Lake Victoria
Lake Victoria is one of the African Great Lakes.
The lake was named after Queen Victoria by the explorer John Hanning Speke, in his reports—the first Briton to document it. Speke accomplished this in 1858, while on an expedition with Richard Francis Burton to locate the source of the Nile River. This expedition was financially sponsored by the Royal Geographic Society.
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 a maximum depth of between and an average 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 percent of this length. The lake's area is divided among three countries: Kenya, Uganda, and Tanzania.
The lake includes many species of fish that appear nowhere else, especially cichlids. Invasive fish, such as the Nile perch, have driven many endemic species to extinction.
Geology
Geologically, Lake Victoria is relatively young at about 400,000 years old. It formed when westward-flowing rivers were dammed by an upthrown crustal block. 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. 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.
Bathymetry
The lake is considered 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 during the last 50 years. 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.
survives in low numbers. Initially feared extinct, when rediscovered it had changed habitat and feeding behavior
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.
Other fish
The non-cichlid native fish include African tetras, cyprinids, airbreathing catfish, bagrid catfish, loach catfish, silver butter catfish, Synodontis squeaker catfish, Nothobranchius killifish, poeciliids, the spiny eel Mastacembelus frenatus, elephantfish, the climbing gourami Ctenopoma muriei and marbled lungfish.At a genus level, most of these are widespread in Africa, but the very rare Xenobarbus and Xenoclarias are endemic to the lake, and the common Rastrineobola is near-endemic.
Crustaceans
Four species of freshwater crabs are known from Lake Victoria: Potamonautes niloticus is widespread in the lake and P. emini has been recorded from the vicinity of Bukoba in Tanzania, but both are also found elsewhere in Africa. The last were first scientifically described in 2017 and very little is known about them: P. entebbe is only known from near Entebbe and P. busungwe only at Busungwe Island in the northwestern part of the lake. The latter likely is the smallest African freshwater crab with a carapace width up to about, although P. kantsyore of Kagera River, and Platythelphusa maculata and P. polita of Lake Tanganyika are almost as small.The only shrimp/prawn is Caridina nilotica, which is common and widespread in Lake Victoria.
Molluscs
Lake Victoria is home to 28 species of freshwater snails, including 12 endemic species/subspecies. There are 17 species of bivalves, including 6 endemic species/subspecies. It is likely that undescribed species of snails remain. Conversely, genetic studies indicate that some morphologically distinctive populations, traditionally regarded as separate species, may only be variants of single species. Two of the snail genera, Biomphalaria and Bulinus, are intermediate hosts of the parasite bilharzia. Human infections by this parasite are common at Lake Victoria. This may increase as a result of the spread of the invasive water hyacinth, and the loss of many snail-eating cichlids in the lake.Fisheries
Lake Victoria supports Africa's largest inland fishery. Initially the fishery involved native species, especially tilapia and haplochromine cichlids, but also catfish, elephantfish, ningu and marbled lungfish. Some of these, including tilapia and ningu, had already declined in the first half of the 20th century due to overfishing. To boost fishing, several species of non-native tilapia and Nile perch were introduced to the lake in the 1950s. Nevertheless, the natives continued to dominate fisheries until the 1970s where their decline meant that there was a strong shift towards the non-native Nile tilapia, non-native Nile perch and the native Lake Victoria sardine. Because of its small size, the abundant open-water Lake Victoria sardine only supported minor fisheries until the decline of other natives. At the peak in the early 1990s, of Nile perch were landed annually in Lake Victoria, but this has declined significantly in later years.Environmental issues
A number of environmental issues are associated with Lake Victoria and the complete disappearance of many endemic cichlid species has been called the "most dramatic example of human-caused extinctions within an ecosystem".Invasive fish
Starting in the 1950s, several species have been introduced to Lake Victoria where they have become invasive and a prime reason for the extinction of many endemic haplochromine cichlids. Among the introductions are several tilapias: redbreast, redbelly, Nile and blue-spotted tilapias. Although these have contributed to the extinction of native fish by causing significant changes to the ecosystem, outcompeted natives and possibly hybridized with the highly threatened native tilapias, the most infamous introduction was the large and highly predatory Nile perch.was introduced to Lake Victoria for fishing, and can reach up to and.
