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Climatology of observations and simulations.
(A to C) Climatology of surface (averaged on 0- to 10-m depth) daily observations (Obs.) and models outputs when CP is enabled (i.e., the reference model, Mod. CP) or muted (Mod. noCP). DOY, Day of Year. The continuous colored line represents the daily average from 1981 to 2021; the dotted lines are the SD. The colored bar above represents seasons (sequentially winter-spring-summer-fall) (A) DIC, (B) pH, and (C) Pco2. The gray line represents Pco2 at equilibrium with the atmosphere. The vertical dotted lines indicate DOY 50, 180, and 300, for which
the vertical Pco2 profiles are exemplified in (D to F). Vertical profiles (daily average ± SD for 1981–2021) over the first 50 m of observed and simulated Pco2 with enabled or muted CP at (D) DOY 50, (E) DOY 180, and (F) DOY 300.
Color codes are the same for (A) to (C).
Credit: Science Advances (2024). DOI: 10.1126/sciadv.ado5924
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 (≃ 150,000 inhabitants). This phenomenon—the production of CO2 by lakes—has been known for years. There is, however, widespread debate as to the mechanisms at work.
Traditional scientific theories suggest that lake CO2 emissions are primarily due to the influx of organic matter from surrounding soils. This material, originating from the decomposition of plant and animal residues, is carried into the lake by rainfall, where it is broken down by microorganisms, leading to the release of CO2. This process is known as respiration.
While this theory accounts for the behavior of some lakes, it doesn't apply to Lake Geneva, which receives very little organic matter from its shores. In theory, its annual carbon balance should be neutral, with winter CO2 production (from organic matter decomposition and water mixing) balanced by summer CO2 absorption (due to algae photosynthesis). So why does Lake Geneva still emit large amounts of CO2?
A team of UNIL 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, 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, not just a biological one, as previously thought.
This 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 UNIL's Faculty of Geosciences and Environment and co-author of the study.
"This issue had been nagging at me since my thesis," she explains. "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 modeling."
Laid out on Lake Geneva, the floating laboratory made it possible to monitor various parameters linked to the carbon cycle, continuously and at high frequency.
In addition to the purely scientific interest of this discovery, this new data is central to the fight against global warming.
"Assessments are carried out every year to identify the emitters (sources) and storages (sinks) of carbon on our planet," explains 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."
The LéXPLORE platform
LéXPLORE is a 10 m x 10 m scientific research platform located on Lake Geneva in Switzerland, almost 600 m from the shore. It is equipped with high-tech instrumentation (109 sensors) and provides continuous measurements, day and night, in all weather conditions.
LéXPLORE brings together five institutions (EPFL, EAWAG, INRAE, UNIL, UNIGE) conducting cutting-edge, multidisciplinary research on the lake and its atmosphere. It is also used as a training and teaching facility, and as a popularization tool for the general public.
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