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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

537: Cuvette Centrale

Major Habitat Type:

tropical and subtropical floodplain rivers and wetland complexes

Author:

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

Countries:

Democratic Republic of Congo

Boundaries:

This ecoregion, rich in fish and other aquatic fauna, encompasses the largest tract of lowland rainforest in Africa and occupies most of the low, flat portion of the Congo basin termed the “Cuvette Centrale” (Bailey 1986). Bordered on the north by a plateau that slopes towards the Congo basin, this ecoregion lies entirely within the Democratic Republic of the Congo (DRC).

Main rivers or other water bodies:

The main channel of the Congo widens as it flows downstream and is surrounded by vast tracts of swamp that connect intermittently with the river. The river flows over its banks into the forest between November and January (Bailey 1986). Several large tributaries join the mainstem Congo as it flows through the Cuvette Centrale ecoregion. The Aruwimi, Itimbiri, and Mongala flow into the Congo from the northeast, and the Lulonga, Ikelemba, and Ruki join from the south.

Topography:

The ecoregion lies at about 300 m asl and is almost totally flat. 

Climate:

The Cuvette Centrale ecoregion has an equatorial and wet climate, with rainfall fairly consistent year round. Average annual rainfall in the Cuvette Centrale is between 1,500 to 2,000 mm and near the equator rain falls throughout the year (Bailey 1986; Lowe-McConnell 1987). Moving away from the equator, rainfall is more periodic, with distinct wet and dry seasons (Bailey 1986). Tributaries from the north flood from August to November, and from the south flood from May to June (Lowe-McConnell 1987). Mean annual temperatures are about 24oC over most of the ecoregion with mean daily maxima of 30oC and mean daily minima of about 20oC (Hughes & Hughes 1992).

Freshwater habitats:

The Cuvette Centrale contains a variety of habitats including open waters, creeks, coves, meadows of aquatic vegetation, permanent swamps, and floodplains (Hughes & Hughes 1992). The Congo accumulates about 70% of its water volume in the Cuvette Centrale as it receives input from many large tributaries (Banister 1986). Downstream, the river braids into a mosaic of islands, sandbanks, and floating Eichornia masses (Bailey 1986). Permanent blackwater swamps with acidic, low-oxygen waters occur in the ecoregion, and many of the forested southern tributaries are blackwater streams. The waters of the main Congo channel are often a deep brown color (Roberts 1973).

The lower valleys of the tributaries all contain swamp forests and Raphia palms. The Itimbiri Valley alone contains over 1,500 km2 of swamp(Hughes & Hughes 1992). Unlike the relatively calm mainstem, many northeastern tributaries, such as the Aruwimi, flow across rapids before joining the Congo (Bailey 1986). 

The ecoregion contains large seasonally and permanently flooded areas. As the Congo flows through the Cuvette Centrale it ranges from 3 to 15 km in width and between 3 to 10 m in depth (Hughes & Hughes 1992). The Congo flows over a broad alluvial plain with seasonally flooded forests that extend beyond its banks for up to 50 km. Seasonally inundated forests also cover islands in the river, and permanent swamp forest grows along the river. Rainfall occasionally inundates areas of forest not flooded by rivers, forming swamps in depressions.

Terrestrial Habitats:

The vegetation of the ecoregion is primarily tropical rain forest with a canopy that can exceed 45 m in height. Although there are a few dominant species, floodplain forests have a rich flora. The vegetation composition varies, largely depending on the soil type (Hughes & Hughes 1992). Mitragyna stipulosa usually dominates on muddy soils along slow flowing water. M. stipulosa is associated with a variety of species, including Alstonia congensis, Macaranga sp., Nauclea diderrichii, N. pobeguinii, and many others. In the understorey, Cyrtosperma senegalense and Marantochloa congensis are common. In areas with sandy soil that experience less flooding, Guibourtia demeusei dominates the forests. These forests have a low canopy of 30 m and a sparse understory. Tall forest vegetation on the islands of the Congo includes Lannea welwitschii, Ficus mucuso, Oxystigma buchholzii, and Pseudospondias microcarpa (Hughes & Hughes 1992).

