Apport de la modélisation tridimensionnelle pour la compréhension du fonctionnement des écosystèmes lacustres et l'évaluation de leur état écologique

Abstract : The quality of ecosystem services provided by lakes is related to the ecosystem structure and functioning. Protecting water bodies is therefore a global goal that requires a better understanding of their function, a monitoring and a water quality assessment. Explaining spatio-temporal heterogeneities of physico-chemical parameters and phytoplankton has been a recurrent ecological and hydrobiological issue. Understanding the dynamics of these heterogeneities is an essential prerequisite for objectively assessing, protecting and restoring freshwater ecosystems. Moreover, three-dimensional (3D) and taking into account and drivers of these heterogeneities are essential prerequisites for theoretical and applied limnology. Concerning the monitoring, spatio-temporal heterogeneities are responsible of uncertainties on the representativeness of the data versus the whole lake which might be questionable and needs to be verified. In Europe, the Water Framework Directive (WFD) initiated in 2000 defines a framework for managing and protecting water bodies in Europe. The classification of water bodies into ecological status is a key issue for the implementation of that framework. For lakes and reservoirs, the assessment of this status is based on biological, physico-chemical and hydro-morphological indicators. Physico-chemical and phytoplankton indicators are calculated based on four observations at an unique sampling station over the growing season, this evaluation being assessed one year for a six-year management plan. In this context, this thesis focuses on the complexity of lakes functioning and the capability of three-dimensional (3D) models to reproduce their functioning. The contribution of 3D models is presented i) for understanding the functioning of lakes of different sizes, ii) coupled to satellite observations, for studying of the influence of wind forcing and hydrodynamics on phytoplankton abundance and spatial heterogeneities, iii) in the context of the WFD, for assessing uncertainties in the lake ecological status assessment. To do that, two 3D models have been created and analyzed, one for Lake Créteil (42 ha) and another for Lake Geneva (580 km2). Lake Créteil 3D model was validated by using high frequency data recorded at three stations. It reproduces well the complex hydrodynamic functioning of the lake, its thermal structure, the alternation between thermal stratification episodes and mixing events, and internal waves. Lake Geneva 3D model was validated by using monthly and bimonthly data at two stations. It reproduces also properly the hydrodynamic functioning of the lake and the seasonal variability of biological and physico-chemical parameters. Simulation results highlight physical and hydrodynamic mechanisms responsible for the occurrence of seasonal hot-spots in phytoplankton abundance. In the context of the WFD, simulation results show also a strong spatial variability of lake ecological status depending on the timing of the four sampling dates as well as the location of the sampling station. These results were also used to assess to representativeness of sampling stations. The results of this thesis suggest that i) the functioning of lakes of different sizes is complex and physical processes generates spatio-temporal heterogeneities, ii) wind and hydrodynamics influence the abundance and the spatial distribution of phytoplankton et iii) spatio-temporal heterogeneities can bias our evaluation of lake ecological status in the WFD
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Frédéric Soulignac. Apport de la modélisation tridimensionnelle pour la compréhension du fonctionnement des écosystèmes lacustres et l'évaluation de leur état écologique. Ingénierie de l'environnement. Université Paris-Est, 2017. Français. ⟨NNT : 2017PESC1197⟩. ⟨tel-01982543⟩

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