Global warming is choking the life out of Lake Tanganyika

Lake Tanganyika in central Africa - where Henry Stanley delivered his immortal question, "Dr Livingstone, I presume?" - is in ecological crisis as a result of global warming.

Studies by two independent teams of scientists have found local temperature rises and climate change have dramatically altered the delicate nutrient balance of the lake, Africa's second largest body of fresh water.

They have discovered that the surface of the lake is getting warmer and that has meant the mixing of vital nutrients in the lake has diminished and cut the lake's fish population.

The effect has had a dramatic impact on the local economy, with fishing yields plummeting by a third or more over the past 30 years and further decreases predicted.

Lake Tanganyika has traditionally supplied between 25 and 40 per cent of the protein needs of the local people, citizens of the four countries bordering the lake, Burundi, Tanzania, Zambia and the Democratic Republic of Congo.

As a tropical lake accustomed to high year-round temperatures, Tanganyika was not obviously vulnerable to the effects of global warming yet this is what the scientists have discovered.

All deep freshwater lakes rely on nutrients in the lower depths periodically coming to the surface where aquatic plants and algae live. This is particularly critical in tropical lakes which have steep temperature gradients that tend to keep the warm, less dense layers on top of the colder, denser water in the lake's depths where the nutrients are stored.

Lake Tanganikya is the second-deepest lake in the world and the second richest in terms of biological diversity; it has 350 species of fish with new ones being discovered regularly. Nutrient mixing has been vital for its biodiversity.

Piet Verburg, of the University of Waterloo, in Canada, and Catherine O'Reilly of the University of Arizona, in Tucson, who led the studies, found warmer temperatures and less windy weather in the region is starving the lake's life of essential salts that contain nitrogen and sulphur.

Dr O'Reilly's study, in the journal Nature, suggests the lake's productivity, measured by the amount of photosynthesis its plant life has done, has diminished by 20 per cent, which could easily account for the 30 per decrease in fish yields.

The scientists say climate change rather than overfishing is largely responsible for the collapse in Tanganyika's fish stocks and the position is likely to get much worse.

"The human implications of such subtle, but progressive, environmental changes are potentially dire in this densely populated region of the world, where large lakes are essential natural resources for regional economies," the scientists say. Dirk Verschuren, a freshwater biologist at Ghent University in Belgium, said both studies could explain why sardine fishing has declined by between 30 and 50 per cent since the late 1970s.

"Since overexploitation is at most a local problem on some fishing grounds, the principal cause of this decline has remained unknown," Dr Verschuren writes in an accompanying Nature article. "Taken together ... the data in the two papers provide strong evidence that the effect of global climate change on regional temperature has had a greater impact on Lake Tanganyika than have local human activities. Their combined evidence covers all the important links in the chain of cause and effect between climate warming and the declining fishery."