Ecoregion Description

533: Southern Gulf of Guinea Drainages

Major Habitat Type:

tropical and subtropical coastal rivers

Author:

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

Reviewers:

John Sullivan, Cornell University, Ithaca, NY, USA

Countries:

Cameroon; Congo; Equatorial Guinea; Gabon

Boundaries:

In the Southern Gulf of Guinea Drainages ecoregion, coastal rivers flow through rainforests on their way to the Gulf of Guinea and the Atlantic Ocean. The ecoregion extends from the Sanaga River in Cameroon south to directly above the Gabon estuary. It ranges about 400 km inland and includes parts of southern Cameroon, Equatorial Guinea, and northern Gabon. 

Drainages flowing into:

Atlantic Ocean

Main rivers or other water bodies:

The main rivers are the lower Sanaga, Nyong, Ntem, Benito, and northern tributaries of the Ogooué (Abanga, Okano, and Ivindo).

Topography:

The topography of the ecoregion is dominated by a sedimentary coastal plain that gives way to an inland plateau. 

Climate:

The climate of this ecoregion is tropical and humid, with a mean annual rainfall of between 1,500 and 2,700 mm. Mean annual temperature varies between 24 and 27°C. Flood regimes within the rivers of the ecoregion are bimodal with peaks in April through June and October through December. 

Freshwater habitats:

The rivers of the ecoregion have a dense network of tributary streams. The Ivindo is a mostly rocky, deep river punctuated by rapids along its course through dense tropical forest.  During peak flows, the rivers overspill their banks and flood the forest. This occurs especially at the head of the Ntem River basin, creating flooded swamps. The Nyong/Doume blackwater swamps are a rare habitat type located along the upper reaches of the Nyong, Doume, and Boumba Rivers. Although these are separate drainages, the characteristics of these rivers are similar and the most upstream portions connect with one another during flooding to create a vast expanse of flooded swamp. During the dry season, the flooded forest remains quite wet with many freshwater pools. These forests have a low diversity of tree species, and swampy meadows of Echinochloa are found close to the rivers. The estuaries support extensive mangrove forests.

Terrestrial Habitats:

The dominant vegetation is littoral forest near the coast and a mixed mesophilous-evergreen forest inland. 

Fish Fauna:

This is a very rich region with observed numbers of fish species exceeding predicted numbers for some river basins (Teugels & Guégan 1994). Over 270 fish species are known from this ecoregion, 58 of which are endemic (Teugels, personal communication). A stretch of approximately forty kilometers on the Dja River includes both the Nki falls and the rapids of Chollet, and separates coastal fish faunas from Congo fish faunas.  Some waterfalls are greater than twenty meters in height. Rapids and falls in this part of the Lower Guinean bioregion are rare. It is estimated that this area alone contains more than 150 fish species. 

Description of endemic fishes:

About one third of the endemic fish are from the genus Aphyosemion. There are also several endemic genera within the Mormyridae (Boulengeromyrus, Ivindomyrus), Cichlidae (Parananochromis), and Clupeidae (Thrattidion) families. The Ivindo River appears to be the center of diversity of a “riverine species flock” of mormyrid fishes of the genus Brienomyrus in which most species are at present undescribed (Sullivan et al. 2002). Twelve distinct species in this genus are recorded in and near the rapids of Loa-Loa on the Ivindo River near Makokou, Gabon (C.D. Hopkins, pers. comm.). Two species have been found only in the region of Nki and Chutes de Chollet: an undescribed characin and an undescribed Steatocranus species. 

The Monts de Cristal is a small coastal mountain range in northern Gabon that has high endemism for fish. The region is forested and at an approximate altitude of 800 m. The ecology of these mountains is similar to that of the Chaillu Massif found further south. It is likely that the Monts de Cristal mountains’ streams support a small number of aquatic species, but several endemic cyprinodonts are known from them. 

Other noteworthy aquatic biotic elements:

Over 20 species of aquatic molluscs live in the ecoregion’s waters. Members of the Neritidae, Ampullariidae, Planorbidae, and other families are present. There are also high numbers of frogs with about 100 species known, including several endemic species.

