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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

549: Lower Congo Rapids

Major Habitat Type:

tropical and subtropical floodplain rivers and wetland complexes

Author:

Victor Mamonekene, Institut de Développement Rural, Université Marien Ngouabi-Brazzaville, Brazzaville – CONGO

Countries:

Congo; Democratic Republic of Congo

Boundaries:

Within the Lower Congo Rapids ecoregion, the Congo River drops 280 m as it travels 350 km between the end of Malebo Pool and the city of Matadi (Beadle 1981). The ecoregion covers parts of the Republic of Congo and the Democratic Republic of Congo.

Drainages flowing into:

Atlantic Ocean

Main rivers or other water bodies:

The largest tributaries to the Congo River in this stretch are the Djoué, Loufoulakari, Inkisi, Kwilu13, and Mpozo. Most of these flow from cliffs, over waterfalls, as they descend to the mainstem river.

Climate:

The climate of the ecoregion belongs to the Guineo-equatorial bas-Congolais type. Rains occur between March and April and between mid-November and mid-December. Precipitation originates offshore as monsoon rains under the influence of the Guinea and Benguela currents.

Freshwater habitats:

As the Congo River descends toward the Atlantic Ocean, the river alternately passes through narrow, deep gorges (about 200 m wide) and wider stretches (about 2 km wide), but runs swiftly through both (Bailey 1986). This stretch of river encompasses 32 falls and rapids within the Mount Cristal Rapids. In addition to rapids with strong currents, there are also some calm sections of river between Brazzaville and Matadi. The fish fauna shows morphological and behavioral adaptations to life in fast running waters.

Terrestrial Habitats:

Gallery forests line the rivers and tree-savanna vegetation covers the landscape. Over the last century the vegetation within this region has changed from lowland forest to wooded savanna (Shumway et al. 2003).

Fish Fauna:

An endemic aquatic fauna occurs in the swift waters of the rapids of this ecoregion. Most of the fishes have morphological and behavior adaptations to fast running water. Specializations among fish include reduction of eye size (micropthalism), a blue or bluish coloration, and modified body form (dorsoventrally depressed heads and bodies). Among the species adapted to fast flowing water are cyprinids of the genera Garra and Labeo; catfishes of the genera Atopochilus, Euchilichthys, Chiloglanis, and Gymnallabes; cichlids of the genera Steatocranus, Teleogramma, Lamprologus, and Leptotilapia; and a group of endemic mastacembelids (Roberts & Stewart 1976). As examples, a mastacembelid eel (Mastacembelus brachyrhinus) and theendemic Lamprologus lethops are both cryptophthalmic, meaning their eyes are reduced in size and partially or completely covered by skin and other tissues (Roberts & Stewart 1976). 

Description of endemic fishes:

Of the 129 fish species that have been recorded from the rapids, 34 are endemic (Roberts & Stewart 1976).

Other noteworthy aquatic biotic elements:

A highly endemic, rheophyllic snail fauna also inhabits the rapids. This fauna includes four endemic monotypic genera (Congodoma, Liminitesta, Septariellina, and Valvatorbis), and 16 endemic from a fauna of 18 species (Brown 1994). Rheophyllic snails exhibit adaptations to fast running water, with the ability to adhere to rocks in the swift current and to tolerate large fluctuations in water level.

Evolutionary phenomena:

The Lower Congo Rapids ecoregion is considered outstanding for its higher taxonomic endemism including 4 endemic aquatic mollusk genera and a fish species radiation (Caecomastacembelus) (Thieme et al. 2005).

Justification for delineation:

This ecoregion is delineated based on the 350-km stretch of rapids in the Lower Congo River and is distinguished by an endemic aquatic fauna adapted to the rapids. It is hypothesized that during the late Pliocene or early Pleistocene, a coastal Lower-Guinean river captured Malebo Pool, connecting the previously interior Congo basin to the ocean. The Congolian nature of the endemic species that occur in the rapids suggests that this fauna evolved before the river capture (Roberts & Stewart 1976). The rapids constitute a barrier to the movement of marine and brackish fishes from the delta into the interior of the Congo Basin.

Level of taxonomic exploration:

Poor. Relatively little is known about the freshwater biota of this ecoregion. There is one paper on fishes of the Lower Congo Rapids (Roberts & Stewart 1976). In its National Biodiversity Action Plan, the government of the DRC identifies rapids and waterfalls as among those habitats most in need of biodiversity surveys (République Démocratique du 1998).

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.

Beadle, L. C. (1981). "The inland waters of tropical Africa" England: Longman Group Limited.

Brown, David (1994). "Freshwater snails of Africa and their medical importance" London, UK: Taylor & Francis.

République Démocratique du, Congo (1998) "Stratégie Nationale et Plan d’Action de la Biodiversité. Project ZAI/96/G31/C/1G/99" <http://bch-cbd.naturalsciences.be/congodr/cdr-fra/contribution/
strataction/strategie/conserv.htm.>(2002)

Roberts, T. R.,Stewart, D. J. (1976). "An ecological and systematic survey of fishes in the rapids of the lower Zaire or Congo River" Bulletin of the Museum of Comparative Zoology, Harvard 147 239-317.

Shumway, C. A., Musibono, D., et al. (2003). "Congo River Environment and Development Project (CREDP) biodiversity survey: systematics, ecology and conservation along the Congo River, September-October 2002" Boston, MA, USA: New England Aquarium Press.

Thieme, M. L., Abell, R., et al. (2005). "Freshwater Ecoregions of Africa and Madagascar: A Conservation Assessment" Washington, D.C., USA: Island Press.

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