Lake Victoria
Lake Victoria is one of the African Great Lakes. With a surface area of approximately, Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after Lake Superior in North America. In terms of volume, Lake Victoria is the world's ninth-largest continental lake, containing about of water. Lake Victoria occupies a shallow depression in Africa. The lake has an average depth of and a maximum depth of. Its catchment area covers. The lake has a shoreline of when digitized at the 1:25,000 level, with islands constituting 3.7% of this length.
The lake's area is divided among three countries: Tanzania occupies 49%, Uganda 45%, and Kenya 6%.
The lake is home to many species of fish which live nowhere else, especially cichlids. Invasive fish, such as the Nile perch, have driven many endemic species to extinction.
Names
Though having multiple local language names, the lake was renamed after Queen Victoria by the explorer John Hanning Speke, the first Briton to document it in 1858, while on an expedition with Richard Francis Burton.Geology
Photojournalist John Reader, writing in his Alan Paton Literary Award-winning Africa: A Biography of a Continent, describes Lake Victoria as being relatively geologically young at about 400,000-years old—having been formed as westward-flowing rivers were backed up "when a fractured block of the Earth's crust tilted along the line of the Great Rift Valley, raising its western edge". A primary study, attempting "fluvial differentiation of the basin of Lake Victoria", draws several relevant tentative conclusions. First, during the Miocene era, what is now the catchment area of the lake was on the western side of an uplifted area that functioned as a continental divide, with streams on the western side flowing into the Congo River basin and streams on the eastern side flowing to the Indian Ocean. Second, as the East African Rift System formed, the eastern wall of the Albertine Rift rose, gradually reversing the drainage towards what is now Lake Victoria. Third, the opening of the main East African Rift and the Albertine Rift downwarped the area between them as the rift walls rose, creating the current Lake Victoria basin.During its geological history, Lake Victoria went through changes ranging from its present shallow depression, through to what may have been a series of much smaller lakes. Geological cores taken from its bottom show Lake Victoria has dried up completely at least three times since it formed. These drying cycles are probably related to past ice ages, which were times when precipitation declined globally. According to another primary source, Lake Victoria last dried out about 17,300 years ago, and it refilled 14,700 years ago—as the African humid period began.
Hydrology and limnology
Lake Victoria receives 80 percent of its water from direct rainfall. Average evaporation on the lake is between per year, almost double the precipitation of riparian areas. Lake Victoria receives its water additionally from rivers, and thousands of small streams. The Kagera River is the largest river flowing into this lake, with its mouth on the lake's western shore. Lake Victoria is drained solely by the Nile River near Jinja, Uganda on the lake's northern shore.In the Kenya sector, the main influent rivers are the Sio, Nzoia, Yala, Nyando, Sondu Miriu, Mogusi, and Migori. The only outflow from Lake Victoria is the Nile River, which exits the lake near Jinja, Uganda. In terms of contributed water, this makes Lake Victoria the principal source of the longest branch of the Nile. However, the most distal source of the Nile Basin, and therefore the ultimate source of the Nile, is more often considered to be one of the tributary rivers of the Kagera River, and which originates in either Rwanda or Burundi. The uppermost section of the Nile is generally known as the Victoria Nile until it reaches Lake Albert. Although it is a part of the same river system known as the White Nile and is occasionally referred to as such, strictly speaking this name does not apply until after the river crosses the Uganda border into South Sudan to the north.
The lake exhibits eutrophic conditions. In 1990–1991, oxygen concentrations in the mixed layer were higher than in 1960–1961, with nearly continuous oxygen supersaturation in surface waters. Oxygen concentrations in hypolimnetic waters were lower in 1990–1991 for a longer period than in 1960–1961, with values of less than 1 mg per litre occurring in water as shallow as compared with a shallowest occurrence of greater than in 1961. The changes in oxygenation are considered consistent with measurements of higher algal biomass and productivity. These changes have arisen for multiple reasons: successive burning within its basin, soot and ash from which has been deposited over the lake's wide area; from increased nutrient inflows via rivers, and from increased pollution associated with settlement along its shores.
Between 2010 and 2022, the surface area of Lake Victoria increased by 15% flooding lakeside communities.
