Boundaries

This ecoregion is defined by Lake Tumba and its adjacent swamp forests and is characterized by migratory species that enter the ecoregion to feed and breed, as well as by swamp-adapted species. The 765 km² lake lies near the equator within the Democratic Republic of the Congo (Marlier 1973).

Freshwater habitats

Located at a low point within the Congo basin, Lake Tumba is flooded for most of the year (Marlier 1973). The lake is extremely shallow and receives much of its nutrient input from small blackwater forest streams that flow from the surrounding, inundated swamps into the lake (Marlier 1973; Lowe-McConnell 1987). The pH of the lake water is low, with values of 4.0-4.9 recorded (Lévêque 1997). The lake is dotted with several small islands, and small deltas have formed at the mouths of some inflowing streams (Hughes & Hughes 1992). The average depth of the lake is only 3 to 5 meters, and the maximum is 8 m (Marlier 1973). Despite its shallow depth, dissolved oxygen is abundant in the entire water column throughout the year due to frequent churning of the waters by strong winds. Lake waters flow into the Congo River via the Irebié Canal, although the flow direction through the canal sometimes reverses during high flood and floodwaters from the Congo enter the lake (Hughes & Hughes 1992). 

Lake Tumba is surrounded by seasonally or permanently inundated forest (Hughes & Hughes 1992). In calm cove waters, mats of Echinochloa pyramidalis and Panicum parviflorum grow. Jardinea congoensis and J. gabonensis grow in beds along the banks and are sometimes interrupted by thickets of Cyrtosperma senegalense and Rhynchospora corymbosa. These thickets occasionally break off and drift freely. Swamp forests grow in areas along the shore that are only minimally exposed at periods of low water. These forests may be submerged up to 4 m in depth during high water (Hughes & Hughes 1992). Irvingia smithii dominates the swamp forests, with Alchornea cordifolia and Cynometra schlechteri also present in high abundance. This forest type merges into Guibourtia and Uapaca forest, which transitions to evergreen rain forest as the inundation zone ends (Hughes & Hughes 1992).

The watercourses of the central Congo basin exhibit two minimum and maximum flows each year, causing the lake to undergo a small flood between May and June and a large flood between September and January (Lowe-McConnell 1987; Hughes & Hughes 1992). Lake level fluctuates both within (about 4 m) and between years.

Terrestrial habitats

Swamp and riverine forests are found in a mosaic with swamp grassland and islands of dryland forest in this ecoregion.

Other noteworthy fishes

Shoaling fish (such as Barbus and Microthrissa) live in the open waters of the lake and feed on small plankton (Hughes & Hughes 1992). Odaxothrissa losera also inhabits the pelagic zone and feeds on small fish. Insects and detritus provide an abundant source of food for fish inhabiting near-shore areas of vegetation. Phago boulengeri eats the fins of other fishes (Hughes & Hughes 1992).

Ecological phenomena

The very low mineral content of Lake Tumba means that allochthonous particulate organic matter from the surrounding forests forms the base of the lake’s food chain (Beadle 1981). Most fish species spawn between August and September, at the start of the main flood, often moving many tens of kilometers upstream and entering inundated forests to breed and feed (Lowe-McConnell 1987).

Justification for delineation

Lake Tumba was formed when a tributary river became blocked close to the point of discharge into the Congo (Hughes & Hughes 1992). Sediments deposited in the main river may have been responsible for blocking the tributary (Bailey 1986). Located in one of the lowest points in the basin, Lake Tumba is also probably a remnant of the large Pliocene lake that existed before the Congo River became connected to the Atlantic Ocean. Thus, it is not surprising that the known species composition of the lake is similar to that of much of the Cuvette Centrale and the larger Congo Basin (Shumway et al. 2003). However, the fauna differs somewhat from that of Lake Mai-Ndombe, which is connected to the Kasai River system rather than to the Congo mainstem. The fauna of Lake Tumba has been little studied recently; a greater understanding of the biogeography of this ecoregion will likely come from further investigation of its fauna.

Level of taxonomic exploration

Poor. Matthes (1964) conducted a survey of the fishes. A survey of the fish and other biodiversity took place in September-October 2002 (Shumway et al. 2003). More surveys are needed both for fishes and other aquatic species.