As early as the 1920s, it was proposed to introduce a large pelagic predator such as the Nile perch to improve the fisheries in the lake. At the same time it was warned that this could present a serious danger to the native fish species and required extensive research into possible ecological effects before done. These warnings primarily concerned the native tilapia O. esculentus, as the smaller haplochromine cichlids were regarded as "trash fish" or even vermin by the British colonial authorities. In the following decades, the pressure to introduce the Nile perch continued, as did warnings about the possible effects of doing it. The first introduction of Nile perch to the region, done by the Uganda Game and Fisheries Department and local African fish guards, happened upstream of Murchison Falls directly after the completion of the Owen Falls Dam in 1954. This allowed it to spread to Lake Kyoga where additional Nile perch were released in 1955, but not Victoria itself. Scientists argued that further introduction should wait until research showed the effect of the introduction in Kyoga, but by the late 1950s, Nile perch began being caught in Lake Victoria. As the species was already present, there were few objections when more Nile perch were transferred to Victoria to further bolster the stock in 1962–63. The origin of the first Victoria introductions in the 1950s is not entirely clear and indisputable evidence is lacking. Uganda Game and Fisheries Department officials denied that they were involved, but circumstantial evidence suggests otherwise and local Africans employed by UGFD have said that they introduced the species in 1954–55 under the directive of senior officials. UGFD officials argued that Nile perch must have spread to Lake Victoria by themselves by passing through the Owen Falls Dam when shut down for maintenance, but this is considered highly unlikely by many scientists. The Nile perch had spread throughout the lake by 1970. Initially the population of the Nile perch was relatively low, but a drastic increase happened, peaking in the 1980s, followed by a decline starting in the 1990s.
Due to the presence of the Nile perch, the natural balance of the lake's ecosystem has been disrupted. The food chain is being altered and in some cases, broken by the indiscriminate eating habits of the Nile perch.
The subsequent decrease in the number of algae-eating fish allows the algae to grow at an alarming rate, thereby choking the lake. The increasing amounts of algae, in turn, increase the amount of detritus that falls to the deeper portions of the lake before decomposing. As a by-product of this the oxygen levels in the deeper layer of water are being depleted. Without oxygen, any aerobic life cannot exist in the deeper parts of the lake, forcing all life to exist within a narrow range of depth. In this way, the Nile perch has degraded the diverse and thriving ecosystem that was once Lake Victoria. The abundance of aquatic life is not the only dependent of the lake: more than thirty million people in Tanzania, Kenya and Uganda rely on the lake for its natural resources.
Hundreds of endemic species that evolved under the special conditions offered by the protection of Lake Victoria have been lost due to extinction, and several more are still threatened. Their loss is devastating for the lake, the fields of ecology, genetics and evolution biology, and more evidently, for the local fisheries. Local fisheries once depended on catching the lungfish, tilapia, carp and catfish that comprise the local diet.
Today, the composition and yields of such fish catches are virtually negligible. Extensive fish kills, Nile perch, loss of habitat and overfishing have caused many fisheries to collapse and many protein sources to be unavailable at the market for local consumption. Few fisheries, though, have been able to make the switch to catching the Nile perch, since that requires a significant amount of capital resources.
Water hyacinth invasion
The water hyacinth has become a major invasive plant species in Lake Victoria.The release of large amounts of untreated wastewater and agricultural and industrial runoff directly into Lake Victoria over the past 30 years has greatly increased the nutrient levels of nitrogen and phosphorus in the lake "triggering massive growth of exotic water hyacinth, which colonised the lake in the late 1990s". This invasive weed creates anoxic conditions in the lake inhibiting decomposing plant material, raising toxicity and disease levels to both fish and people. At the same time, the plant's mat or "web" creates a barrier for boats and ferries to maneuver, impedes access to the shoreline, interferes with hydroelectric power generation, and blocks the intake of water for industries. On the other hand, water hyacinth mats can potentially have a positive effect on fish life in that they create a barrier to overfishing and allow for fish growth, there has even been the reappearance of some fish species thought to have been extinct in recent years. The overall effects of the water hyacinth, however, are still unknown.
