Benin and Burkina Faso, and eventually merges with the Oti River after entering Ghana.

As with rainfall, flows vary substantially throughout the basin. Many of the tributaries and headwater streams of the Volta are intermittent in their upper reaches, but become permanent in their lower reaches. In the middle of the basin extensive floodplains occur along the savanna lined rivers. The Oti and its tributaries, in particular, support large amounts of inundated low-lying plains (Petr 1986; Hughes & Hughes 1992; Lévêque 1995). 

In 1964 the Volta River was dammed at Akosombo and Kpong, creating Lake Volta, the largest artificial lake in Africa with a surface area of approximately 8,300 km2 (Biney 1990). Since 1966, the lake has inundated the confluences of all of its major tributaries. Before the dam was built, the Black and White Voltas merged to become the Volta River. About 175 km downstream, the Oti joined the Volta, and 200 km downstream further the river flowed through the quartzite walls of Ajena-Akosombo Gorge. Below the gorge, the river spilled onto adjacent floodplains and into coastal lagoons and ponds during the flood season before it flowed into the sea (Petr 1986).

Terrestrial Habitats:

The vegetative cover follows the rainfall pattern with dry xerophytic savanna grasslands predominant in the north, savanna woodland covering the middle of the basin, and rainforest in the southwest (Hughes & Hughes 1992).

Fish Fauna:

Dominated by Cyprinids, mormyrids, mochokid catfishes, and characins, approximately 145 fish species are known to inhabit the Volta. The vast majority of these riverine species are insectivores, substrate feeders, or fish predators (Payne 1986). Many, including species of Alestes, Citharinus, Distichodus, and Labeo are adapted to the seasonal floods, moving upstream and onto the floodplains to spawn and feed at the onset of flooding (Lowe-McConnell 1985). Many species that have adaptations to riverine habitats have declined or been lost from the system as a result of damming.

Description of endemic fishes:

The Volta basin has low endemism and moderate richness among all taxa. Only nine fish, one crab, and one frog are known endemics. The endemic fish, Steatocranus irvinei, is a rheophyllic species with ventral fins modified as suckers to attach itself to rocks in rapidly flowing waters. This riverine species was originally found in Senchi Rapids and rapids of the Bui Gorge on the Black Volta (Petr 1986). 

Other noteworthy aquatic biotic elements:

About 40 aquatic reptiles and 25 aquatic molluscs inhabit the fresh waters of the Volta. Aquatic mammals present in the ecoregion are marsh mongoose (Atilax paludinosus), African clawless otter (Aonyx capensis), spot-necked otter (Lutra maculicollis), hippopotamus (Hippopotamus amphibius), and the vulnerable West African manatee, (Trichechus senegalensis). The single endemic frog, Phrynobatrachus francisci, congregates along floodplains and ponds to spawn during the rainy season (Rödel 2000). The freshwater prawn, Palaemon palucidens, was once abundant in the lower Volta and supported an active fishery, but has declined since dam construction (Petr 1986).

Justification for delineation:

This ecoregion is defined by the boundaries of the Volta River basin. The freshwater ichthyofauna of the Volta basin is very similar to that of the Niger, with which faunal exchanges have probably occurred during the Holocene (Lévêque 1997). The Black Volta may have previously been a tributary of the Niger River between Gao and Niamey, and only more recently been captured by the Volta (Lowe-McConnell 1985). Similarly, the Pendjari River was probably connected to the Niger, but now feeds into the Volta (Lévêque 1997). Despite these relatively recent connections, several fish species are endemic to the ecoregion, including Irvineia voltae, Steatocranus irvinei, Synodontis arnoulti, and Barbus bawkuensis.

Level of taxonomic exploration:

Fair

References/sources:

Biney, C. A. (1990). "A review of some characteristics of freshwater and coastal ecosystems in Ghana" Hydrobiologia 208 45-53.

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

Lévêque, C. (1995)"River and stream ecosystems of northwestern Africa" In Cushing, C.E.;Cummings, K.W.;Minshall, G.W. (Ed.). River and stream ecosystems. (pp. 519-536) Netherlands: Elsevier Science B. V..

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.

Payne, A. I. (Ed.) (1986). "The ecology of tropical lakes and rivers" U.K.: John Wiley & Sons.

Petr, T.,Mitrofanov, V. P. (1998). "The impact on fish stocks of river regulation in Central Asia and Kazakhstan. Lakes & Reservoirs" Research and Management 3 143-164.

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