ID

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521

Author(s)

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Dalmas Oyugi, Kenya National Museum, Nairobi, Kenya

Countries

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Burundi
Democratic Republic of Congo
Kenya
Rwanda
Tanzania
Uganda

Reviewer(s)

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Lauren Chapman, Department of Zoology, University of Florida, Gainesville, Florida, USA

Major Habitat Type

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Large lakes

Main rivers to other water bodies

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With an area of approximately 68,800 km², Lake Victoria is the largest tropical lake in the world as well as the second largest freshwater lake in the world (Spigel & Coulter 1996). Stretching 412 km from north to south and 355 km from west to east, Lake Victoria spans the borders of Tanzania, Uganda and Kenya. Its massive catchment (over 193,000 km²), reaches well into Rwanda and Burundi (Hughes & Hughes 1992). Lake Victoria occupies a shallow depression 1,134 m asl, between the west and east African rifts. The water balance of the lake is maintained primarily through rainfall and evaporation, rather than inflows and outflows (Spigel & Coulter 1996). Due to this dependence on rainfall and evaporation, the residence time of water in Lake Victoria is 23 years (Cohen et al. 1996; Spigel & Coulter 1996). Lake level in Victoria has varied by about 2 meters in the last century in response to changes in rainfall and evaporation. The Lake basin itself is about 400,000 years old, but several recent studies suggest that the lake was completely dry for several thousand years and re-filled only 15,000 years ago (Johnson et al. 1996; Johnson et al. 2000; Talbot & Laerdal 2000).

Numerous rivers and streams drain into Lake Victoria. The principal affluent is the Kagera River, which enters the lake along its western shore, draining the highlands of Burundi and Rwanda. The Kagera River is about 360 miles long and the Ruvuvu River is its principal tributary. A series of swamps (2-18 km wide) and small lakes occur along the course of the Kagera River with several water falls in its upper reaches (De Vos et al. 2001). The Nzoia River is also a perennial affluent of Lake Victoria. It drains the Elgon Massif, the Cherangani Hills and Sergoit, entering the lake in the northeast. Inflows from rivers in the northwestern and southeastern portion of the ecoregion constitute the remainder of the riverine input. Rivers entering the lake from the northeast tend to be swift flowing whereas, rivers of the northwest tend to be sluggish and perennial (Hughes & Hughes 1992). 

The only outlet from Lake Victoria is the Victoria Nile River, which flows through Lake Kyoga and then to Lake Albert, to the north and northwest, respectively (Hughes & Hughes 1992). Passing through extensive areas of swampland, the Victoria Nile enters Lake Kyoga from the south. Lake Kyoga is part of a permanently flooded series of shallow lakes and swamps called the Kyoga Lake-Kwania Swamp complex, which contains 3,416 km² of open water and shallow lakes and 2,184 km² of permanently flooded swamp (Hughes & Hughes 1992).

Located on the border of Rwanda and the Democratic Republic of Congo (DRC), Lake Kivu lies in the Western Rift Valley, at an altitude of 1,463 m asl and is bordered by steep slopes that rise to elevations of over 2,000 m. It is 100 km long, and has a maximum width of 50 km. The lake is deep with a depth of about 480 meters and it is the most completely stratified lake in Africa. Lake Kivu formerly drained into Lake Edward. Volcanic activity severed this connection and Lake Kivu now drains into Lake Tanganyika via the Ruzizi River (Worthington & Lowe-McConnell 1994).

