Reservoirs: A neglected source of methane emissions

When they first saw the data, environmental chemist Tonya Del Sontro and her PhD supervisor Professor Bernhard Wehrli were skeptical. But the unexpectedly high values stood up to careful analysis: on average, daily emissions of methane (CH₄) from Lake Wohlen amount to more than 150 mg per m² surface area. This is by far the highest emission rate recorded to date for a temperate reservoir. At a water temperature of 17°C the rate is twice as high, which makes it comparable to the emission rates observed for tropical reservoirs.

Equivalent to 25 million car kilometres

Overall, the reservoir on the Aare produces 150 tonnes of methane a year. This is about the same amount as is emitted annually by around 2000 cows; in terms of global warming potential, it is equivalent to the carbon dioxide emissions from 25 million car kilometres, since methane is about 25 times as potent a greenhouse gas as CO₂. "So hydropower isn't quite as climate-neutral as people have assumed in the past," says Del Sontro. At the same time, she does not wish to dramatize the findings of her research: even if the Aare hydropower plant is taken to be responsible for all the methane emissions from the reservoir, and these are expressed in CO₂-equivalents, a coal-fired power station with the same output produces 40 times as much CO₂. However, the emissions from Lake Wohlen do indicate that run-of-the-river reservoirs can also be significant sources of methane emissions in temperate regions. As Wehrli notes, "That's something which has previously been overlooked in greenhouse gas budgets."

Bubbling sediments

Methane production in Lake Wohlen is attributable to organic matter transported by the Aare, e.g. from Lake Thun. In the reservoir, the organic matter settles rapidly, undergoing microbial fermentation in the sediments. "In the summer," says Del Sontro, "the water in Lake Wohlen sometimes looks like champagne, with masses of gas bubbles rising to the surface." To study these bubbles, the Eawag scientists used custom-made gas traps in the form of inverted funnels. The analyses revealed that they were mainly composed of methane.

During the winter, on account of the cold temperatures, methane emissions are minimal. According to Wehrli, this explains why climate researchers have previously focused largely on the huge reservoirs lying in tropical regions: here, temperatures are warm throughout the year, and inundated forest areas provide a large supply of nutrients for microbes. Likewise, low water temperatures and low nutrient inputs mean that large Alpine reservoirs do not give rise to significant methane emissions. The researchers now plan to study other run-of-the-river reservoirs on the Central Plateau so as to determine whether Lake Wohlen is a special case, or whether methane budgets need to be revised for Switzerland as a whole.