Surface waters in Lake Tanganyika, the second-oldest and second-deepest lake in the world, are currently warmer than at any time in the previous 1,500 years, according to a study published recently online issue of the journal Nature Geoscience.

“This result is in addition to those from other African lakes showing that changes in regional climate have a significant impact on the lakes, and on the human populations that depend on the lakes' resources,” said Paul Filmer, program director in the U.S. National Science Foundation's Division of Earth Sciences, which funded the research.

The scientists took core samples from the lakebed that laid out a 1,500-year history of the lake's surface temperature.

The resulting data showed that 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 largest temperature change in the 20th century. The change has affected its unique ecosystem, which relies upon nutrients from the depths to jumpstart the food chain on which fish survive.

“Our data show a consistent relationship between lake surface temperature and productivity such as that of fish stocks,” said Jessica Tierney of Brown University, the paper's lead author.

Decline in productivity

“As the lake gets warmer, we expect productivity to decline, and we expect that it will affect the fishing industry.” The lake, one of the richest freshwater ecosystems in the world, is divided into two levels.

Most of the animal species live in the upper 100 meters, including valuable sardines. Below that, the lake holds less and less oxygen, and at certain depths, it has no oxygen.

The lake depends on wind to churn its waters and send nutrients from the depths toward the surface.

These nutrients are food for algae, which supports the lake's entire food web. But as Lake Tanganyika warms, the mixing of waters is lessened; fewer nutrients are funneled from the depths to the surface.

More warming at the surface magnifies the difference between the two lake levels; even more wind is needed to churn the waters enough to ferry nutrients toward the 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, an important food source.