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.

Summary Blue‐green algae dominated the open waters of Lake Victoria during an algal bloom between February and August 1986. Microcystis sp. accounted for more than 90% of the bloom organisms with algal counts reaching a high density of 34,000 colonies per ml and chlorophyll a concentration at 77‐6 μg 1‐1. Secchi disc visibilities and the 1 % light penetration were reduced to 0–2 m and 1 m, respectively. The release of nutrients from river inflows, from upwelling and from sediments into the euphotic zone, coupled with high temperatures, produced the observed blooms. The blooms subsequently declined as a result of physical flushing, temperature reduction associated with the rainy season and nutrient exhaustion. Résumé Les algues bleues et vertes ont prédominé dans les eaux libres du Lac Victoria durant une éclosion massive d'algues, de février à août 1986. Microcystis représentait plus de 90% des organismes éclos, avec des chiffres atteignant plus de 34,000 colonies par ml et une concentration en chlorophylle de 77‐6 mug 1‐1. La visibilité du disque de Secchi et la pénétration d'un pour cent de la lumière étaient réduits à 0–2 m et 1 m respectivement. l'apport de nutriments par les rivières, par les sources et par les sédiments vers la zone exposée à la lumière, combiné avec de hautes températures a causé le phénomène observé. Cette explosion diminua ensuite à cause de phénomènes physiques: une réduction de température associée à la saison des pluies et à l'épuisement des nutriments.

Lake Victoria until recently harbored the world’s second richest lacustrine fish assemblage, but it is now experiencing a mass extinction. Here we report on the current status of the endemic cichlid fishes from a preliminary biotic inventory conducted as part of limnological reconnaissance in Kenyan and Ugandan waters during 1989–1992. Sixty‐two haplochromine taxa were recovered, half of them new. Some are local endemics, while others are more widely distributed, based on review of earlier collections. Comparison with data from 1982–1984 and 1988 suggests that some shoreline assemblages have been locally stable over the past decade. In addition to littoral rock and vegetation, four kinds of refugia for indigenous fishes are recognized: (1) schools of Rastrineobola, (2) benthic microbial mats in deep water, (3) satellite lakes, (4) the oxycline. Reconnaissance by remote operated vehicle (ROV) documented fish kills due to deoxygenation of the water column, indicating the risk of using the oxycline as a refugium. Several species formerly abundant in Lake Victoria were found only in satellite lakes, revealing the importance of these lakes in the conservation of indigenous species. Recent events in Lake Victoria shed light on mechanisms of speciation and mass extinction, insights relevant to conservation planning for the valuable remnants of the lake’s indigenous fauna.

The morphometry and hydrology of the Nyanza Gulf of Lake Victoria can be greatly affected by violent storms. This can result in a condition in which nutrient-rich bottom mud is mixed with the sediment-laden runoff water from neighbouring marshes and rivers. This situation can lead to massive fish kills. It is such an event in 1984 which will be described that caused the sudden death of over 400 000 fish weighing over 2400 tonnes.The unusually low precipitation in the catchment area during that year resulted in a drop in the lake level from 12.4 to 11.8 m. This low water level, combined with a severe storm created the conditions which caused this large fish kill. The species affected were Lates niloticus and Oreochromis niloticus. Mortality was attributed to several factors which included high levels of suspended material in the water column (detritus and algae) which clogged the gills of the fish, low dissolved oxygen, low pH, and high concentrations of algae.

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