DEOXYGENATION OF THE DEEP-WATER OF LAKE VICTORIA, EAST-AFRICA

The annual cycle of the vertical distribution of oxygen concentrations at a deep water station on Lake Victoria in 1990-1991 was compared with data collected in 1960-1961. Oxygen concentrations in the mixed layer are higher now, with nearly continuous oxygen supersaturation in surface waters. Oxygen concentrations in hypolimnetic waters are lower now for a longer period, with values < 1 mg liter-1 occurring in water as shallow as 40 m compared with a shallowest occurrence of > 50 m in 1961. The changes in oxygenation are consistent with measurements of higher algal biomass and productivity. The causes for the eutrophication of Lake Victoria are as yet undetermined, although higher nutrient loading, altered climate, and food-web changes are all potentially involved. The persistent and areally extensive deoxygenation of the hypolimnion must negatively affect the formerly productive and species-rich demersal fish community.