A scientific study of Lake Wohlen in central Switzerland found "unexpectedly high" emissions of methane, a powerful greenhouse gas, the Swiss Federal Institute of Acquatic Science and Technology (EAWAG) said on Monday.

The 150,000 tonnes of methane bubbling up from sediment in the retention lake on the river Aare over a year are the equivalent of emissions from 2,000 cows, or 25 million kilometres (15.5 million miles) travelled by cars, EAWAG added in a statement.

"So hydropower isn't quite as climate-neutral as people have assumed in the past," said one of the scientists involved, Tonya Del Sontro.

"In the summer, the water in Lake Wohlen sometimes looks like champagne with masses of gas bubbles rising to the surface," she added.

The peer-reviewed research by scientists at Swiss, German and Israeli institutes was published in the US journal Environmental Science and Technology.

Joint author Bernhard Wehrli, a professor of aquatic sciences at Zurich's Federal Institute of Technology (ETHZ), said the study mirrored initial findings in research on tropical reservoirs, notably at the Kariba dam in Zambia.

Preliminary studies there had found "very high" inflows of carbon-rich material, forming sediment in the reservoir that released greenhouse gases, Wehrli told AFP.

However, he underlined that the complex process was highly dependent on temperature, depth and the amount of carbon rich organic material -- such as vegetation -- that accumulated in the sediment on the reservoir floor.

"We have done a study of high Alpine reservoirs that are in a better state in terms of methane emissions," he added. High altitude mountain reservoirs are generally in colder, more rocky and sparsely vegetated surroundings.

In the shallow Lake Wohlen, on the populated 500-metre (1,640 foot) high plateau near the Swiss capital Bern, the reservoir captures large amounts of organic material that would flow along the rive Aare.

"It's not the fault of the reservoir builders that there are emissions, but because the flow of the river is slowed down," Wehrli explained.

That allows the organic material to ferment and produce gas that bubbles up to the surface.

In Lake Wohlen the methane emissions were "minimal" in winter, while some deeper lakes may also store carbon, Wehrli said.

The research paper nonetheless concluded that the methane emissions discovered in the 90-year-old reservoir "indicate that temperate water bodies can be an important but overlooked methane source."

Hydroelectric power generated from dams and waterways is often regarded as a clean energy source.

But methane is one of the gases blamed for global warming and most industralised countries are required to cut emissions under the Kyoto Protocol.

Scientists regard the gas, which is produced naturally by humans, farm animals and rotting material as well as by industry, as up to 25 times more effective in trapping heat than carbon dioxide (CO2).

The research is due to be extended to other river reservoirs on the Swiss central plateau, to examine whether Switzerland might need to revise its "methane budgets," EAWAG said.

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