The average temperature of European lakes has risen by more than a degree since 1960. These climate-driven changes in water temperature have a major impact on the ecosystem of lakes and might lead to more clear water in European lakes.

These results were obtained by the Dutch ecologist Marten Scheffer of Wageningen University, who analyzed massive data sets of observations from European lakes in collaboration with colleagues from German and Dutch research institutes. Their findings will be published in the next issue of Limnology and Oceanography.

The weather in Europe is affected by the so-called "North Atlantic Oscillation," much as El Niño drives oscillations on the southern part of the globe.

The difference in air pressure between Portugal and Iceland is used as an index for the state of the North Atlantic Oscillation. The winter value of this index has a remarkably strong predictive power for the temperature of lakes in the subsequent year.

The scientists demonstrated that the climate-driven changes in water temperature have a major impact on the ecosystem. In warmer years the chances for having clear water in spring are higher. This water clarity is caused by zooplankton that filter the water.

Importantly, submerged aquatic plants benefit from the improved light conditions in clear water and the high temperature. Since these plants themselves enhance water clarity, this can lead to a positive feedback. Research in shallow lakes has shown that this effect is strong enough to permanently stabilize the clear state once it has been reached.

The new results therefore imply that extremes in the climatic oscillation may flip turbid lakes into a stable clear water condition.

This work provides the first evidence that in addition to pollution, climatic change may strongly affect the condition of lakes.

(Reference: The paper "Climatic warming causes regime shifts in lake food webs" will be published in Limnology and Oceanography, Vol. 46, No. 7, November 2001. The paper, illustration material and links to relevant websites are available at this URL.)

[Contact: Prof.dr. Marten Scheffer]

19-Sep-2001