Lakes Edward and George lie in the western portion of the ecoregion. Surrounded by extensive swamps, Lake George straddles the equator and is fed by numerous rivers, which drain the eastern slopes of the Rwenzori Mountains, the highlands of the Western Rift Valley and the Virunga Massif. In general, Lake George is well mixed, though it does have a diurnal stratification cycle (Thompson 1976; Hughes & Hughes 1992). Lake George’s main inflows are the Rivers Nsong, Mbuku, and Bumlikwesi (Beadle 1981). The enormous swamp surrounding Lake George is dominated bypapyrus (Cyperus papyrus), which typically comprises over 95% of the plant biomass (Thompson 1976). Lake Edward is connected to Lake George by the Kazinga Channel, which 40 km long and has a maximum width of less than 1 km (Beadle 1981). Just 5 km off its western shore in the DRC, Lake Edward plummets to a depth of 112 meters, and then slopes gradually up to its eastern shore in Uganda (Hughes & Hughes 1992). The major outflow from Lake Edward is the Semliki River and its main inflows are the Rivers Nyamugasani, Ishasha, Rutshuru and Rwindi, with a smaller contribution by the Kazinga Channel (Beadle 1981).

The Lake Victoria ecoregion is also endowed with several small satellite lakes. A few of these include Lakes Kanyaboli, Sare, Namboyo in Kenya; Lakes Nabugabo, Gigati, and Agu in Uganda; and Lakes Ikimba and Burigi in Tanzania (Aloo 2003). These lakes may act as valuable sources of biodiversity, since many of them are still relatively undisturbed by human activities.

Lakes Victoria, Edward and George are all part of the greater Nile catchment. Separated from the lower Nile catchment by Murchinson Falls, evidence suggests that a connection between these lakes existed in the recent past, within the last several thousand years (Worthington & Lowe-McConnell 1994). During the uplift of the mid-Pleistocene, it is theorized that the rapid “back ponding” of the Kagera and Katonga Rivers overspilled the basin of Lake Victoria, draining into the Edward basin well into the late-Pleistocene (Beadle 1981). Even today, the Katonga River is able to flow in either direction, enabling species communication between all three lakes (Worthington & Lowe-McConnell 1994). Lakes Edward and George have had a very dynamic history. In the early Pleistocene (2 million years ago), there may have been passable connections between Lake Edward and Albert basins, now separated by a section of rapids (300 m descent). Lake Albert is not included in this ecoregion because this separation has likely caused its fauna to remain distinctly riverine and Nilotic. The faunas of lakes Edward and George were very similar: fossil remains of Lates, Hydrocynus, and Crocodylus niloticus in the Edward basin link the two faunas. Subsequently, the two lakes may have gone through a number of mass extinction and recolonization events (Beadle 1981). The last of these events probably occurred between 8,000 and 10,000 years ago and was associated with the eruption of volcanoes which may have deposited enormous amounts of toxic ash on the lake, killing species such as Lates which require well-oxygenated water (Beadle 1981; Schofield & Chapman 2000). Similarly, the Nile crocodiles (Crocodylus niloticus) were no longer present in the areas of Lakes Edward and George after the eruption of neighboring volcanoes (Beadle 1981), presumably kept from colonizing the lake by difficulties in passing the Semliki rapids. Some species in Lakes Edward and George are also found in Lake Albert (e.g., Bagrus docmac, Oreochromis niloticus), while other species (e.g., Polypterus senegalus, Hydrocynus spp.), and even families (such as Mastacembelidae, Characidae, Schilbeidae) that typify the Nilotic fauna from Lake Albert are absent (Greenwood 1966). Many of the cichlids from Lakes Edward and George are closely related to species in Lake Victoria (Greenwood 1966; Greenwood 1973; Beadle 1981; Kaufman et al. 1997); however, the number and distribution of endemic cichlids in the lakes is uncertain, and holds the key to our understanding of the relationship between land form and evolution of cichlid faunas in the Lake Victoria Basin.

A recent study on the phylogeny of the Lake Victoria-Edward cichlid species flock suggests, in contrast to the results of previous studies, that the flock is derived from the Congolese-Nilotic genus Thoracochromis and not from the East African riverine Astatotilapia (Seehausen et al. 2003). The rivers feeding Lake Victoria were tributaries of the Congo until uplifting of the region to its west about 400,000 years ago and it is possible that ancestry with the Congolese Thoracochromis predates this event. Seehausen et al. (2003) also question the monophyly of the Lake Victoria-Edward flock and raise the possibility that the flock has arisen from hybrid swarms.