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Ecoregion Description
550: Lower Congo
Major Habitat Type:
tropical and subtropical floodplain rivers and wetland complexes
Author:
Victor Mamonekene, Institut de Développement Rural, Université Marien Ngouabi Brazzaville, Brazzaville – CONGO
Ashley Brown and Robin Abell, Conservation Science Program,
Reviewers:
Gordon McGregor Reid, North of England Zoological Society, Zoological Gardens, Chester, UK
Countries:
Angola; Democratic Republic of Congo
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.
Drainages flowing into:
Atlantic Ocean
Main rivers or other water bodies:
The main water body in this ecoregion is the Congo River, with a few large tributaries, such as the Mpozo and Lufu, entering at its left bank. The largest of these tributaries is the Inkisi River. The subterranean Thysville Caves complex consists of two suites of caves (Banister 1986) located on two separate Congo tributary systems. Although the fauna inhabiting the caves is quite similar, they do not appear to possess subterranean connections to each other.
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.
Climate:
The climate of the ecoregion is tropical and humid with a rainy season from October to May and a dry season from June to September. The rainfall regime is bimodal with a peak in November and another in March. Average annual rainfall is 1,888 mm and average annual temperature is 23.8°C.
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.
Fish Fauna:
About 200 fish are known from this ecoregion, twelve of which are endemic. Mormyrids, cichlids and cyprinids are particularly speciose. An estuarine ichthyofauna occurs in the Congo River upstream to Matadi, where rapids stop brackish water species from moving further inland. The lower Congo River has a mangrove-lined estuary at its mouth with many euryhaline fishes and freshwater representatives of marine families like clupeids, gobies, and tetraodonts. The Thysville Caves system hosts two fish species, one of which is subterranean Congo blind barb (Caecobarbus geertsii). The other is the pinkfin alestes (Alestes grandisquamis), a pelagic surface water fish with a relatively widespread distribution in the Congo Basin.
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).
Other noteworthy aquatic biotic elements:
The ecoregion is moderately rich in fish, aquatic herpetofauna, and aquatic molluscs. Over thirty frogs and reptiles are known from this ecoregion’s fresh waters and seventeen molluscs. Five of the molluscs are endemic (Lanistes congicus, L. intortus, Funduella incisa, Hydrobia luvilana, and Potadoma schoutendeni).
Several aquatic mammals, molluscs, and waterbirds live above ground in Thysville Caves region. Inhabitants include the marsh mongoose (Atilax paludinosus) and the molluscs Etheria elliptica and Lentorbis benguelensis. Birds include the Egyptian goose (Alopochen aegyptiacus), spur-winged goose (Plectropterus gambensis), and African pygmy-goose (Nettapus auritus).
Evolutionary phenomena:
The Thysville Caves complex of the Lower Congo is globally outstanding for its higher taxonomic endemism; one of two fish genera (Caecobarbus) is endemic.
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).
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.
Kamdem-Toham, A., D'Amico, J., et al. (2003) "Biological priorities for conservation in the Guinean-Congolian forest and freshwater region: Report of the Guinean-Congolian forest and freshwater region workshop, Libreville, Gabon, March 30-April 2, 2000". Libreville, Gabon. WWF.
Mutambwe, Shango (1984). "Contribution à l’étude de l’écologie de la rivière Luki (sous-affluent du fleuve Zaïre): Bassin versant-Poissons" Unpublished Thesis. .