Fish Fauna:

The Cuvette Centrale has a rich fish fauna, with between 300 and 400 fish species expected, about 240 species documented and at least 12 known endemics (Poll & Gosse 1963). However, few biological studies have been conducted in this ecoregion and further surveys should locate additional species and new endemics, particularly because rapids and waterfalls isolate the Cuvette Centrale fish fauna from the lower and upper sections of the river.

The base of the food chain in many of the nutrient-poor blackwater rainforest streams begins with allochthonous organic material that falls into the water. In addition to woody matter, terrestrial input in the form of insects, seeds or fruits provides an important source of food for several species of fish. Large characoids and cyprinids feed on terrestrial insects, seeds or fruits that have fallen in the water and several species may feed primarily on these items (Beadle 1981; Lowe-McConnell 1987). For example, in the streams flowing into the Congo near Yangambi, of 670 individuals in 16 families, 84% were feeding almost exclusively on terrestrial insects (Lowe-McConnell 1987). 

The main river channel supports a wide variety of species adapted to pelagic, benthic and shoreline habitats. Pelagic species include schooling plankton feeders such as Microthrissa and Barbus (Lowe-McConnell 1987). Schools of Microthrissa migrate upstream and downstream seasonally (Hughes & Hughes 1992). Catfish (bagrids and mochokids) are found in the slow flowing portions of the Congo’s main channel. Larger species of Mormyridae, Characidae, Citharinidae, and Cyprinidae are often found in the main channel, with smaller species of these families found in streams (Lowe-McConnell 1987). Barbus as well as Hydrocynus vittatus (which preys on Barbus) are also found in the open waters (Hughes & Hughes 1992). The benthic fish fauna of the main river channels includes many insectivores as well as some detritus-feeders, omnivores, and piscivores (Lowe-McConnell 1987). Insect and detritus feeders include many Barbus, Chrysichthys, Synodontis,and Tylochromis species. Some piscivores such as many Chrysichthys, Mormyrops, and some Polypterus also feed near the bottom (Hughes & Hughes 1992). Large catfish (bagrids and mochokids) live primarily in the large river channels. Clariids live on the muddy bottoms of rivers, streams, and swamps (Lowe-McConnell 1987).

Due to the relative stability of flow levels within the central portion of the Congo River, different habitat types (such as swampy inundated areas) are present year-round. The relative stability of these habitats has allowed many fish species to become specialized to narrow niches. For example, Eugnathichthys spp. are fin eaters, attacking larger fish, Campylomornyrus spp. use their long rostral probe to feed on bottom-dwelling insects among rocks or in the mud (Roberts 1973). Despite the specialization of some species, many Congo River species are able to survive a wide range of temperature conditions. The wide main river channel receives less shade from streamside forests and is more affected by wind than smaller tributary streams, and as a result has higher water temperatures (between 22.5 –33o C) than the smaller streams (Lowe-McConnell 1987).

Many fish species live in the swamps and marginal waters between the rivers and inundated forests  (Lowe-McConnell 1987). Many clariids, anabantids (Ctenopoma and Microctenopoma) and lungfish (Protopterus) are able to withstand highly anoxic conditions and live in the swamps that border the middle Congo and adjacent shallow blackwater lakes (Beadle 1981). Several species are adapted to these conditions and either use accessory breathing organs to survive in the low-oxygen waters or live directly under the water surface, where oxygen is in greater supply (Banister 1986). Clariidae have aborescent accessory breathing organs in the gill chamber and Ctenopoma (Anabantidae) have labyrinthine accessory breathing organs. Protopterus dolloi can build nests in mud and decaying vegetation, with a tunnel “chimney” to the atmosphere, through which the fish is able to breathe. Other species such as Hepsetus odoe stay near the water surface. Most swamp fish feed on insects or other fish (Banister 1986; Lowe-McConnell 1987).

Many fish leave the rivers and spawn in the forest during high water. Juveniles often remain in the floodplains to feed and grow, despite the receding floodwaters (Hughes & Hughes 1992). The majority of species spawn when the floods originating from the northern tributaries occur in September to October, although another breeding season occurs between April and June. The fish hatch within several hours to two days, and grow quickly in the inundated zone (Lowe-McConnell 1987). Parental care is present to a small extent. Lungfish construct a nest that the male guards and aerates. The air-breathers notopterids, osteoglossoids, and anabantids also guard their eggs and/or young (Roberts 1973). However, little is know about the reproductive biology of most of the species.