Growth of the water hyacinth in Lake Victoria has been tracked since 1993, reaching its maxima biomass in 1997 and then declining again by the end of 2001. Greater growth was observed in the northern part of the lake, in relatively protected areas, which may be linked to current and weather patterns and could also be due to the climate and water conditions, which are more suitable to the plants growth. The invasive weed was first attempted to be controlled by hand, removed manually from the lake; however, re-growth occurred quickly. Public awareness exercises were also conducted. More recently, measures have been used such as the introduction of natural insect predators, including two different water hyacinth weevils and large harvesting and chopping boats, which seem to be much more effective in eliminating the water hyacinth. A green power plant that uses harvested water hyacinth was constructed in Kisumu County in 2013. In addition to the biogas it produces, its by-product can be used as fertilizer.
Other factors which may have contributed to the decline of the water hyacinth in Lake Victoria include varying weather patterns, such as El Niño during the last few months of 1997 and first six months of 1998 bringing with it higher levels of water in the lake and thus dislodging the plants. Heavy winds and rains along with their subsequent waves may have also damaged the plants during this same time frame. The plants may not have been destroyed, instead merely moved to another location. Additionally, the water quality, nutrient supply, temperature, and other environmental factors could have played a role. Overall, the timing of the decline could be linked to all of these factors and perhaps together, in combination, they were more effective than any one deterrent would have been by itself. The water hyacinth is in remission and this trend could be permanent if control efforts are continued.
Pollution
Pollution of Lake Victoria is mainly due to discharge of raw sewage into the lake, dumping of domestic and industrial waste, and fertiliser and chemicals from farms.The Lake Victoria basin while generally rural has many major centres of population. Its shores in particular are dotted with the key cities and towns, including Kisumu, Kisii, and Homa Bay in Kenya; Kampala, Jinja and Entebbe in Uganda; and Bukoba, Mwanza, and Musoma in Tanzania. These cities and towns also are home to many factories that discharge some chemicals directly into the lake or its influent rivers. Likewise, set up of small beaches and local authorities around the lake, lack proper sewage treatment facilities therefore allowing pollutants find their way into the water. Large parts of these urban areas also discharge untreated sewage into the river, increasing its eutrophication that in turn is helping to increase the invasive water hyacinth. Increased logging and act of deforestation has led to environmental degradation around the region reducing the absorption of polluting chemicals and deteriorating the water quality.
Environmental data
As of 2016, an environmental data repository exists for Lake Victoria. The repository contains shoreline, bathymetry, pollution, temperature, wind vector, and other important data for both the lake and the wider Basin.History and exploration
The first recorded information about Lake Victoria comes from Arab traders plying the inland routes in search of gold, ivory, other precious commodities, and slaves.The lake existed and was known to many Africans in the catchment area long before it was sighted by a European in 1858 when the British explorer John Hanning Speke reached its southern shore while on his journey with Richard Francis Burton to explore central Africa and locate the Great Lakes. Believing he had found the source of the Nile on seeing this "vast expanse of open water" for the first time, Speke named the lake after Queen Victoria. Burton, who had been recovering from illness at the time and resting further south on the shores of Lake Tanganyika, was outraged that Speke claimed to have proved his discovery to have been the true source of the Nile River, which Burton regarded as still unsettled. A very public quarrel ensued, which not only sparked a great deal of intense debate within the scientific community of the day, but also much interest by other explorers keen to either confirm or refute Speke's discovery.
In the late 1860s, the famous British explorer and missionary David Livingstone failed in his attempt to verify Speke's discovery, instead pushing too far west and entering the River Congo system instead. Ultimately, the Welsh-American explorer Henry Morton Stanley, on an expedition funded by the New York Herald newspaper, confirmed the truth of Speke's discovery, circumnavigating the lake and reporting the great outflow at Ripon Falls on the lake's northern shore.
Nalubaale Dam
The only outflow for Lake Victoria is at Jinja, Uganda, where it forms the Victoria Nile. The water since at least 12,000 years ago drained across a natural rock weir. In 1952, engineers acting for the government of British Uganda blasted out the weir and reservoir to replace it with an artificial barrage to control the level of the lake and reduce the gradual erosion of the rock weir. A standard for mimicking the old rate of outflow called the "agreed curve" was established, setting the maximum flow rate at 300 to 1,700 cubic metres per second depending on the lake's water level.In 2002, Uganda completed a second hydroelectric complex in the area, the Kiira Hydroelectric Power Station, with World Bank assistance. By 2006, the water levels in Lake Victoria had reached an 80-year low, and Daniel Kull, an independent hydrologist living in Nairobi, Kenya, calculated that Uganda was releasing about twice as much water as is allowed under the agreement, and was primarily responsible for recent drops in the lake's level.