Boundaries

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.

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.

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).

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).

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.