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African Lake¿s Fish Yields Plummet as Global Temperatures Rise

Lake Tanganyika



ANDREW COHEN, UNIVERSITY OF ARIZONA AT TUCSON/ NSF
A lake in East Africa that houses 18 percent of the world's freshwater is suffering as the world gets warmer, scientists say. The productivity of Lake Tanganyika, which borders the Democratic Republic of Congo, Tanzania, Zambia and Burundi, has dropped 20 percent since the 1950s, resulting in a 30 percent decrease in fish yields. According to a report published today in the journal Nature, the findings indicate that the impact of regional effects of global climate change can be larger than that of local anthropogenic activity or overfishing.

As the second largest lake, Lake Tanganyika is one of the world's great freshwater ecosystems and provides 25 to 40 percent of the annual animal protein supply to the countries bordering it. Catherine M. O'Reilly of Vassar College and her colleagues used local records, sediment cores and water measurements to track the water body's history over the past 80 years. They found that the air temperature in the region increased by about 0.6 degree Celsius, whereas the deep waters of the lake warmed by 0.31 degree. At the same time, the scientists determined that wind velocities in the region have decreased by 30 percent since the 1970s. Together, these factors reduced the rate at which organic matter is created, thus lowering the number of fish that the lake can sustain. "Our research provides the strongest link to date between long-term changes in lake warming in the tropics, recorded by instruments, and declining productivity of the lake's ecosystem, as seen in sediment cores," says study co-author Andrew Cohen of the University of Arizona. "This work provides a clear indication of the regional effects of global climate change, and especially global warming, on tropical lake ecosystems."

Modeling forecasts for the area predict additional temperature increases of around 1.5 degrees C within the next 80 years, which could have dire consequences for the inhabitants of the region who depend on the lake for food, the researchers note. "To date, most studies have found significant effects of climate change in the northern hemisphere," O'Reilly observes, "while our study indicates that substantial warming is also occurring in the tropics, and that it is having a negative impact on some ecosystems."

"The Killing Lakes," by Marguerite Holloway (Scientific American, July 2000), is available for purchase from Scientific American Digital.
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