Boundaries

The ecoregion extends from northern Gabon through the eastern portion of Congo and Cabinda (Angola) and ends above the Congo River Basin in Democratic Republic of Congo. Its boundaries are defined by the basins of the Kouilou-Niari and Nyanga rivers and the mainstem of the Ogooué River. The waters of this rainforest ecoregion are exceptionally rich in freshwater species

Topography

Moving east, this ecoregion’s coastal plain gives way to the Mayombe Range, which is interrupted by the Tchibanga and Ndéndé-Mouila gorges in which savanna vegetation dominates. Beyond the Mayombe Range rises the Chaillu Massif with its highest peak being Mt. Iboundji at 972 m asl.

Freshwater habitats

Along the rivers, open marshes with Papyrus and Vossia are common. Mangrove forests thrive in the estuaries and lagoons that extend along the coast. The Ogooué, Nyanga, and Kouilou-Niari rivers experience a bimodal flood regime. The first flood period occurs in November and the second in April. Mean annual discharge of the Ogooué River is about 4,758 m³/s and that of the Kouilou River is about 913 m³/s.

Terrestrial habitats

Phytogeographically, this ecoregion is within the Guineo-Congolian region, which is characterized by a dense moist forest. In many places where the native vegetation has been cleared, secondary forest or agricultural lands now occur. Dry grasslands predominate on sandy soils near the coast. More inland and in the north, around Koulamoutou and Lastourville, the moist forest extends interspersed with secondary forest (White 1983).

Description of endemic fishes

About one-quarter of the fish species are endemic to this ecoregion (Teugels, personal communication). There is an especially high diversity of snoutfishes (Mormyridae) and killifishes (Aplocheilidae) in the Ogooué basin. The Ogooué (including its tributary, the Ivindo [819]) is likely the center of speciation for the mormyrid genus Brienomyrus (Sullivan et al. 2002).

Additionally, the Kouillou-Niari region is a contact zone between the Ogooué and Congo rivers; though unstudied, it is suspected to be rich in freshwater species and endemics (Kamdem-Toham et al. 2003). The Chaillu Massif in Gabon has steep gradients and dense forest and is also suspected to harbor many endemic small stream fishes. Several endemic cyprinodonts are known to live in these mountain streams, and surveys may reveal more endemic fish plus other taxa (Kamdem-Toham et al. 2003).

Justification for delineation

The northern tributaries of the Ogooué River (Abanga, Okano, and Ivindo) are excluded from this ecoregion because their fauna is more similar to that of rivers in the Southern Gulf of Guinea Drainages [533] ecoregion than to that of 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). During climatic fluctuations of the Pleistocene period, the forested West Coast Equatorial bioregion (Lower Guinea) is considered to have been a refuge for many species. Various Amphiliidae, Mochokidae, and Cyprinidae, in addition to some anguilliform fish are restricted to the Guinean rivers and adapted to the cooler waters and hydrological regimes of this forested region (Lévêque 1997).

Level of taxonomic exploration

Poor. Few data are available to describe the biodiversity of this ecoregion’s freshwater systems. Many rivers and streams have not yet been sampled, such as the Nyanga and upper Kouilou Rivers as well as many small coastal basins (Daget & Stauch 1968; Dowsett & Dowsett-Lemaire 1991; Teugels et al. 1991; Mamonekene & Teugels 1993). Little information is available on the life history and ecology of freshwater species and data on species distributions are scarce for aquatic taxa other than fish.

The Muséum National d\'Histoire Naturelle’s extensive fish collections cover only parts of the Ogooué Basin and various sections of the Ogooué River are in need of specific studies. The middle Ogooué rapids between Njolé and Lastoursville are shallow gradient rapids for over 200 km of the main channel. Rapids are particularly difficult to sample, but they likely have their own fauna. The lakes and swamps of northern Lambarene and Ngomo in the Ogooué River basin are possibly important examples of large lowland equatorial swamp, and surveys of odonata are needed. The large Ogooué Delta, between Port-Gentil and Lambarene, is a priority survey area for aquatic mammals and waterbirds.