Hydrology and Limnology
Lake Victoria receives almost all (80%) of its water from direct precipitation. 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. 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. 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. 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.
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% 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. 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.
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