Bathymetry
The lake is a shallow lake considering its large geographic area with a maximum depth of approximately and an average depth of. A 2016 project digitized ten-thousand points and created the first true bathymetric map of the lake. The deepest part of the lake is offset to the east of the lake near Kenya and the lake is generally shallower in the west along the Ugandan shoreline and the south along the Tanzanian shoreline.Native wildlife
Mammals
Many mammal species live in the region of Lake Victoria, and some of these are closely associated with the lake itself and the nearby wetlands. Among these are the hippopotamus, African clawless otter, spotted-necked otter, marsh mongoose, sitatunga, bohor reedbuck, defassa waterbuck, cane rats, and giant otter shrew.Reptiles
Lake Victoria and its wetlands has a large population of Nile crocodiles, as well as African helmeted turtles, variable mud turtles, and Williams' mud turtle. The Williams' mud turtle is restricted to Lake Victoria and other lakes, rivers, and swamps in the upper Nile basin.Cichlid fish
Lake Victoria formerly was very rich in fish, including many endemics, but a high percentage of these became extinct since the 1940s. The main group in Lake Victoria is the haplochromine cichlids with more than 500 species, almost all endemic, and including an estimated 300 that still are undescribed. This is far more species of fish than any other lake in the world, except Lake Malawi. These are the result of a rapid adaptive radiation in the last circa 15,000 years. Their extraordinary diversity and speed of evolution have been the subjects for many scientists studying the forces that drive the richness of life everywhere. The Victoria haplochromines are part of an older group of more than 700 closely related species, also including those of several smaller lakes in the region, notably Kyoga, Edward–George, Albert, and Kivu.Most of these lakes are relatively shallow and part of the present-day upper Nile basin. The exception is Lake Kivu, which is part of the present-day Congo River basin, but is believed to have been connected to Lakes Edward and Victoria by rivers until the uplifting of parts of the East African Rift. This deep lake may have functioned as an "evolutionary reservoir" for this haplochromine group in periods where other shallower lakes in the region dried out, as happened to Lake Victoria about 15,000 years ago. In recent history only Lake Kyoga was easily accessible to Victoria cichlids, as further downstream movement by the Victoria Nile is prevented by a series of waterfalls, notably Murchison. In contrast, the Owen Falls between Victoria and Kyoga were essentially a series of rapids that did not effectively block fish movements between the two lakes.
The Victoria haplochromines are distinctly sexually dimorphic, and their ecology is extremely diverse, falling into at least 16 groups, including detritivores, zooplanktivores, insectivores, prawn-eaters, molluscivores and piscivores. As a result of predation by the introduced Nile perch, eutrophication and other changes to the ecosystem, it is estimated that at least 200 species of Lake Victoria haplochromines have become extinct, including more than 100 undescribed species. Initially it was feared that this number was even higher, by some estimates 65 percent of the total species, but several species that were feared extinct have been rediscovered after the Nile perch started to decline in the 1990s. Several of the remaining species are seriously threatened and additional extinctions are possible. Some species have survived in nearby small satellite lakes, have survived in refugias among rocks or papyrus sedges, or have adapted to the human-induced changes in the lake itself. Such adaptions include a larger gill area, changes in the feeding apparatus, changes to the eyes and smaller head/larger caudal peduncle. The piscivorous, molluscivorous and insectivorous haplochromines were particularly hard hit with many extinctions. Others have become extinct in their pure form, but survive as hybrids between close relatives. The zooplanktivores have been least affected and in the late 1990s had reached densities similar to, or above, the densities before the drastic declines, although consisting of fewer species and often switching their diet towards macroinvertebrates. Some of the threatened Lake Victoria cichlid species have captive "insurance" populations in zoos, public aquaria and among private aquarists, and a few species are extinct in the wild.
Before the mass extinction that has occurred among the lake's cichlids in the last 50 years, about 90 percent of the native fish species in the lake were haplochromines. Disregarding the haplochromines, the only native Victoria cichlids are two critically endangered tilapia, the Singida tilapia or ngege and Victoria tilapia.
In 1927–1928 Michael Graham conducted the first ever systematic Fisheries Survey of Lake Victoria. In his official report of the expedition, Graham wrote that "The ngege or satu Tilapia esculenta, is the most important food fish of the lake, whether for native or non-native consumption. No other fish equals it in the quality of the flesh. It is convenient size for trade, travels well and is found in much greater numbers than other important fish, such as semutundu, Bagrus sp.". Furthermore, Graham noted that the introduction of the European flax gill net of 5 inch mesh had undoubtedly caused a diminution in the number of ngege in those parts of the Kavirondo Gulf, the northern shore of the lake, the Sesse Islands and Smith's Sound which are conveniently situated close to markets. Survey catches in 1927–28 included several Haplochromis species that are now thought to be extinct, including: Haplochromis flavipinnis, Haplochromis gowersii, Haplochromis longirostris, Haplochromis macrognathus, Haplochromis michaeli, Haplochromis nigrescens, Haplochromis prognathus.
As well as being due to the introduction of Nile perch, the extinction of cichlids in the genus Haplochromis has also been blamed on the lake's eutrophication. The fertility of tropical waters depends on the rate at which nutrients can be brought into solution. The influent rivers of Lake Victoria provide few nutrients to the lake in relation to its size. Because of this, most of Lake Victoria's nutrients are thought to be locked up in lake-bottom deposits. By itself, this vegetative matter decays slowly. Animal flesh decays considerably faster, however, so the fertility of the lake is dependent on the rate at which these nutrients can be taken up by fish and other organisms. There is little doubt that Haplochromis played an important role in returning detritus and plankton back into solution. With some 80 percent of Haplochromis species feeding off detritus, and equally capable of feeding off one another, they represented a tight, internal recycling system, moving nutrients and biomass both vertically and horizontally through the water column, and even out of the lake via predation by humans and terrestrial animals. The removal of Haplochromis, however, may have contributed to the increasing frequency of algal blooms, which may in turn be responsible for mass fish kills.