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).

Topography

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

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).

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