Lake Geneva threatened by suffocating algae

Lake Geneva Keystone

Scientists are warning that dams on the river Rhone could turn Lake Geneva into a weed-covered dead zone. They say disruption to the river is starving the lake of oxygen, threatening wildlife and exposing the waters to colonisation by noxious algae.

The warnings, published in New Scientist magazine, make alarming reading. Two researchers from the University of Geneva have been examining the impact of dams on Lake Geneva, and have concluded that the Lake is slowly suffocating to death.

They say that the nine hydro-electric dams, built on tributaries of the Rhone over the past 40 years, are seriously disrupting the seasonal flow of the river. The effect is to deprive the lake of vital supplies of dissolved oxygen, which are carried into the lake during the summer floods.

And the problem is getting worse. The two biologists, Jean-Luc Loizeau and Janusz Dominik, calculated that the number of days when the lake receives sufficient oxygenated water has fallen from 55 to only 15 days a year.

The reason the oxygen is so vital is because it creates a barrier, which prevents phosphorus lying at the bottom of the lake from escaping into the water. According to Loizeau, “Phosphorus over-fertilises the water, sparking a process called eutrophication, where algae form smelly toxic tides which suffocate the lake.”

“It’s important for there to be a minimum of four to five milligrams of oxygen per litre of water at the lake’s bottom,” Loizeau added. “Anything less could trigger a release of phosphorus.”

If that happens, a runaway process could take place. Loizeau and Dominik warn that an increase in phosphorous in the water will also trigger the growth of plankton. This in turn will consume more oxygen, allowing even greater quantities of phosphorous to escape.

swissinfo with agencies