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Ecoregion Description

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Species Richness

# of Endemic Species


547: Mai Ndombe

Major Habitat Type:

tropical and subtropical floodplain rivers and wetland complexes


Emily Peck, Conservation Science Program, WWF-US, Washington, DC


Uli Schliewen, Zoologische Staatssammlung München, Munich, Germany


Democratic Republic of Congo


Mai Ndombe is a dynamic lacustrine system that occupies the lowest interior elevation of the biologically diverse Congo River basin. Situated entirely within the Democratic Republic of Congo, the ecoregion extends from the southwest part of Salonga National Park in the north, to its confluence with the Lukenie-Fimi River in the south. Its boundaries are defined by the extent of Lake Mai Ndombe and surrounding flooded forest.

Drainages flowing into:

Lake Mai Ndombe connects with the Lukenie-Fimi River, which joins with the Kasai and eventually flows into the Congo River.

Main rivers or other water bodies:

Lake Mai Ndombe, formerly called Lake Inongo and Lake Léopold II, covers an area of approximately 2,300 km2, although the lake fluctuates in size according to season and rainfall (Welcomme 1972). The irregularly-shaped lake has a north-south orientation, a length of 135 km, a width ranging from 17 to 55 km, and an average depth of 3 meters (Hughes & Hughes 1992). The ecoregion probably contains the largest block of shallow blackwater lake and flooded forest habitat in the eastern Congo basin.  


Located just south of the equator, the ecoregion is subject to a hot and humid climate. The mean annual temperature is close to 25 %u02DAC, and relative humidity remains high at about 80-90% throughout the year. Rainfall varies with latitude and season. A mean annual rainfall of 1,900 mm in the north decreases slightly to 1,700 mm in the south. The wet season occurs during October and November, with a maximum monthly rainfall of up to 225 mm documented in October. The dry season occurs from June through August, with a minimum monthly rainfall of 10-50 mm recorded in July (Hughes & Hughes 1992).

Freshwater habitats:

Extensive areas of permanent swamp forest occur in the northeast of the ecoregion and extend southeast, along the Lokoro River. Permanent swamp forests also line the banks of Mai Ndombe’s major affluents. Dense Raphia swamps proliferate along the shores of the wettest of these forests. These permanently inundated swamp forests grade into less permanently inundated forests of Oubanguia africana and Guibourtia demeusei, which eventually grade into evergreen rainforest in unflooded areas (Hughes & Hughes 1992).

Seasonal flooding of tributaries to the lake, combined with seasonal rainfall, have a marked effect on the hydrologic cycle and ecology of Mai Ndombe. Fourteen major affluents flow across swampy lands and into the lake; the largest of these, Lokoro, Loti, and Olongo-Lule, are located at the northern end of the lake. Following the onset of heavy rains in October, floodplains along the tributaries and around the lake become inundated. In the north, floodwaters flow into permanent swamp forests, bringing oxygenated water and nutrients. In the south, flooding of the Lukenie River periodically inundates swamp forests along the southern portion of the lake (Hughes & Hughes 1992). Flooding serves to increase the productivity of the lake’s shallow waters, characterized by high acidity and humic content. During years of particularly heavy flooding, the swamp can connect with Lac Tumba to the north.

At the end of the wet season, floodwaters leave Lake Mai Ndombe via the Lukenie-Fimi River. The Lukenie River flows from the east to its confluence with the waters leaving Mai Ndombe; after this junction, the river is called the Fimi. Further downstream, the Kasai and Fimi Rivers join together and become the Kwa, which eventually joins the Congo. Mai Ndombe’s black waters do not mix readily with the clear waters of the Lukenie-Fimi, and are distinct for many kilometers downstream of the confluence of the Fimi and Kasai Rivers (Hughes & Hughes 1992; Remane 1997). 

Terrestrial Habitats:

Rainforest and swamp forest border Lake Mai Ndombe.

Fish Fauna:

Over 30 fish species are described from the lake, but the few studies and historical collections of Mai Ndombe suggest that species richness may be much higher than currently recorded (Roberts 1973). Representatives from several families of fish are common in the lake, including Alestiidae, Amphiliidae, Cichlidae, Claroteidae, Clariidae, Mochokidae, andMormyridae.

Description of endemic fishes:

Three fish species, Amphilius opisthophthalmus, Hemichromis cerasogaster, and Nanochromis transvestitus, are considered endemic.

