Lake Tanganyika

Lake Tanganyika () is an African Great Lake. It is the second-oldest freshwater lake in the world, the second-largest by volume, and the second-deepest, in all cases after Lake Baikal in Siberia. It is the world's longest freshwater lake. The lake is shared among four countries—Tanzania, the Democratic Republic of the Congo (DRC), Burundi, and Zambia, with Tanzania (46%) and DRC (40%) possessing the majority of the lake. It drains into the Congo River system and ultimately into the Atlantic Ocean.

Etymology

"Tanganika" was the name of the lake that Henry Morton Stanley encountered when he was at Ujiji in 1876. The name first originated from the Bembe language when they arrived in South Kivu around the 7th century, they discovered the lake and started calling it “êtanga ‘ya’ni’â” which means “a big river” in their Bantu language. Stanley found also other names for the lake among different ethnic groups, like the Kimana, the Yemba and the Msaga.

An alternate etymological source of the name is found in the Luvale language of Zambia & Angola, where the word "tanganyika" literally means "star", pointing to the lake having supernatural or cosmological significance to the surrounding tribes.

Geography and geological history

Lake Tanganyika is situated within the Albertine Rift, the western branch of the East African Rift, and is confined by the mountainous walls of the valley. It is the largest rift lake in Africa and the second-largest lake by volume in the world. It is the deepest lake in Africa and holds the greatest volume of fresh water on the continent, accounting for 16% of the world's available fresh water. It extends for in a general north–south direction and averages in width. The lake covers , with a shoreline of , a mean depth of and a maximum depth of (in the northern basin). It holds an estimated .

The catchment area of the lake is . Two main rivers flow into the lake, as well as numerous smaller rivers and streams (whose lengths are limited by the steep mountains around the lake). The one major outflow is the Lukuga River, which empties into the Congo River drainage. Precipitation and evaporation play a greater role than the rivers. At least 90% of the water influx is from rain falling on the lake's surface and at least 90% of the water loss is from direct evaporation.

The major river flowing into the lake is the Ruzizi River, formed about 10,000 years ago, which enters the north of the lake from Lake Kivu. The Malagarasi River, which is Tanzania's second largest river, enters the east side of Lake Tanganyika. The Malagarasi is older than Lake Tanganyika, and before the lake was formed, it probably was a headwater of the Lualaba River, the main Congo River headstream.

The lake has a complex history of changing flow patterns, due to its high altitude, great depth, slow rate of refill, and mountainous location in a turbulently volcanic area that has undergone climate changes. Apparently, it has rarely in the past had an outflow to the sea. It has been described as "practically endorheic" for this reason. The lake's connection to the sea is dependent on a high water level allowing water to overflow out of the lake through the Lukuga River into the Congo. When not overflowing, the lake's exit into the Lukuga River typically is blocked by sand bars and masses of weed, and instead this river depends on its own tributaries, especially the Niemba River, to maintain a flow.

Due to the lake's tropical location, it has a high rate of evaporation. Thus, it depends on a high inflow through the Ruzizi out of Lake Kivu to keep the lake high enough to overflow. This outflow is apparently not more than 12,000 years old, and resulted from lava flows blocking and diverting the Kivu basin's previous outflow into Lake Edward and then the Nile system, and diverting it to Lake Tanganyika. Signs of ancient shorelines indicate that at times, Tanganyika may have been up to lower than its present surface level, with no outlet to the sea. Even its current outlet is intermittent, thus may not have been operating when first visited by Western explorers in 1858.

The lake may also have at times had different inflows and outflows; inward flows from a higher Lake Rukwa, access to Lake Malawi and an exit route to the Nile have all been proposed to have existed at some point in the lake's history.

Lake Tanganyika is an ancient lake. Its three basins, which in periods with much lower water levels were separate lakes, are of different ages. The central began to form 9–12 million years ago (Mya), the northern 7–8 Mya and the southern 2–4 Mya.