Other noteworthy aquatic biotic elements:

The ecoregion also hosts a rich avifauna, with several hundred species in the Congo basin overall. Hartlaub’s duck (Pteronetta hartlaubii), threatened elsewhere by habitat loss due to deforestation, is found in this ecoregion (Hughes & Hughes 1992; Scott & Rose 1996).

In addition to the hippopotamus (Hippopotamus amphibius), several other aquatic mammals inhabit the Cuvette Centrale. The giant otter shrew (Potamogale velox) and the Congo clawless otter (Aonyx congica) depend on the ecoregion’s rivers and streams. These two species feed on fish, crabs, frogs, and aquatic molluscs. Allen’s swamp monkey (Allenopithecus nigroviridis) inhabits the ecoregion as well, feeding on fruits, leaves, and invertebrates including crabs. Sitatunga (Tragelaphus spekei) and chevrotain (Hyemoschus aquaticus) also live along watercourses in the Cuvette Centrale. The chevrotain relies on the water as a refuge from predators (Kingdon 1997). 

Eight species of molluscs occur in the ecoregion. The distribution of snails in the ecoregion depends primarily on the acidity of the water. While few molluscs can be found in brown-colored acidic waters, molluscs are common in the more nutrient-rich Congo mainstem and in northern tributaries that receive water from streams and rivers draining less impoverished upland soils (Brown 1994). 

Justification for delineation:

This ecoregion is defined by the relatively flat central region of the Congo River from the Upper Congo Rapids [539] downstream to Lake Tumba [538]. The Cuvette Centrale is located in an ancient continental basin, invaded several times by the sea during the Mesozoic (Beadle 1981). Subsequently, an uplifting of land around the basin obstructed this drainage to the coast, forming a large endorheic lake system during the Pliocene. About 400,000 years ago, a coastal river, which is now called the Congo, cut back to capture the lake water (Beadle 1981). Most of the lake was drained, leaving the large swampy and seasonally flooded area still present today along the river in the flat basin, as well as the two major lakes Tumba and Mai-Ndombe at the lowest part of the saucer-shaped Cuvette Centrale. While dry phases during the Pleistocene altered the vegetation of the basin, the river system has remained relatively stable throughout the Quaternary (Hughes & Hughes 1992). The environmental stability of the area over a long period, the range of available habitats in the bioregion, and the long isolation from other bioregions are thought to be conditions that favored the evolution of a rich biota, largely endemic to the Congo basin (Beadle 1981).

Level of taxonomic exploration:

Poor

References/sources:

Bailey, R. G. (1986)"The Zaire River system" In Davies, B.R.;Walker, K.F. (Ed.). The ecology of river systems. (pp. 201-214) Dordrecht, The Netherlands: Dr W. Junk Publishers.

Banister, K. E. (1986)"Fish of the Zaire system" In Davies, B.R.;Walker, K.F. (Ed.). Ecology of river systems. (pp. 215-224) Dordrecht, The Netherlands: Dr W. Junk Publishers.

Beadle, L. C. (1981). "The inland waters of tropical Africa" England: Longman Group Limited.

Brown, David (1994). "Freshwater snails of Africa and their medical importance" London, UK: Taylor & Francis.

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

Kingdon, Jonathan (1997). "The Kingdon field guide to African mammals" San Diego, CA, USA: Academic Press.

Lowe-McConnell, R. H. (1987) Ecological studies in tropical fish communities. Cambridge, UK: Cambridge University Press.

Poll, M.,Gosse, J. P. (1963). "Contribution a l’etude systematic de la faune ichthyologique du Congo Central" Annales du Musee royal d’Afrique centrale (Sciences Zoologiques) 116 43-111.

Roberts, T. R. (1973)"Ecology of fishes in the Amazon and Congo basins" In Meggers, B.J.;Ayensu, E.S.A.;Duckworth, W.D. (Ed.). Tropical Forest Ecosystems in Africa and South America: A Comparative Revue. (pp. 239-254) Washington, DC, USA: Smithsonian Institution Press.

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.

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