Carbon Cycle in Lake Geneva

Closing the carbon budget on Lake Geneva Inland waters are extremely reactive and they are known to both transport a lot of carbon and also emit and sequester it, yet research has historically focused on boreal rather than clearwater lakes. We spoke to Professor Marie-Elodie Perga about her research into the carbon cycle on Lake Geneva, in which she aims to find out more about the processes responsible for this reactivity and transport.

There are typically two types of lakes in mountainous areas like Switzerland. There are the smaller lakes that are found at high altitudes and which are usually relatively unproductive, then there are very large lakes at lower altitudes. “Lake Geneva for example is only about 370 metres above sea level,” says Marie-Elodie Perga, an Associate Professor in the Institute of Earth Surface Dynamics at the University of Lausanne in Switzerland. These lakes tend to have quite ‘hard’ waters, with a higher dissolved mineral content (especially in calcium and bicarbonates) – related to the presence of chalk deposits – than ‘soft’ waters, and the lakes themselves tend to be very clear. “We have a very limited understanding of carbon cycling in these clear, hardwater lakes,” continues Professor Perga. “A lot of work has been done on boreal lakes, for example in Canada or Sweden. However, those lakes are completely different – they usually have very soft waters, and there is a lot of organic carbon that gives the waters a brown-ish appearance.”

Lake Geneva The majority of the studies on carbon cycling in lakes so far have been conducted on boreal lakes, while the clearwater lakes are relatively under-represented. This issue is at the heart of Professor Perga’s work as the Principal Investigator of a research

interface between two environments, two chemistries. “Remote sensing images show a point where the lake turns a blue, chalky colour, which is caused by an in-lake calcite precipitation event that we call whiting,” outlines Professor Perga. “This whiting is created where the Rhone comes into the

The metabolism of a lake is certainly important, but it’s not the full picture. There are many other processes that may be responsible for the overall CO2 concentration in lakes, for example all the processes involving inorganic carbon. project in which she is investigating carbon cycling in Lake Geneva, looking to close the carbon budget and account for the complexities of the lake. “We are working on the pelagic area of the lake, the off-shore area, where the water is quite deep. We are also looking at the interaction between the Rhone river, which is the main tributary, and the lake,” she explains. This represents the

lake. It’s an interface that introduces a lot of spatial variability.”

A further point of variability is between the littoral area of the lake, which is relatively shallow and has more concentrated chemistry, and the off-shore area. Professor Perga and her colleagues take full account of the varying nature of these different environments. “We look at the lake not

The Lexplore platform on Lake Geneva. © Pascal perolo

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as a uniform, single place but as a mosaic of different environments with different reactivities,” she says. In terms of temporal variability, geochemical processes can take place over very different timescales. “When you think about the rate of photosynthesis for example, it changes every time a cloud obscures the sun. There is also variability at larger scales, at the diel, seasonal and interannual scales,” points out Professor Perga.

“We try to look at the processes at different timescales – from minute timescales, using the Lexplore platform on the lake, up to 10year timescales or higher, when we look at the sediment cores. We are looking at how these timescales are embedded into each other.”

This would help researchers to understand why the carbon cycle has changed to the extent that it has over recent decades, and also to forecast what is likely to happen in future. The metabolism of the lake, the balance between the amount of photosynthesis in the lake and the total respiration of organisms within it, is one factor which drives the CO2 concentration.

“If there is more photosynthesis than respiration, then CO2, concentration will decrease, and the other way round it will increase,” says Professor Perga. The metabolism has been the central focus of carbon cycle research on inland waters, but Professor Perga says the general perspective is shifting. “The metabolism is certainly important, but it’s not the full picture. There are many other processes that may be responsible for the overall CO2 concentration in lakes, for example all the processes involving inorganic carbon (those bicarbonates coming from the catchment and their processing in the lake through whiting),” she continues.

The aim now is to gain a fuller picture of the factors that affect the carbon budget on Lake Geneva, which has changed significantly over recent years. Researchers have about 50 years worth of monitoring data on many different components of the carbon cycle, but Professor Perga says it is difficult to

Preparation and analysis of water samples from Lexplore. © Didier Jezequel

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Satellite image of a whiting event, June 29 2019, (credit photo Sentinel-2A, © ESA).

explain why it has changed so much. “There are many candidates. It may be because the catchment has changed a lot, because the hydrology has changed, because the trophic level of the lake has changed,” she explains.

The goal in the project is to essentially solve this puzzle, and to identify which factors are driving changes in the carbon cycle. “Is it essentially driven by physical processes?

Or is it primarily driven by catchment processes?” says Professor Perga. “We are doing data analysis to investigate this. We develop models at very fine scales, and then we try to upscale them, mechanistically, to the whole lake.”