Islands

Of the several islands in Lake Tanganyika, the most important are:

* Kavala Island (DRC)
* Mamba-Kayenda Islands (DRC)

Milima Island
(DRC)
* Kibishie Island (DRC)
* Mutondwe Island (Zambia)
* Kumbula Island (Zambia)

Water characteristics

The lake's water is alkaline with a pH around 9 at depths of . Below this, it is around 8.7, gradually decreasing to 8.3–8.5 in the deepest parts of Tanganyika. A similar pattern can be seen in the electric conductivity, ranging from about 670 μS/cm in the upper part to 690 μS/cm in the deepest.

Surface temperatures generally range from about in the southern part of the lake in early August to in the late rainy season in March—April. At depths greater than , the temperature is very stable at . The water has gradually warmed since the 19th century and this has accelerated with global warming since the 1950s.

The lake is stratified and seasonal mixing generally does not extend beyond depths of . The mixing mainly occurs as upwellings in the south and is wind-driven, but to a lesser extent, up- and downwellings also occur elsewhere in the lake. As a consequence of the stratification, the deep sections contain "fossil water". This also means it has no oxygen (it is anoxic) in the deeper parts, essentially limiting fish and other aerobic organisms to the upper part. Some geographical variations are seen in this limit, but it is typically at depths around in the northern part of the lake and in the south.Wright, J.J.; and L.M. Page (2006). Taxonomic revision of Lake Tanganyikan Synodontis (Siluriformes: Mochokidae). Florida Mus. Nat. Hist. Bull. 46(4): 99–154.Lowe-McConnell, R.H. (1987). Ecological Studies in Tropical Fish Communities. . The oxygen-devoid deepest sections contain high levels of toxic hydrogen sulphide and are essentially lifeless, except for bacteria.

Biology

Reptiles

Lake Tanganyika and associated wetlands are home to Nile crocodiles (including famous giant Gustave), Zambian hinged terrapins, serrated hinged terrapins, and pan hinged terrapins (last species not in the lake itself, but in adjacent lagoons).Spawls, Howell, Drewes, and Ashe (2002). A Field Guide to the Reptiles of East Africa. Academic Press, London. . Storm's water cobra, a threatened subspecies of banded water cobra that feeds mainly on fish, is only found in Lake Tanganyika, where it prefers rocky shores.

Cichlid fish

The lake holds at least 250 species of cichlid fishWest, K. (prepared by) (2001).
Lake Tanganyika: Results and Experiences of the UNDP/GEF Conservation Initiative (RAF/92/G32) in Burundi, D.R. Congo, Tanzania, and Zambia.
' Lake Tanganyika Biodiversity Project. and undescribed species remain.Mortiff, C
Lake Tanganyika and its Diverse Cichlids.
Cichlid-Forum. Retrieved 1 March 2017. Almost all (98%) of the Tanganyika cichlids are endemic to the lake and it is thus an important biological resource for the study of speciation in evolution. Some of the endemics do occur slightly into the upper Lukuga River, Lake Tanganyika's outflow, but further spread into the Congo River basin is prevented by physics (Lukuga has fast-flowing sections with many rapids and waterfalls) and chemistry (Tanganyika's water is alkaline, while the Congo's generally is acidic). The cichlids of the African Great Lakes, including Tanganyika, represent the most diverse extent of adaptive radiation in vertebrates.

Although Tanganyika has far fewer cichlid species than Lakes Malawi and Victoria which both have experienced relatively recent explosive species radiations (resulting in many closely related species), its cichlids are the most morphologically and genetically diverse. This is linked to the high age of Tanganyika, as it is far older than the other lakes. Tanganyika has the largest number of endemic cichlid genera of all African lakes. All Tanganyika cichlids are in the subfamily Pseudocrenilabrinae. Of the 10 tribes in this subfamily, half are largely or entirely restricted to the lake (Cyprichromini, Ectodini, Lamprologini, Limnochromini and Tropheini) and another three have species in the lake ( Haplochromini, Tilapiini and Tylochromini). Others have proposed splitting the Tanganyika cichlids into as many as 12–16 tribes (in addition to previous mentioned, Bathybatini, Benthochromini, Boulengerochromini