Modelling Littoral–Pelagic Exchange Using a Coupled Hydrodynamic–Biogeochemical Model

The fringes of lake shorelines (the littoral zone), rich in plant growth and highly productive, play a crucial role in methane emission. Yet, these zones between land and water have largely been overlooked in studies of greenhouse gas budgets. This work presents the development of a coupled three-dimensional hydrodynamic–biogeochemical model to simulate the transport dynamics between the littoral and pelagic (open water) zones of Lake Erken. The aim was to enhance our understanding of the lateral exchange between the littoral and pelagic zones. