Global warming is reducing thermal stability and mitigating the effects of eutrophication in Lake Victoria (East Africa)

Abstract

The impacts of global warming have been reported from several deep lakes in the African Rift Valley and in each thermal gradients within the water column have increased thus strengthening already existing oxyclines, below which the water is permanently anoxic^1,2,3,4^. The temperature of Lake Victoria rose by 0.9°C between 1960 and 1990 raising fears that thermal stability would increase resulting in more extensive and severe anoxia in the deeper waters^5^. This is of concern because of the eutrophication of the lake, which began in the 1960s,^6,7^ and led to dense blooms of sometimes toxic cyanobacteria, increased deoxygenation of the bottom waters, and fish kills in some parts of the lake^8,9,10^. Here we show that thermal gradients in the water column have weakened over the last decade and that deoxygenation of deeper waters is less pronounced than expected. Since 1927 the temperature of the deeper waters has risen by 1.3°C compared to only 1.0°C in the surface layers, thereby decreasing thermal and density differentials in the water column. This contradicts the view that eutrophication would increase deoxygenation of the water column perhaps to the point where fish production could not be sustained^11^. Our results suggest that the impacts of global warming on tropical lakes are likely to highly variable and may not, in the short term at least, be uniformly detrimental.