2 Thèses sur le cycle du Carbone dans le lac Leman à Lausanne

Deux bourses de thèse pour 4 ans sont ouvertes à l’Université de Lausanne pour travailler sur le cycle du carbone dans le Lac Leman à l’Institut de Dynamique des Surfaces Terrestres dans le cadre du projet CARBOGEN.

Carbogen project: If inland waters have recently been acknowledged as significant reactors of the global carbon cycle, light has been essentially shed on wetlands, permafrost and humic lakes, under the overwhelming paradigm that lake supersaturation with CO2 arises from metabolic processes. Within this picture, large and clearwater lakes have been largely overlooked, considered as neutral to the atmosphere. As a result, our knowledge about the carbon cycle in such lakes is clearly deficient. For instance, in Switzerland, a heuristic carbon budget attempted on the heavily studied and highly monitored Lake Geneva ended up unbalanced, C outputs being twice higher than the inputs. Estimated CO2 outgassing reaches surprisingly high numbers. Rough estimates suggest that the 10 largest Swiss lakes emit as much CO2 as fossil fuel bustion of total Swiss agriculture. The example of Switzerland shows that large and clearwater lakes could be a central feature of a national carbon budget and plead for a revision of our C conception in such environments.
Project CARBOGEN assumes that (1) key processes in lake carbon cycling are inaccurately scaled or remain missing, (2) C is controlled by intricated physical and biogeochemical processes which relative contributions depend on the time and space scales of observation, (3) lakes carbon cycling is highly sensitive to human and climate disturbances. The motive of CARBOGEN is therefore to address the carbon cycle of Lake Geneva through two objectives. The first one is to close the lake carbon budget by refining flux estimates accounting for the large temporal and spatial variability of the carbon processes and by identifying and quantifying missing sources. The second aims at untying the mechanisms behind the long-term C variability, and therefore to quantify the human contribution to such changes. For that purpose, CARBOGEN relies on an integrated, process oriented perspective on the carbon cycle of Lake Geneva, combining field surveys, high-frequency monitoring, bioassays and modelling.

Sujet 1: Long- and short term drivers of CO2 in Lake Geneva

Within CARBOGEN, the research topic of the PhD candidate will focus on the short- and long term drivers of Lake Geneva CO2 concentrations, with a more specific attention on the role played by vertical and lateral transport. CO2 concentrations in lakes have long been thought to be essentially
controlled by lake metabolism but this long-standing paradigm is being considerably challenged, while the role of chemical and hydrological processes are just being reconsidered. Previous works on Lake Geneva have shown that the drivers of CO2 variability in Lake Geneva, such as the underlying mechanisms, appear to differ depending on the time and space scales of observations. They highlight the necessity for a process-oriented approach to understand C-cycling in Lake Geneva, that fully integrates
biogeochemistry and hydrodynamics.

The PhD candidate is expected to untie the drivers of the vertical and temporal (at short and longer time scale) of Lake Geneva CO2, using pluri-decennial monitoring dataseries and high-frequency measurements form an automated platform. Data use and interpretation will be based from both deductive (data mining) and inductive approaches (simple 1D modelling combining physics to biogeochemical processes).

The PhD candidate is expected to work in strong collaboration with a second PhD candidate from the CARBOGEN project focusing on spatial variability in C processes in Lake Geneva. This project implies a true co-supervision shared between Prof Perga and Dr Bouffard, thereby requiring regular trips
between their institutions.

Sujet 2: A balanced Carbon budget for Lake Geneva

In the frame of the CARBOGEN project, the PhD candidate will focus on the estimation of dominant C fluxes in Lake Geneva accounting for spatial and seasonal variability. The applicant will rely on two high-frequency monitoring platforms (one inshore and a second off shore), frequent field surveys, and laboratory experiments in order to explore the unconsidered role of inorganic carbon such as nonlinear reactions of organic pools. These inputs would finally be upscaled in an attempt to reach a balanced carbon budget.

The PhD candidate is expected to work in strong collaboration with a second PhD candidate from the CARBOGEN project focusing on CO2 variability on short and long-time scales. The project also implies a strong collaboration with A. Borges (University of Liège, Belgium).

Contact details and application

For any questions, contact marie-elodie.perga@unil.ch.
Applications are to be sent directly by e-mail and should include a CV and motivation letter along with the name of three references.

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