Lake Tanganyika's surface waters are currently warmer than at any time in the previous 1,500 years, a University of Arizona researcher and his colleagues report online in Nature Geoscience.
The rise in temperature during the 20th century is driving a decline in the productivity of the lake, which hosts the second-largest inland fishery in Africa.
"People throughout south-central Africa depend on the fish from Lake Tanganyika as a crucial source of protein," said study co-author Andrew S. Cohen, a UA professor of geosciences. "This resource is likely threatened by the lake's unprecedented warming since the late 19th century and the associated loss of lake productivity."
This is the first detailed record of temperature and its impacts on a tropical African ecosystem that allows scientists to compare the last 100 years with the previous 1,400 years, Cohen said.
The team attributes the lake's increased temperature and the decreased productivity during the 20th century to human-caused global warming.
"We've got a global phenomenon driving something local that has a huge potential impact on the people that live in the region and on the animals that live in the lake," he said.
The annual catch of the Lake Tanganyika fishery is estimated at about 198,000 tons per year, more than 20 times greater than the U.S. commercial fishery in the Great Lakes, he said. The nations of Burundi, Tanzania, Zambia and the Democratic Republic of Congo border the lake, which is the longest lake in the world and the second deepest.
The surface waters of Lake Tanganyika are the most biologically productive part of the lake. For the 1,400 years before 1900, those waters were no warmer than 75.7 F (24.3 degrees C). Since 1900, the lake's surface waters warmed 3 degrees F, reaching 78.8 degrees F (26 degrees C) in 2003, the date of the researchers' last measurement.
The researchers used sediment cores from the lake bed to reconstruct the 1,500-year history of the lake. The scientists analyzed the cores for chemicals produced by microbes and left in the sediments to determine the lake's past temperature and productivity.
Because sediment is deposited in the lake in annual layers, the cores provide a detailed record of Lake Tanganyika's past temperatures and productivity and of the regional wildfires.
The instrument record of lake temperatures from the 20th century agrees with the temperature analyses from the cores, Cohen said.
The cores were extracted as part of the UA's Nyanza Project, a research training program that brought together U.S. and African scientists and students to study tropical lakes. The National Science Foundation funded the project.
"A big part of our mandate for the Nyanza Project was looking at the interconnectivity between climate, human activity, resources and biodiversity," said Cohen, who directed the multi-year project.
Lake Tanganyika and similar tropical lakes are divided into two general levels. Most of the fish and other organisms live in the upper 300 feet (about 100 meters). At depths below that, the lake waters contain less and less oxygen. Below approximately 600 feet, the lake water, although nutrient-rich, has no oxygen and fish cannot live there.
During the region's windy season, the winds make the lake's surface waters slosh back and forth, mixing some of the deep water with the upper layers. This annual mixing resupplies the lake's food web with nutrients and drives the lake's productivity cycle, Cohen said.
However, as Lake Tanganyika warms, the upper waters of the lake become less dense. Therefore, stronger winds are required to churn the lake waters enough to mix the deeper waters with the upper layer. As a result, the upper layers of the lake are becoming increasingly nutrient-poor, reducing the lake's productivity.
In addition, warmer water contains less dissolved oxygen, reducing the quality of the habitat for some fish species.
Other lakes in Africa are showing similar effects to those the team found in Lake Tanganyika, he said.
The finding has implications for lakes in more temperate climates.
"Increasingly, lakes in the U.S. are warming and they're behaving more like these African lakes," Cohen said. "There's a potential for learning a lot about where we're going by seeing where those lakes already are."
The team's article, "Late twentieth-century warming in Lake Tanganyika unprecedented since AD 500," will be published in the June issue of Nature Geoscience. Cohen's co-authors on the paper are first author Jessica E. Tierney of Brown University in Providence, R.I.; Marc T. Mayes, Natacha Meyer and James M. Russell, also at Brown University; Christopher Johnson, a former University of Arizona student now at the University of California, Los Angeles; and Peter W. Swarzenski of the U.S. Geological Survey in Santa Cruz, Calif. The National Science Foundation funded the research.
Andrew S. Cohen
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Andrew S. Cohen
UA Department of Geosciences