Tuesday, August 27, 2013

Lake Victoria

English: Source of the Nile launches (Victoria...
 
Lake Victoria (Nam Lolwe in Luo; Victoria Nyanza in Bantu[1] ) is one of the African Great Lakes. The lake was named after Queen Victoria, by John Hanning Speke, an officer in the British Indian Army.
With a surface area of 68,800 square kilometres (26,600 sq mi), Lake Victoria is Africa’s largest lake by area, and it is the largest tropical lake in the world. Lake Victoria is the world's 2nd largest freshwater lake by surface area; only Lake Superior in North America is larger. In terms of its volume, Lake Victoria is the world's ninth largest continental lake, and it contains about 2,750 cubic kilometers (2.2 billion acre-feet) of water.
 
Lake Victoria receives its water primarily from direct precipitation and thousands of small streams. The largest stream flowing into this lake is the Kagera River, the mouth of which lies on the lake's western shore. Two rivers leave the lake, the White Nile (known as the "Victoria Nile" as it leaves the lake), flows out at Jinja, Uganda on the lake's north shore, and the Katonga River flows out at Lukaya on the western shore connecting the lake to Lake George.[2]
 
Lake Victoria occupies a shallow depression in Africa and has a maximum depth of 84 m (276 ft) and an average depth of 40 m (130 ft).[3] Its catchment area covers 184,000 square kilometers (71,040 sq mi). The lake has a shoreline of 4,828 km (3,000 mi), with islands constituting 3.7% of this length,[4] and is divided among three countries: Kenya (6% or 4,100 km2 or 1,600 sq mi), Uganda (45% or 31,000 km2 or 12,000 sq mi) and Tanzania (49% or 33,700 km2 or 13,000 sq mi)
Lake Victoria has, during its geological history, gone through changes ranging from its present shallow depression, through to what may have been a series of much smaller lakes.[4] Geological cores taken from its bottom show Lake Victoria has dried up completely at least three times since it formed.[6] These drying cycles are probably related to past ice ages, which were times when precipitation declined globally.[6] Lake Victoria last dried out 17,300 years ago, and it refilled beginning about 14,700 years ago. Geologically, Lake Victoria is relatively young – about 400,000 years old – and it formed when westward-flowing rivers were dammed by an upthrown crustal block.[6]
 
This geological history probably contributed to the dramatic cichlid speciation that characterises its ecology, as well as that of other African Great Lakes,[7] although some researchers dispute this, arguing while Lake Victoria was at its lowest between 18,000 and 14,000 years ago, and it dried out at least once during that time, there is no evidence of remnant ponds or marshes persisting within the desiccated basin. If such features existed, then they would have been small, shallow, turbid, and/or saline, and therefore markedly different from the lake to which today's species are adapted.[8]
The shallowness of Lake Victoria, its limited river inflow, and its large surface area compared to its volume make it vulnerable to the effects of climate changes.

Hydrology and limnology

Lake Victoria receives almost all (80%) of its water from direct precipitation.[4] Average evaporation on the lake is between 2.0 and 2.2 metres (6.6 and 7.2 ft) per year, almost double the precipitation of riparian areas.[9] In the Kenya Sector, the main influent rivers are the Sio, Nzoia, Yala, Nyando, Sondu Miriu, Mogusi and the Migori. Combined, these rivers contribute far more water to the lake than does the largest single inflowing river, the Kagera, which enters the lake from the west.[10] The lake outflows into the White Nile and the Katonga River, both part of the upper Nile river system.
 
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 (i.e. the layer of water that lies below the thermocline, is noncirculating, and remains perpetually cold) were lower in 1990–1991 for a longer period than in 1960–1961, with values of less than 1 mg per litre (< 0.4 gr/cu ft) occurring in water as shallow as 40 metres (130 ft) compared with a shallowest occurrence of greater than 50 metres (160 ft) in 1961. The changes in oxygenation are considered consistent with measurements of higher algal biomass and productivity.[11] These changes have arisen for multiple reasons: successive burning within its basin,[12] soot and ash from which has been deposited over the lake's wide area; from increased nutrient inflows via rivers,[13] and from increased pollution associated with settlement along its shores.
 
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.[4][14] 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.[14] There is little doubt that Haplochromis played an important role in returning detritus and plankton back into solution.[15][16][17] With some 80% 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 and humans.[15] The removal of Haplochromis, however, may have contributed to the increasing frequency of algal blooms,[13][16][17] which may in turn be responsible for mass fish kills.[13].[5]
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