Other noteworthy fishes:

Mai Ndombe provides habitat for the peculiar cichlid species Nanochromis transvestitus (Stewart & Roberts 1984). The females of this fish show a striking coloration, with bright hues and contrasts, whereas the males are completely dull. This form of sexual dimorphism is uncommon in cichlids and hints at an unusual breeding system. Historical collections include other cichlids with peculiarities (e.g., the thick-lipped endemic Hemichromis cerasogaster).  

The permanent swamp forests surrounding Lake Mai Ndombe contain fish species with adaptations to survive in the anoxic, semi-stagnant water. Clarias buthupogon and Clarias gabonensis are two such species (Lowe-McConnell 1987; Hughes & Hughes 1992).

Other noteworthy aquatic biotic elements:

Reconnaissance surveys of the region suggest that it is rich in freshwater mammals and waterbirds. Freshwater mammal species includemarsh mongoose (Atilax paludinosus), giant otter-shrew (Potamogale velox), Cameroon clawless otter (Aonyx congicus), spotted-necked otter (Lutra maculicollis), and African water rat (Colomys goslingi). The rare kingfisher (Corythornis leucogaster) has been recorded adjacent to the lake (Hughes & Hughes 1992). Ten species of aquatic frogs, 3 species of aquatic turtles, 3 species of crocodiles, and several semi-aquatic snakes have also been recorded.

Ecological phenomena:

Occupying the lowest point within the Congo River basin, the lake receives large volumes of organic matter from the surrounding catchment during the rainy season. The influx of organic matter results in high levels of fermentation and decomposition, which reduce the amount of dissolved oxygen in the lake water. However, the influx of floodwaters also brings oxygentated water to the flooded areas (Roberts 1973; Remane 1997). During heavy rains in October and November, fish move into the seasonally inundated swamp forests to feed and breed. Because Mai Ndombe is a blackwater lake, most of the food available to fish comes from terrestrial sources or floating vegetation, both of which are more commonly encountered during the flood season. The swamp forest’s warm, still, shallow water, its dense vegetation, and its rich organic mud provide not only an abundance of fish food and ideal conditions for breeding, but also protection for juvenile fish from predators (Roberts 1973; Lowe-McConnell 1987).

Evolutionary phenomena:

Although rather similar ecologically to Lake Tumba, the species composition of the two lakes differs in several aspects, indicating different biogeographic histories. However, the ichthyofauna of the Lukenie-Fimi River, which receives Mai Ndombe’s outflow, is amongst the poorest studied parts of the Congo Basin, so any theories about the origin of the Mai Ndombe ichthyofauna remain speculative.

Justification for delineation:

This ecoregion is defined by the extent of Lake Mai Ndombe and surrounding flooded forest; and is distinguished by the lacustrine faunal elements that occur in its waters. Mai Ndombe sits within the Cuvette Centrale, a saucer-shaped depression in the central Congo River Basin. During the late Miocene and early Pliocene, a large endhoreic lake occupied the entire Cuvette Centrale. Lake Mai Ndombe is thought to be a remnant of this great interior lake, although geological details supporting this are lacking. As a large and ancient lacustrine area Mai Ndombe may have played an important role in the evolution of the endemic fish fauna of the Congo Basin. It may also have served as a refuge for the ancient lacustrine faunal elements that inhabited the larger former inland lake system (Lowe-McConnell 1987; Shumway 1999). Lake Mai-Ndombe is connected to the Kasai River system such that its fauna is more related to the Kasai than to the Congo mainstem.

Level of taxonomic exploration:

Poor. As only a few historical collections exist for the lake and its tributaries, species richness and endemism are most likely underestimated. A survey should include all affluent streams and the entire flooded forest, with priority given to fish and molluscs.


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

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

Remane, K. (1997). "African inland fisheries, aquaculture, and environment" Oxford, UK: Fishing News Books.

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.

Shumway, C. A. (1999) Forgotten waters: Freshwater and marine ecosystems in Africa. Strategies for biodiversity conservation and sustainable development. Alexandria, VA, USA: Global Printing, Inc..

Stewart, D. J.,Roberts, T. R. (1984). "A new specis of dwarf cichid fish with reversed sexual dichromatism from Lac Mai-Ndombe, Zaire" Copeia 1984(1) 82-86.

Welcomme, Robin L. (1972) The inland waters of Africa. Rome, Italy: FAO.

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