Boundaries

Lake Rukwa is a saline lake of over 85,000 km² located in the Rift Valley. The lake lies within the southwest corner of Tanzania, east of Lake Tanganyika’s [542] southern tip. The Lake Rukwa ecoregion, comprised of the lake and its catchment, is bordered in the south east by the Mbeya Range, in the west by the slopes of the Ufipa escarpment and Mbizi (up to 2,664 m asl), and in the north-east by rocky cliffs and rolling hills that reach as high as 1,707 m at Mount Sange (Hughes & Hughes 1992; Baker & Baker 2001; Baker & Baker 2002).

Topography

Lake Rukwa lies in a rift within the Western Rift Valley, and the lake itself sits at 790 m asl. 

Freshwater habitats

At high water, there is a single lake, but at lower levels the lake splits into two basins that differ in both size and depth. The water levels are currently high (Baker & Baker 2002), although there is evidence that the level is once again falling. The lakebed slopes toward the east, resulting in greatest depths on the eastern shore. The south basin has maximum depths reaching about 10-15 m. The north basin is shallower, and it occasionally dries completely. The water depth of the swamp barrier between the two basins is usually 1 m or less (Hughes & Hughes 1992; Seegers 1996). 

The lake stretches lengthwise for about 165 km, with widths of 37 km in the north basin and a maximum width of 48 km near the middle of the lake (Seegers 1996). Along the northern and western shores of the lake lie extensive wetlands that include both permanent swamps and temporary floodplains. The presence of dead or dying trees under water is evidence that these areas were flooded relatively recently (Seegers 1996). At the deltas of the Luika, Songwe, Momba, and Chambue Rivers (south lake), and Kavu and Rungwa Rivers (north lake), there are also large swampy areas (Seegers 1996). The waters of the lake have high levels of sodium and high alkalinity (pH between 8.0 and 9.0) (Seegers 1996). The less saline swamps that surround the lake contain Cyperus papyrus and Phragmites mauritianus, among other species. C. papyrus can also be found along with Oryza sp., Leersia sp., Vossia cuspidata and Typha sp. in deeper waters of the lake. 

Terrestrial habitats

Floodplain vegetation surrounding Lake Rukwa is primarily grassland, with short grasses dominated by the salt-tolerant Diplachne fusca, Sporobolus spicata, and S. robustus (Hughes & Hughes 1992). Gallery forests of woodland, with many Acacia species, grow along the tributaries that feed the lake and along the lake’s edges (Hughes & Hughes 1992). 

Description of endemic fishes

This ecoregion hosts an aquatic fauna that includes endemic fish species flocks of the cichlid genus Haplochromis and the catfish genus Chiloglanis (family Mochokidae). Both flocks have six known species (Seegers 1996). 

Justification for delineation

This ecoregion is defined by the Lake Rukwa basin with its two endemic fish species flocks. Recent connections existed between the Rukwa basin and both the Malagarasi system [543] and the Chambeshi (which drains to Lake Bangweulu [544]). There is also evidence of connections between the headwaters of the Manda and Kalambo Rivers, which presently drain into Lake Tanganyika, and the Mfwizi River of the Rukwa drainage (Seegers 1996).

Level of taxonomic exploration

Good. Important historical references for this ecoregion include Ricardo (1939), Verheyen (1939), Vesey-FitzGerald and Beesley (1960) and Vesey-FitzGerald (1964)(Ricardo 1939; Verheyen 1939; Vesey-FitzGerald and Beesley 1960; Vesey-FitzGerald 1964). The fish species of the lake are well known with Seeger’s (1996) volume on the fish of the Lake Rukwa basin. Additionally, technical reports available at the Regional Natural Resources Office in Mbeya, such as Davenport (2000) review the status of the basin’s ecosystems and provide comprehensive species lists of flora and fauna of the area(Davenport 2000).