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Lake ecosystems in a changing climate : strategy for coupled hydrodynamic and biogeochemical modelling

Abstract : The ecological state of freshwater ecosystems worldwide has deteriorated along the past decades. Anthropogenic pressures have altered their physical and biogeochemical dynamics, acting both within their watershed and on the climatic conditions. Eutrophication and climate change contributed to the increase of algal blooms, and in particular of toxic cyanobacteria blooms , which currently constitute one of the main concern in the management of water resources.With the advance of urbanization, an increasing number of lakes are located in metropolitan areas. The high loads of nutrients and pollutants coming from the watershed often lead urban lakes to eutrophic conditions and cyanobacteria blooms, that cause bathing bans and restrictions for aquatic sports. Responsive surveys and long-term climate change impact studies are essential for the management of such sites, but rarely addressed.In this respect, modelling tools are of central importance to better understand the functioning of aquatic ecosystems, the factors triggering harmful algal blooms, and to support the management of water resources. However, aquatic ecological models are often complex and highly parametrized, and their implementation and calibration are challenging. Automated strategies for parameters calibration are available but are rarely applied. Furthermore, data from traditional periodical limnological survey do not allow to test the models on dynamics quicker than the span between two successive campaigns, and to thoroughly assess the uncertainty of their outcomes.In this context, this PhD thesis focuses on the use of deterministic models to reproduce the thermal dynamics and phytoplankton dynamics, notably cyanobacteria, in a small and shallow urban lake on different time-scales. To do so, two coupled hydrodynamic and biogeochemical three-dimensional (3D) models are implemented and analysed. The models used here are the FLOW and BLOOM modules from the Delft3D modelling suite, and the biogeochemical library Aquatic EcoDynamics coupled with the hydrodynamic model TELEMAC3D. The models are applied on Lake Champs-sur-Marne, an urban lake located in the East of Paris that suffers from strong cyanobacterial blooms and for which an extensive data set is available.This work aims to address in detail three strategic elements in lake ecosystem modelling:•The impact of climate change on the thermal regime of small and shallow lakes, and its relation to cyanobacterial growth. This is assessed through long-term 3D hydrodynamic simulations that allowed to hindcast the evolution of the study site during the past six decades.• The applicability and the benefits of automated calibration for complex biogeochemical models. This is done through an innovative methodology for parameter estimation: Approximate Bayesian Computation (ABC), tested here for the first time on a complex, highly-parametrized model.•The coupling and the feedbacks between hydrodynamic and biogeochemical models focusing on different time scales, and the importance of an extensive data set, that includes continuous high-frequency observations.The results show that the impact of climate change on small and shallow lakes can be severe, with consequences on the stratification dynamics and that thermal conditions increasingly favourable for cyanobacterial growth have established over time in the study site. This suggests that cyanobacteria dominance could become a widespread issue in the near future, if such trends are confirmed. Furthermore, this work proves that automated calibration strategies, and ABC in particular, can be profitably applied to complex physically-based biogeochemical models in order to improve their results over the period chosen for calibration. Eventually, this work also highlights the importance of an extensive data set to set-up a coupled 3D hydrodynamic / biogeochemical model, and analyse and exploit its results over different time scales.
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Submitted on : Monday, February 14, 2022 - 2:01:22 PM
Last modification on : Friday, August 5, 2022 - 2:38:11 PM
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  • HAL Id : tel-03572828, version 1


Francesco Piccioni. Lake ecosystems in a changing climate : strategy for coupled hydrodynamic and biogeochemical modelling. Environmental Engineering. École des Ponts ParisTech, 2021. English. ⟨NNT : 2021ENPC0018⟩. ⟨tel-03572828⟩



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