Several large-bodied aquatic vertebrates, such as the West African black forest terrapin (Pelusios niger), West African mud turtle (Pelusios castaneus), and African slender-snouted crocodile (Crocodylus cataphractus) frequent the ecoregion’s rivers. The dwarf crocodile (Osteolaemus tetraspis) also lives in the ecoregion, primarily in permanent pools in swamps and areas of slow-moving freshwater in the rain forests. The Atlantic hump-backed dolphin (Sousa teuszii) occasionally enters the lower stretches of these coastal rivers.

Justification for delineation:

This ecoregion is defined by coastal rivers of the West Coast Equatorial bioregion (lower Sanaga, Nyong, Ntem, Benito, and northern tributaries of the Ogooué) which are characterized by similar fish species assemblages. The short stretch of falls on the Dja River separates the fish fauna characteristic of the Southern Gulf of Guinea Drainages from that of the Sangha [534] ecoregion. The northern tributaries of the Ogooué River (Abanga, Okano, and Ivindo) are included in this ecoregion because their fauna is more similar to that of other rivers in the Southern Gulf of Guinea Drainages ecoregion than to the mainstem Ogooué. For example, possibly due to a 50-km stretch of rapids and falls that separates the Ivindo and Ogooué Rivers, as well as to previous river captures, the fauna of the Ivindo is more similar to the Ntem than to the Ogooué (Thys van den Audenaerde 1966; Sullivan et al. 2002). Questions remain concerning the evolutionary history of the Ivindo River fauna, including how it arose and why it is more similar to the Ntem River in Cameroon. During climatic fluctuations of the Pleistocene, this ecoregion may have been a refuge for ichthyofauna and other aquatic-dependent taxa of the West Coast Equatorial (or Lower Guinea) bioregion (Lévêque 1997). 

Level of taxonomic exploration:

Fair. Roman (1971)) (Roman 1971)published a book on the fishes of Rio Muni (Equatorial Guinea) and Kamdem Toham (1998) and Kamdem Toham and Teugels (1998) (Kamdem Toham 1998; Kamdem Toham and Teugels 1998)studied the fish biodiversity of the Ntem River. Other taxa are less well studied.

References/sources:

Kamdem Toham, A. (1998). "Fish biodiversity of the Ntem river basin (Cameroon): Taxonomy, ecology and conservation" Unpublished Thesis. Katholieke Universiteit Leuven.

Kamdem Toham, A.,Teugels, G. G. (1998). "Diversity patterns of fish assemblages in the Lower Ntem River Basin (Cameroon), with notes on potential effects of deforestation" Archiv für Hydrobiologie 141(4) 421-446.

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.

Lévêque, C. (1997) Biodiversity dynamics and conservation: The freshwater fish of tropical Africa. Cambridge, UK: Cambridge University Press.

Roman, B. (1971). "Peces de Rio Muni, Guinea Ecuatorial (Aguas dulces y salobres)" Barcelona, Spain: Fundation la Salle de Ciencias Naturales.

Sullivan, J. P., Lavoué, S., et al. (2002). "Discovery and phylogenetic analysis of a riverine species flock of African electric fishes (Mormyridae: Teleostei)" Evolution 56 597-616.

Teugels, G. G.,Guégan, J. F. (1994). "Diversité biologique des poissons d’eaux douces de la Basse-Guinée et de l’Afrique Centrale in Biological Diversity in African Fresh-and Brackish water Fishes, Geographical reviews, Symposium PARADI" Annales du Musée royal de l’Afrique Central, Sciences Zoologiques 275 67-85.

Thys van den Audenaerde, D. F. E. (1966). "Les Tilapia (Pisces, Cichlidae) de Sud-Cameroun et du Gabon étude systematique, Annales du Musée royal de l’Afrique Central, 8o - Sciences Zoologiques, 153" Tervuren, Belgium: Musée Royal de l’Afrique Central.