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. 

Topography

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

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. 

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. 

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.