Changes in Oxygen Levels in Lake Victoria1.2S 32.7E
This thumbnail image focuses on the jagged shoreline of the southern half of Lake Victoria, although the entire body of water can be observed upon opening the full image. The lake occupies a shallow depression in Africa and has a maximum depth of 84 metres (276 ft) and an average depth of 40 metres (130 ft). Its catchment area covers 184,000 square kilometers (71,040 sq mi). The lake has a shoreline of 4,828 kilometres (3,000 mi).
Lake Victoria receives almost all (80%) of its water from direct precipitation. Average evaporation on the lake is between 2–2.2 metres (6 ft 7 in–7 ft 3 in) per annum, almost double the precipitation of riparian areas.
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. 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.