One type of model is mechanistic-driven, with equations for every type of process that may happen, which is then applied to the lake. If this proves ineffective in simulating the lake properly, it provides a hint as to where researchers should focus their attention, while Professor Perga is also using another approach, which builds on ideas from machine learning. “We are also using data-driven approaches – so we get all the data we can, and we use those models as a kind of black box. The model will try to simulate the output, based on the number of inputs,” she outlines. The idea here is to combine the mechanistic data with the machine learning models to get a deeper picture of the carbon cycle in Lake Geneva. “We know that the mechanistic models based on physics work very well in some situations. But we also know that when it comes to biological processes the mechanistic models are less effective,” continues Professor Perga. “We are trying to combine the best of two worlds, the mechanistic models and the machine learning approach.”

Maintenance of CO2 sensors, from Lexplore. © Pascal perolo

Sediment coring.

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Carbon Cycle in Lake Geneva Carbon Cycle in Lake Geneva

Project Objectives

Solving the conundrum of carbon cycling in lake Geneva: -combing long-term approaches (paleoecology, long-term monitoring data) to high-frequency observations and modelling from the LéXPLORE platform -Addressing the role of catchment-wide versus in-lake processes -elucidating the role of inorganic carbon cycling versus organic carbon processes -combining data-driven and mechanistic models

Project Funding

Funded by the Swiss National Science Foundation SNSF 200021-175530 (Cycle du carbone dans le Léman) and Bourses d’excellence de la confédération suisse (to Dr Rantala)

Project Partners

• LéXPLORE (https://lexplore.info/fr/accueil/) • APHYS (EPFL) • SURF (EAWAG)

Contact Details

Project Coordinator, Marie-Elodie Perga Associate Professor UNIL-Mouline, Géopolis, bureau/office 5885 Lausanne, Suisse T: +41 21 692 44 27 E: marie-elodie.perga@unil.ch W: http://wp.unil.ch/lakes/ marie-elodie.perga@unil.ch Perolo, P., Fernández Castro, B., Escoffier, N., Lambert, T., Bouffard, D., and Perga, M.-E.: Accounting for surface waves improves gas flux estimation at high wind speed in a large lake, Earth Syst. Dynam. Discuss. [preprint], https://doi.org/10.5194/esd-2021-30 Wüest, A., Bouffard, D., Guillard, J., Ibelings, B. W., Lavanchy, S., Perga, M.-E., & Pasche, N.(2021). LéXPLORE: A floating laboratory on Lake Geneva offering unique lake research opportunities. Wiley Interdisciplinary Reviews: Water, e1544. https://doi.org/10.1002/wat2.1544

Lexplore platform The Lexplore platform plays an important role in this research. Established by a scientific consortium including the University of Lausanne and several other Swiss partners, Professor Perga says Lexplore is a unique research infrastructure.

“There is nothing like it elsewhere in the world. It’s a floating laboratory, so we can operate and work from the platform, and there are many sensors around,” she outlines. The sensors of particular relevance to Professor Perga’s project are related to CO2 concentration in the air and the water, as well as pH and conductivity.

“We have a chamber that measures the fluxes of CO2 at the interface between the water and the atmosphere. From those high-frequency measurements, we could see for instance that wave events, despite being quite rare (6% of the time) contribute to a quarter of the lake’s CO2 emissions to the atmosphere,” she continues.

This data helps researchers delve deeper into the underlying reasons behind changes in the carbon cycle, and gain more detailed insights. The historical picture is also very important in this respect, and researchers are using sediment cores to reconstruct past CO2 concentrations on Lake Geneva, as well as others. “We have done this on 6-7 different lakes around Switzerland, not just Lake Geneva. In all lakes, phosphate

pollution has driven very important changes in the CO2 concentration, but not always in the same direction,” outlines Professor Perga. This then affects the quantity of algae on the lake, which is a prominent consideration in Professor Perga’s research.

“Algae photosynthesise, and so this affects how much CO2 gets into the lake, and how much is transferred and transported,” she explains. “We know phosphate pollution has an impact, but we cannot really explain in which direction and by how much.”

Enhanced weathering Professor Perga is looking into the possibility of an extension. Professor Perga is also interested in enhanced weathering approaches, which hold the potential of removing CO2 from the atmosphere. “The principle of enhanced weathering is that CO2 in the atmosphere can be captured when it reacts to certain rocks,” she explains. Switzerland counts on carbon capture solutions such as enhanced weathering to achieve carbon neutrality by 2050. “We are wondering if the lake would help make enhanced weathering more efficient,” says Professor Perga. “Enhanced weathering remains very uncertain, but it would be an interesting way to widen the impact of the research that has been done on carbon processes and carbon cycling in the environment.” © Pascal perolo

Rantala, M. V., Bruel, R., Marchetto, A., Lami, A., Spangenberg, J. E., & Perga, M.-E. (2021). Heterogeneous responses of lake CO2 to nutrients and warming in perialpine lakes imprinted in subfossil cladoceran δ13C values. Science of the Total Environment, 782, 146923. doi:https://doi.org/10.1016/j.scitotenv.2021.146923

Professor Marie-Elodie Perga

Marie-Elodie Perga is Associate professor at UNIL (since 2017), after working in France (INRAE) and Canada (UVIc). She studies biogeochemical processes in high-altitude lakes, and large swiss lakes at the foothill of the alps, connecting them to physical and ecological processes.

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