Are our lakes on the brink of suffocation?

In order to gain insight into how lakes breathe, EPFL scientists have studied oxygen depletion in the depths of Lake Geneva - the first time such research has been carried out. By collecting key data, they were able to enhance their understanding of the lake's ecosystem and how it is likely to evolve over time. In the autumn, lakes undergo their annual water exchange as high winds, inflows from rivers and variations in water temperature mix up the water. During these colder months, the surface water, which is rich in oxygen, cools, sinks and mixes with the deeper layers, which brings fresh oxygen into the lakes' depths. But because Lake Geneva is so deep - going down 310 meters - the water exchange is only partial. The lake gets fully re-oxygenated only every eight years or so. A long-term shortage of oxygen could destroy the flora and fauna living in the lake's depths. To better understand how lakes breathe, Robert Schwefel from EPFL's Physics of Aquatic Systems Laboratory probed the depths of Lake Geneva in order to measure oxygen levels there. He took samples at different depths using a robot capable of descending several hundred meters. Thanks to the robot's microprobes, he was able to measure oxygen concentrations micrometer by micrometer, in both the water and the sediment. Samples were taken at seven locations, and the results were quite telling: "We were able to collect data that is essential for estimating oxygen depletion in the sediment - something that had never before been measured out in the field." 175 natural lakes in Switzerland and as many reasons to be concerned