Unlike oceans, lakes are major emitters of CO2. Why is this so, and what mechanisms are at work? Scientists at the University of Lausanne have succeeded in explaining the complete carbon cycle in Lake Geneva for the first time, generating a model that can be applied to several of the world’s great lakes.
Like most lakes in the world, Lake Geneva is an emitter of greenhouse gases, particularly carbon dioxide (CO2). Annually, it produces as much CO2 as the automobile transport of the city of Lausanne (â 150’000 inhabitants). This phenomenon - the production of CO2 by lakes - has been known for years. There is, however, considerable debate as to the mechanisms at work.
The basics of a carbon enigma
Traditional scientific theories attribute lake CO2 emissions to the massive influx of organic matter from surrounding soils into the lake. This matter, resulting from the decomposition of plant and animal residues, would be drained into the water by rainfall, then decomposed once in the lake by the micro-organisms living there, generating CO2. This is known as the respiration process. Although this theory can explain the activity of certain lakes, it doesn’t work in the case of Lake Geneva: it receives very little organic matter from its shores. Its balance over a one-year period should be neutral, with CO2 production in winter (decomposition of organic matter, water mixing), offset by CO2 absorption in summer (algae photosynthesis). How then can we explain the fact that it exceeds the neutral CO
A mechanism finally identified
At the University of Lausanne, a team of scientists has just deciphered the mechanisms involved. Most of the emissions actually come from the natural erosion of rocks in the lake’s upstream basin. When rainwater hits the rocks, it releases bicarbonate and calcium ions, which then find their way into the lake. In summer, under the effect of heat and the growth of algae - which change the pH of the water and act as a catalyst - the ions form microparticles of limestone. This is known as calcite precipitation. This chemical reaction releases CO2, giving the lake its milky blue-green appearance in the warm season. Algae continue to absorb CO2 because they are present in large numbers, but this is not enough to compensate for the massive production resulting from rock erosion. The additional emissions are therefore the result of a geological process, and not just a biological one, as previously thought.
The discovery was published in Science Advances. Our results not only explain the carbon cycle in Lake Geneva, they also reveal a universal process that applies to several of the world’s great lakes," explains Marie-Elodie Perga, Professor of Limnology at the University of Lausanne’s Faculty of Geosciences and Environment, and co-author of the study. This was an issue that had been nagging at me since my thesis", she explains. By using a scientific infrastructure that is unique in the world - the LéXPLORE platform - we were able to observe, model and equate these processes on a very fine scale, providing the missing piece to traditional carbon cycle modelling. Laid out on Lake Geneva, the floating laboratory enabled us to monitor various parameters linked to the carbon cycle, continuously and at high frequency.
A significant contribution to the fight against global warming
In addition to the purely scientific interest of this discovery, these new data are significant in helping to properly combat global warming, in particular. Every year, assessments are carried out to identify the emitters (sources) and storages (sinks) of carbon on our planet", explains Marie-Elodie Perga. It’s very important to have in-depth knowledge of how CO2 is naturally transported, stored and transformed between continents, water and the atmosphere. Only a global vision will enable us to take effective action to combat global warming. ’