African Lake Loses Third of Its Fish to Climate Warming

ARLINGTON, Virginia, August 14, 2003 (ENS) - An African lake that holds 18 percent of the world's liquid freshwater is suffering from the Earth's warming climate, according to new research released today. Climate change in the region is harming Lake Tanganyika's ecosystem, reducing fish populations by one-third over the past 80 years, an international team of scientists, whose study was supported by the U.S. National Science Foundation, report in the latest issue of the journal "Nature."

Lake Tanganyika is large and deep, filling the chasm of a rift valley bordering the Democratic Republic of Congo, Tanzania, Zambia and Burundi. The lake supports many types of fish. Only a few species are eaten by people, yet they supply 25 to 40 percent of the animal protein for the communities of that region.

Scientists Catherine O'Reilly of Vassar College, Andrew Cohen of the University of Arizona, Simone Alin of the University of Washington, Pierre-Denis Plisnier of the Royal Museum for Central Africa in Belgium, and Brent McKee of Tulane University in Louisiana found "multiple lines of evidence" directly linking climatic warming with the survival of organisms in Lake Tanganyika.

"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," said Cohen. "This work provides a clear indication of the regional effects of global climate change, and especially global warming, on tropical lake ecosystems."

Lake Tanganyika coastline at Gombe Stream National Park. (Photo courtesy Andrew Cohen, University of Arizona at Tucson, National Science Foundation)

Temperatures have increased 0.6 degrees Celsius in the air above the lake, with a proportional increase in the water temperature, while wind velocities have decreased, the scientists found.

Those temperature changes stabilize the water column in lakes, especially in the tropics where, unlike in temperate regions, winter cooling and mixing is absent.

The increased stability decreased circulation, hampering the re-supply of nutrients from the deep water to the surface waters of the lake where they help algae grow. The algae, which form the base of Lake Tanganyika's food chain, ultimately feed the commercially important fish.

"The fisheries of Lake Tanganyika currently yield approximately 200,000 tons of fish per year, and are far and away the most important source of animal protein for human consumption in this region of Central Africa," said Cohen. Given the already significant problems of malnutrition and civil conflict in central Africa, a significant decline in fishing yields resulting from climate change could lead to extremely serious consequences for the region's food supply," he said.

Future predictions for this region indicate a roughly 1.5 degree Celsius rise in air temperature, said O'Reilly, who preducts the warming will further stabilize the lake and reduce mixing, with potentially devastating effects on fish stocks.

"Continued climate warming has some severe implications for the nutrition and economy of the region's people, who depend heavily on the lake as a natural resource," said O'Reilly.

Fishing boats on Burundi Lake Tanganyika Beach (Photo courtesy Andrew Cohen, University of Arizona at Tucson, National Science Foundation)

The researchers also analyzed organic matter from well dated lake sediment cores and found clues that life in the Lake Tanganyika ecosystem has been on the decline.

They found that algae abundance declined 20 percent over the 80 year period for which data exists. The decline is a direct result of the reduction in lake circulation. Based on earlier studies of other lakes, that decline would lead to a 30 percent reduction in fish stocks, in addition to an possible effects of overfishing, researchers said.

"This is an important study that demonstrates the dramatic response of a lake ecosystem to changes in climatic and environmental conditions over a relatively short period of time," said Jarvis Moyers, director of the Division of Atmospheric Sciences at the National Science Foundation.

The National Science Foundation (NSF) supported this research through the Nyanza Project, an interdisciplinary research training program for undergraduate and graduate students and secondary school teachers based at the University of Arizona at Tucson.

This project, part of the NSF Research Experiences for Undergraduates program, supported O'Reilly and Alin, who were graduate students when they conducted the field and lab work. O'Reilly is now visiting assistant professor of environmental science at Vassar College and a faculty member of the Nyanza Project.

The project also supported undergraduates who collected lake water temperature data. Major funding for the broader lake research came from the United Nations Global Environmental Fund's Lake Tanganyika Biodiversity Project.