Unprecedented Warming in Lake Tanganyika

Well here we go again, you know the drill. Global warming at fault, other possibilities ignored, multiple press releases. Lake Tanganyika is the second largest lake in the world for fresh water, so naturally any change it is cause for “alarm”. Unfortunately in these press releases there is no mention of a possible increase in turbidity due to human action on and around the lake, decreasing the albedo to absorb more sunlight on the lake surface, warming it. At least somebody has already asked that question previously in peer reviewed literature where they describe the Lake Tanganyika problem as “watershed deforestation, road building, and other anthropogenic activities result in sediment inundation…“.

But in our current press releases, there is this hat tip to anthropogenic: “The team attributes the lake’s increased temperature and the decreased productivity during the 20th century to human-caused global warming.”

First from Brown University:

Brown Geologists Show Unprecedented Warming in Lake Tanganyika

PROVIDENCE, R.I. [Brown University] — Lake Tanganyika, the second oldest and the second-deepest lake in the world, could be in for some rough waters.

Geologists led by Brown University have determined the east African rift lake has experienced unprecedented warming during the last century, and its surface waters are the warmest on record. That finding is important, the scientists write in the journal Nature Geoscience, because the warm surface waters likely will affect fish stocks upon which millions of people in the region depend.

The team took core samples from the lakebed that laid out a 1,500-year history of the lake’s surface temperature. The data showed the lake’s surface temperature, 26 degrees Celsius (78.8°F), last measured in 2003, is the warmest the lake has been for a millennium and a half. The team also documented that Lake Tanganyika experienced its biggest temperature change in the 20th century, which has affected its unique ecosystem that relies upon the natural conveyance of nutrients from the depths to jumpstart the food chain upon which the fish survive.

“Our data show a consistent relationship between lake surface temperature and productivity (such as fish stocks),” said Jessica Tierney, a Brown graduate student who this spring earned her Ph.D. and is the paper’s lead author. “As the lake gets warmer, we expect productivity to decline, and we expect that it will affect the [fishing] industry.”

The research grew out of two coring expeditions sponsored by the Nyanza Project in 2001 and 2004. Cores were taken by Andrew Cohen, professor of geological sciences at the University of Arizona and director of the Nyanza project, and James Russell, professor of geological sciences at Brown, who is also Tierney’s adviser.

Lake Tanganyika is bordered by Burundi, the Democratic Republic of Congo, Tanzania, and Zambia — four of the poorest countries in the world, according to the United Nations Human Development Index. An estimated 10 million people live near the lake, and they depend upon it for drinking water and for food. Fishing is a crucial component for the region’s diet and livelihood: Up to 200,000 tons of sardines and four other fish species are harvested annually from Lake Tanganyika, a haul that makes up a significant portion of local residents’ diets, according to a 2001 report by the Lake Tanganyika Biodiversity Project.

Lake Tanganyika, one of the richest freshwater ecosystems in the world, is divided into two general levels. Most of the animal species live in the upper 100 meters, including the valuable sardines. Below that, the lake holds less and less oxygen, and at certain depths, it is anoxic, meaning it has no oxygen at all. What this all means is the lake is highly stratified and depends on wind to churn the waters and send nutrients from the depths toward the surface as food for algae, which supports the entire food web of the lake. But as Lake Tanganyika warms, the mixing of waters is lessened, the scientists find, meaning less nutrients are funneled from the depths toward the surface. Worse, more warming at the surface magnifies the difference in density between the two levels; even more wind is needed to churn the waters enough to ferry the nutrients toward the fish-dwelling upper layer.

The researchers’ data show that during the last 1,500 years, intervals of prolonged warming and cooling are linked with low and high algal productivity, respectively, indicating a clear link between past temperature changes and biological productivity in the lake.

“The people throughout southcentral Africa depend on the fish from Lake Tanganyika as a crucial source of protein,” noted Cohen, an author on the paper. “This resource is likely threatened by the lake’s unprecedented warming since the late 19th century and the associated loss of lake productivity.”

Climate change models show a general warming in the region, which, if accurate, would cause even greater warming of the Lake Tanganyika’s surface waters and more stratification in the lake as a whole. “So, as you move forward, you can imagine that density gradient increasing,” said Russell, an author on the paper.

Some researchers have posited that the declining fish stocks in Lake Tanganyika can be attributed mainly to overfishing, and Tierney and Russell say that may be a reason. But they note that the warming in the lake, and the lessened mixing of critical nutrients is exacerbating the stocks’ decline, if not causing it in the first place. “It’s almost impossible for it not to,” Russell said.

Other authors on the paper are Brown graduates Marc Mayes and Natacha Meyer; Christopher Johnson at the University of California, Los Angeles; and Peter Swarzenski, with the United States Geological Survey. The National Science Foundation and the Nyanza Project funded the research.

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Here is the University of Arizona version

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Twentieth-Century Warming in Lake Tanganyika is Unprecedented

An artisanal fisher sails on Lake Tanganyika. (Photo credit: Andrew S. Cohen)

UA geosciences professor Andrew S. Cohen (in the pink shirt) and students in the UA’s Nyanza Project look at a sediment core from the bottom of Lake Tanganyika, the world’s second deepest lake. (Photo credit: Laura Wetter)

Warming in the last century threatens one of Africa’s largest inland fisheries.

By Mari N. Jensen, College of Science, May 17, 2010

Lake Tanganyika’s surface waters are 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.