United Press International - August 21, 2003
Lidia Wasowicz, UPI Senior Science Writer

Researchers reading through 200 years of history, diligently recorded in undisturbed layers of sediment at the bottom of Lake Tanganyika, have detected a direct link between global warming and demise of organisms in the second deepest, largest and most biologically diverse fresh body of water on Earth.

Their studies revealed increasing temperatures fueled by the emission of so-called greenhouse gases and sinking algae and fish stocks over the past half-century or so.

The 30-percent decline in fish catches estimated over that period carries immense economic, environmental and ecological implications, given Lake Tanganyika holds 18 percent of the world's liquid fresh water and is a critical food source for the surrounding countries of Burundi, Tanzania, Zambia and the Democratic Republic of Congo.

"The changes in productivity over the past 50 to 80 years appear to be something completely new to this lake," said lead author Catherine O'Reilly, who conducted the research at the University of Arizona in Tucson but since has transferred to Vassar College in Poughkeepsie, N.Y.

She and colleagues blame climate warming of between 0.5 and 0.8 degrees Celsius, coupled with reduced wind speeds, for the drop in fish catches. The absence of gales strong enough to whip up primary nutrients from the depths of the one-mile-deep, 400-mile-long lake left the top predators on the food chain high and dry, the international team proposes.

Mud-packed records support the theory, pointing to a 20-percent drop in base food sources since the 1950s, and a 30-percent decline in the fish that eat them. Thus an environmental disaster has befallen a population already beset by pestilence, civil unrest, famine and a growing AIDS epidemic in one of the poorest areas of the world.

"There are already serious food shortages in the region as a result of wars in the Congo and Burundi and AIDS," said Andrew Cohen, a paleolimnologist -- a scientist who studies ancient lakes -- at the University of Arizona in Tucson. She has been examining Lake Tanganyika for 20 years.

"A significant long-term decline in this (fish) resource, coming on the heels of all the civil unrest ... could be catastrophic."

Evidence points to a similar state of affairs in Malawi, Albert and other East Africa Rift Valley lakes.

"(The result will be) less food, higher fish prices (and) fewer jobs fishing," said Cohen, co-author of the study published in the British journal Nature. "Currently 45,000 people are employed by fishing around the lake directly and millions depend on this indirectly."

Second only in depth and volume to Siberia's Lake Baikal -- which holds 20 percent of Earth's liquid fresh water -- Tanganyika boasts a rich biological diversity that spans endemic fish, snails and crustaceans. But the lake's once-opulent productivity is ebbing.

"The main significance of these findings is that this very large tropical lake ecosystem, which by its size and depth is lucky to be somewhat buffered from the negative effects of human activities in its drainage basin (deforestation and agriculture, soil erosion, etc.), evidently cannot escape the pervasive reach of human impact through man's influence on global climate," said biologist Dirk Verschuren of Ghent University in Ghent, Belgium.

The findings seem to explain the observed reduction in fish catches in the area, attributed to unknown environmental factors rather than overfishing. Since the late 1970s, the annual catch of sardines -- the dominant commercial fish -- has plummeted 30 percent to 50 percent, to 165,000 to 200,000 tons. Larger lake fish have decreased even more, from composing up to 60 percent of the catch to less than 2 percent.

Of the more than 700 species inhabiting the 10-million-year-old lake, six are caught commercially, including two sardine varieties and four larger predators.

The fish bring in tens of millions of dollars in trade, provide up to 40 percent of the animal protein consumed by area residents and serve as a major source of nutrition for the surrounding human population.

"It is one of the most interesting lakes in the world, both because of its age, its huge number of species found nowhere else in the world (evolved in the lake) and its potential for giving us very clear records of climate and environmental change in central Africa," Cohen told UPI.

U.S. planners and policymakers would be well-served to pay close attention to Lake Tanganyika, he added.

"All our models used to manage lakes and their resources have come out of temperate zones, but it is important for the U.S. society to understand tropical lakes as we enter an era when the climate is warming," Cohen said in a telephone interview.

"They are in effect bellwethers of things to come over the next 50 years in terms of our own freshwater resources, as our own lakes (especially in the southeastern states) behave more and more like tropical systems."

Though most U.S. commercial fishing occurs in marine waters, lakes provide recreation, fresh water supplies and spots for some of America's 34 million sports fishermen to cast their lines.

Lakes serve as particularly valuable tools for forecasters because, unlike river or sea beds which scour away sediment over the years, they accumulate a continuous, undisturbed record of their history.

"A lake is like a tape recorder that captures even subtle changes over decades, centuries, millennia," Cohen said. "What this record shows us is that temperature increases clearly can make lakes less productive."

The researchers measured and compared past records of lake and air temperatures and wind velocities, factors that help determine how well water circulates. A good mixing distributes nutrients, which 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 in Washington, which helped fund the study.

The team detected a heating trend, starting in 1900 -- apparently fueled by increased emission of the greenhouse gas carbon dioxide, which has been implicated in global warming.

Average annual temperatures in the 1950s hovered at 23.7 degrees C. at the north end of the lake, compared to the current 24.2 degrees C., O'Reilly said. The greatest air temperature change has occurred since the 1970s, she added.

Climatologists predict air temperatures will rise 1.3 to 1.7 degrees C. in the East African region over the next 80 years.

"Right now the most alarming thing is what is likely to happen," O'Reilly cautioned. "If the relatively small temperature increase over the past 50 years is responsible for up to a 30-percent decrease in fish stocks, future reductions in fish yields could be dramatically larger."

That bodes ill for the economy of an area where a fisherman's income can be twice that of his peers.

"This study indicates that global warming can have ... consequences other than just higher temperature," O'Reilly said. "Governments should be thinking very carefully about the consequences of continued warming, and that international cooperation on reducing the inputs of greenhouse gases could be very helpful in reducing some of these consequences."

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