ARLINGTON, Va.—In an important new study directly linking climatic warming with the survival of lake organisms, researchers have found multiple lines of evidence showing that increasing air and water temperatures and related factors are shrinking fish and algae populations in a major lake. The lake holds 18 percent of the world's liquid freshwater and is a critical food source in East Africa.
Reporting in the August 14, 2003, issue of the journal Nature, 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, announce that climate change in the region is harming Lake Tanganyika's ecosystem, decreasing fish stocks by as much as 30 percent over the past 80 years.
Lake Tanganyika is large and deep, filling the chasm of a rift valley bordering the Democratic Republic of Congo, Tanzania, Zambia and Burundi. An ecosystem unto itself, 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. Recently, the fish supplies have diminished, and catches are shrinking.
"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."
The researchers measured lake water temperatures, along with air temperatures and wind velocities, and compared data to equivalent records from the past eight decades. Those factors help determine how well water circulates within the lake, a critical factor for the distribution of nutrients that support life in the lake's food chain.
"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 (NSF), the independent federal agency that supports fundamental research and education across all fields of science and engineering and a sponsor of this research.
Ultimately, O'Reilly and her colleagues found that 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.
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.
Future predictions for this region indicate a roughly 1.5 degree Celsius rise in air temperature, said O'Reilly—further stabilizing the lake and reducing 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. "To date, most studies have found significant effects of climate change in the northern hemisphere," she added, "while our study indicates that substantial warming is also occurring in the tropics, and that it is having a negative impact on some ecosystems."
In addition to finding evidence of warming in lake water temperatures and decreased windiness from instrument records, the researchers analyzed organic matter from well-dated lake sediment cores and found clues that life in the ecosystem has been on the decline.
Information from the sediments indicated that algae abundance declined 20 percent over the 80-year period for which data exists. The researchers believe 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 any possible effects of over-fishing.
"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 added.
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.
-NSF-
The National Science Foundation (NSF) is an independent federal agency that supports fundamental research and education across all fields of science and engineering, with an annual budget of nearly $5.3 billion. NSF funds reach all 50 states through grants to nearly 2,000 universities and institutions. Each year, NSF receives about 30,000 competitive requests for funding, and makes about 10,000 new funding awards. The NSF also awards over $200 million in professional and service contracts yearly.
Nature 424, 766 - 768 (14 August 2003); doi:10.1038/nature01833
Climate change decreases aquatic ecosystem productivity of Lake Tanganyika, Africa
CATHERINE M. O'REILLY1,*, SIMONE R. ALIN1,*, PIERRE-DENIS PLISNIER2, ANDREW S. COHEN1 & BRENT A. MCKEE3 http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v424/n6950/abs/nature01833_r.html
1 Department of Geosciences, University of Arizona, Tucson Arizona 85721, USA
2 Royal Museum for Central Africa, 3080 Tervuren, and Department of Biology, Namur University, B-5000 Namur, Belgium
3 Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA
* Present addresses: Environmental Science Program, Vassar College, Poughkeepsie, New York 12603, USA (C.M.O.); Large Lakes Observatory, University of Minnesota, Duluth, Minnesota 55812, USA (S.R.A.)
Although the effects of climate warming on the chemical and physical properties of lakes have been documented, biotic and ecosystem-scale responses to climate change have been only estimated or predicted by manipulations and models. Here we present evidence that climate warming is diminishing productivity in Lake Tanganyika, East Africa. This lake has historically supported a highly productive pelagic fishery that currently provides 25–40% of the animal protein supply for the populations of the surrounding countries. In parallel with regional warming patterns since the beginning of the twentieth century, a rise in surface-water temperature has increased the stability of the water column. A regional decrease in wind velocity has contributed to reduced mixing, decreasing deep-water nutrient upwelling and entrainment into surface waters. Carbon isotope records in sediment cores suggest that primary productivity may have decreased by about 20%, implying a roughly 30% decrease in fish yields. Our study provides evidence that the impact of regional effects of global climate change on aquatic ecosystem functions and services can be larger than that of local anthropogenic activity or overfishing.
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.
This NASA satellite file image shows Lake Tanganyika in East Africa. Global warming is wrecking Africa's Lake Tanganyika, inflicting a catastrophic decline in fish catches, a study says. (AFP-NASA/File)
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."
