Lake Tanganika

Lake Tanganyika ( ; ) is an African Great Lake. It is the world's second-largest freshwater lake by volume and the second deepest, in both 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 (the DRC), Burundi, and Zambia—with Tanzania (46%) and the DRC (40%) possessing the majority of the lake. It drains via the Lukuga River into the Congo River system, which ultimately discharges at Banana, Democratic Republic of the Congo into the Atlantic Ocean.

Geography

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 freshwater 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 of water.

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.

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, one of only twenty more than a million years old. 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.

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 sulfide and are essentially lifeless, except for bacteria.

Biology

Reptiles

Lake Tanganyika and its 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 fishes

Lake Tanganyika is home to at least 250 endemic species of cichlids, and undescribed species likely remain to be discovered. Almost all (roughly 98%) of the lake's cichlid species are found solely there, and nowhere else, thus making it a precious biological resource for the study of speciation and evolution. The cichlids of the African Great Lakes, including Tanganyika, represent the most diverse extent of adaptive radiation in vertebrates. Some of the endemic species do occur slightly into the upper Lukuga River (Lake Tanganyika's outflow), but any further distribution into the Congo River basin is stemmed purely by geography and chemistry—Tanganyika's lake environment is far more stable and consistent than the rapids and fast-flowing sections of the Congo River. Additionally, Lake Tanganyika's water is alkaline and of a higher pH (which the cichlids prefer), containing a higher calcium and mineral content than the Congo's acidic, sediment-rich waters, which collect much organic detritus from the surrounding rainforests. In areas of the Congo away from rapids or whitewater, the accumulated sediment and organic matter creates sections of "blackwater", with a high concentration of tannins from dissolving wood and leaves, creating an environment in which cichlids simply do not thrive. Likewise, many tropical riverine species would likely suffer if exposed to the crisp, alkaline lake water.

Although Lake Tanganyika has fewer cichlid species than Lakes Malawi or 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 maturity 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), while 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 (Bathybatini, Benthochromini, Boulengerochromini, Eretmodini, Greenwoodochromini, Perissodini and Trematocarini, in addition to the aforementioned tribes).

Most Tanganyika cichlids live along the shoreline, down to a depth of , but some deep-water species regularly descend to . ''Trematocara'' species have, exceptionally, been found at more than , which is deeper than any other cichlid known. Some of the deepwater genera (e.g., ''Bathybates'', '' Gnathochromis'', '' Hemibates'' and ''Xenochromis'') have been caught in places virtually devoid of oxygen, and how they are able to survive there is unclear. Tanganyika cichlids are generally benthic (found at or near the bottom) and/or coastally.Lindqvist, O.V.; H. Mölsä; K. Solonen; J. Sarvala, editors (1999). From Limnology to Fisheries: Lake Tanganyika and Other Large Lakes. pp. 213–214. Springer. No Tanganyika cichlids are truly pelagic ''and'' offshore, except for some of the piscivorous ''Bathybates''. Two of these, '' B. fasciatus'' and '' B. leo'', mainly feed on Tanganyika sardines. Tanganyika cichlids differ extensively in ecology, and include species that are herbivores, detritivores, planktivores, insectivores, molluscivores, scavengers, scale-eaters and piscivores. These dietary specializations, however, have been shown to be variable and subject to opportunistic changes. That is, many species of Tanganyikan cichlid with specialized diets showed opportunistic, episodic exploitation of '' Stolothrissa tanganicae'' and ''Limnothrissa miodon'' when prey concentrations were unusually high. The fishes' breeding behavior falls into two main groups: the substrate- or sand-spawners (often in caves or rock crevices) and the mouthbrooders.Schliewen, U. (1992). Aquarium Fish. Barron's Educational Series. . Among the endemic species are two of the world's smallest cichlids, ''Neolamprologus multifasciatus'' and '' N. similis'' (both shell dwellers), measuring up to , and one of the largest, the giant cichlid'', ''Cyprichromis'', ''Eretmodus'', ''Julidochromis'', ''Lamprologus'', ''Neolamprologus'', ''Tropheus'' and ''Xenotilapia'', are popular aquarium fish due to their bright colors and patterns, and interesting behaviors. Recreating a Lake Tanganyika biotope to host those cichlids in a habitat similar to their natural environment is also popular in the aquarium hobby, with many species today being bred in captivity successfully.

File:Bathybates ferox.jpg, Bathybatini (E): ''Bathybates ferox'' is benthic and piscivorous, but the genus also includes pelagic species. The tribe is sometimes split in three, others being Hemibatini and Trematocarini
File:Benthochromis tricoti.jpg, Benthochromini (E): ''Benthochromis horii'' was scientifically described in 2008, but has often been misidentifed as '' B. tricoti''
File:DKoehl Boulengerochromis microlepis.jpg, Boulengerochromini

HOME


Content is Copyleft
Website design, code, and AI is Copyrighted (c) 2014-2017 by Stephen Payne


Consider donating to Wikimedia



As an Amazon Associate I earn from qualifying purchases