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Valentin Essert thesis defense

Valentin Essert->] will present his PhD thesis on Monday 23 May at 2 pm in room -107 M, Chrono-environnement laboratory (UFR ST, Propédeutique building, 25000 Besançon). His work concerns Sources and transfer pathways of carbon in lake pelagic food webs based on the analysis of carbon isotopic signatures of daphnids and their subfossil remains, a combined neo- and paleolimnological approach.

Jury composition
Heiri Oliver, Rapporteur, University of Basel
Dupuy Christine, Rapporteur, University of La Rochelle
Grosbois Guillaume, Examiner, Université de Québec en Abitibi-Témiscamingue
Latour Delphine, Examiner, University of Clermont-Auvergne
Millet Laurent, Thesis Director, CNRS
Masclaux Hélène, Co-supervisor, University of Franche-Comté

Thesis summary
Lakes are crucial ecosystems for the regulation of the global carbon cycle, being able to fix inorganic carbon via photosynthesis and mineralize organic carbon to produce CO2 and CH4. Recent intensification of anthropogenic pressures has led to a disruption of the biogeochemical carbon cycle for many lakes, resulting in an intensification of organic matter mineralisation processes. These dynamics can lead to an imbalance in the balance between emission, storage and transport processes in carbon food webs. However, the controlling factors and mechanisms of carbon transfer in food webs remain poorly understood, particularly in relation to pelagic food webs. This thesis therefore aims to study the sources and pathways of carbon transfers to pelagic consumers at variable temporal scales, and to assess the influence of disturbances affecting lake systems. The strategy implemented combines synchronous and diachronic approaches and is based on the analysis of stable carbon isotopes in daphnia and their subfossil remains, the ephippia. The results acquired during this thesis have contributed to show a seasonal variability of the carbon sources consumed by daphnids with, in particular, sustained transfers of methanogenic carbon (C-CH4) during the winter. Our results also show that the isotopic signature of ephippia reflects the winter transfers of C-CH4 to a greater or lesser extent depending on their production period in relation to the period of autumnal lake mixing. We have thus proposed an original strategy based on the use of ephippia combined with other subfossil remains with a broader temporal integration for the study of the carbon cycle in paleolimnology. Finally, the neo- and palaeolimnological approaches developed in this thesis highlight the influence of thermal stratification, watershed occupation and lake trophic level on the production and transfer processes of CO2 and CH4 in pelagic food webs

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