Boundaries

Lake Tanganyika lies in a deep graben within eastern Africa’s Great Rift Valley and is split between Burundi, Democratic Republic of Congo, Tanzania, and Zambia. The lake has one of the richest lake faunas on earth (Worthington & Lowe-McConnell 1994). Its fauna exhibits extraordinarily high levels of endemism within several taxonomic groups at the species and genus levels. This ecoregion comprises the lake and its drainage basin.

Freshwater habitats

Lake Tanganyika is a meromictic lake, meaning that the non-circulating hypolimnion (bottom layer) does not mix with the circulating upper layer (epilimnion). The mixed, oxygenated layer extends to about 50-250 m (depending on season), limiting the distributions of aquatic life to this depth (Lowe-McConnell 1993). Beyond this depth, the environment is anaerobic and accumulated particulate matter makes the water much denser than the above layer (Patterson & Makin 1998). Permanent stratification keeps much of the nutrients in the hypolimnion (Lowe-McConnell 1987). 

The deep lake is narrow (about 650 km long by 50 km wide on average) with steep sides. The Kalemie shoal separates the lake into two main basins. The mean depth over the shoal is 500 m and the depth on either side of the shoal is greater than 1,000 m (Spigel & Coulter 1996). The volume of freshwater within the lake is equivalent to one-sixth of the earth’s freshwater. The southern end of the lake has gentle sloping sides, and oxygenated waters will extend down to a depth of about to 240 m during mixing of the circulating upper layer of water, allowing the benthic fish community to be more productive in this portion of the lake (Coulter & Mubamba 1993). 

Rainfall and evaporation largely determine the water balance of the lake. Between 800-1,200 mm/yr of rain falls in the vicinity of the lake (mean 1,050 mm/yr) and evaporation averages 1,530 mm/yr (Spigel & Coulter 1996; Patterson & Makin 1998). The many rivers and streams that enter the lake, including its major tributaries — the Rusizi, Malagarasi and Lugufu rivers — play a small role in the lake’s water balance. The only major outflow from the lake is the River Lukuga, which discharges to the Congo River via the Lualaba (Patterson & Makin 1998). Given the small influence of rivers, the flushing time of water in the lake (lake volume divided by river outflow) is an incredible 7,000 years (Spigel & Coulter 1996).

The deep (more than 500 m) rift valley lake habitat type is considered globally rare because there are fewer than eight such lakes worldwide (Thieme et al. 2005).

Description of endemic fishes

Seventy-nine percent of 287 described fish species are endemic, with greater than 95% endemism within the Cichlidae family. Forty-nine of 57 cichlid genera (86%) are endemic to the lake. The lake also supports species flocks in the Claroteidae and Mochokidae catfish families, the Centropomidae (Lates), and the Mastacembelidae spiny eels (Worthington & Lowe-McConnell 1994).

Other noteworthy fishes

The pelagic fish community of Lake Tanganyika is unique among Africa’s great lakes. Two species of endemic clupeids, Limnothrissa miodon and Stolothrissa tanganicae, feed on zooplankton in the pelagic zone and, in turn, provide food for the four predatory centropomids, Lates angustifrons, L. mariae, L. microlepis, and L. stappersii. These fish are the basis of the off-shore fishery on the lake.

Justification for delineation

Lake Tanganyika’s great depth has presumably given its fauna an evolutionary advantage. Over its long history (the lake is probably 9-12 million years old), Lake Tanganyika served as a refuge for aquatic organisms during extremely dry periods when other water bodies desiccated (Cohen et al. 1993). The split into two or three separate basins during low lake levels seems to have had important effects on the evolution and distribution of the ichthyodiversity and facilitated allopatric speciation (Coulter 1991; Snoeks 2000). The age of the lake has also facilitated a further differentiation of the fish fauna as compared to the faunas of Lakes Malawi and Victoria. For example, the Lake Tanganyika cichlids are considered to have evolved from eight ancestral lineages, more than in Lakes Malawi or Victoria (Salzburger et al. 2002).

The fish fauna of Lake Tanganyika has strong affinities with the Congo Basin (Roberts 1975; De Vos & Snoeks 1994) and the two systems are still connected hydrologically. In the late Miocene/early Pliocene, the Congo Basin contained a large internal lake that covered the ‘Cuvette Centrale’ (Coulter 1991). It is believed that Lake Tanganyika’s fauna had its origins in this ancient environment, although the details and timeline are not complete. The 23 fish families of Lake Tanganyika are all present in the Congo basin fauna.

Level of taxonomic exploration

Fair