Warming trend affects African ecosystem. (Slow Turnover).

Over the past 90 years, rising water temperatures in Lake Tanganyika have dramatically reduced populations of the aquatic microorganisms at the base of the lake's food chain, a new analysis shows.

More than 650 kilometers long and up to 50 km wide, Lake Tanganyika is by volume the world's second-largest body of fresh water, surpassed only by Russia's Lake Baikal. Lake Tanganyika winds through southeastern Africa's Great Rift Valley and in spots is more than 1 km deep.

Although dissolved nutrients are scarce in the lake's shallow waters, they're abundant in waters so deep that there's no plant life to consume them. Therefore, near-surface microbes such as phytoplankton depend largely on the upwelling of nutrient-rich waters, says Piet Verburg, a marine biologist at the University of Waterloo in Ontario.

This water movement is often driven by winds that sweep surface waters away from shore, allowing underlying water to rise. That sort of mixing, however, has been stifled in recent years by lake warming.

Since 1913, the average temperature at the bottom of the lake's north basin has risen by about 0.2[degrees]C, but water only 100 meters below the surface has warmed about 0.9[degrees]C. Because warm water is less dense than cool water, the increasing temperature spread has made it more difficult for the underlying nutrient-rich water to upwell, says Verburg.

This decline in circulation has affected populations of aquatic microbes, especially in the past few decades. Biomasses of several plankton species measured in spring of 2001 and summer of 2000 averaged only 30 percent of those tallied during spring and summer of 1975, says Hedy Kling of the Freshwater Institute in Winnipeg, Manitoba. She, Verburg, and Robert E. Hecky of the University of Waterloo report their findings in an upcoming Science.

Although dramatic, the slump in phytoplankton populations isn't unprecedented, according to analyses of Lake Tanganyika sediments. Since the last ice age, there's been at least one extended period--between 4,000 and 2,400 years ago--when the lake's plankton productivity declined even below today's measure, says Simone Alin of the University of Minnesota in Duluth.