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 wi th data collected in 1960-1961. Oxygen concentrations in the mixed lay er are higher now, with nearly continuous oxygen supersaturation in su rface waters. Oxygen concentrations in hypolimnetic waters are lower n ow 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 h igher algal biomass and productivity. The causes for the eutrophicatio n of Lake Victoria are as yet undetermined, although higher nutrient l oading, altered climate, and food-web changes are all potentially invo lved. The persistent and areally extensive deoxygenation of the hypoli mnion must negatively affect the formerly productive and species-rich demersal fish community.