Boundaries

The Lower Congo River ecoregion extends from the mouth of the Congo River upstream to just below the Mounts Cristal rapids at Matadi, and also includes left-bank tributaries to the Congo River mainstem between Matadi and Kinshasa. The ecoregion extends from Banana to Matadi and is located in the southern part of the Mayombe Mountains, which run parallel to the coast all the way from northern Angola to Gabon. Formerly classified as separate ecoregions, the Lower Congo ecoregion now includes the Thysville Caves complex located just south of the mainstem Congo and downstream of Malebo Pool.

Topography

The cave region lies at about 600-800 m above sea level, while the surrounding land is relatively even and flat and sits at about 500 m above sea level. 

Freshwater habitats

The Lower Congo River below the Mounts Cristal rapids has a discharge rate that varies between 3-5 x 10⁴ m³/sec at low flow and 6-7.5 x 10⁴ m³/sec at high flow. The river is 9.5 km wide at its mouth and its freshwater flow continues through a deep channel into a submarine canyon offshore. The Congo River discharges an estimated 35,427,000 tons of dissolved substances into the ocean each year, and a plume of brown river water extends for over 80 km offshore (Bailey 1986). The flood regime of the tributaries depends on local rains. However, when the Congo River is in full flood, it backs up the inflowing water from the tributaries and creates large pools in the tributary mouths (Mutambwe-Shango 1984). The flood regime of the Congo follows the bimodal pattern of the rains, with flood flows from October to December and from February to April.

In the dark environment of the subterranean streams of the Thysville caves, photosynthesis is not possible; the energy supply providing the foundation for the cave’s food chain comes instead from allochthonous inputs from the surrounding forest.

Terrestrial habitats

Vegetation is a mixture of dense Guinean-Congolian moist forest and Guinean-Congolian grasslands. 

Description of endemic fishes

The most famous inhabitant of the Thysville Caves complex is the endemic Congo blind barb (Caecobarbus geertsii). There is speculation that Barbus holotaenia is the closest surface water ancestor of this hypogean species (Banister 1986). However, as B. holotaenia lives outside of the ecoregion, some researchers suggest that the true ancestor could actually be a more localized, as-yet undiscovered species from a nearby surface stream, most of which are as of yet poorly surveyed. Populations in the two separate caves appear to have the same characteristics, but it is unknown if the populations are in fact the same species and able to interbreed (Banister 1986). C. geertsii is the only species in its genus and one of only three species of blind cavefish known from tropical West Africa (Beadle 1981).

Justification for delineation

This ecoregion is defined by the lower course of the Congo River below the Mounts Cristal rapids, including the Thysville Caves system, and is characterized by a mix of brackish and freshwater species.

Level of taxonomic exploration

Poor. Reasonably good collections exist for this ecoregion but they need to be studied and catalogued. Mutambwe-Shango (1984) completed a hydrobiological study of the Luki Reserve, located north of Matadi.The Thysville Caves are poorly known scientifically and should be re-surveyed (Kamdem-Toham et al. 2003).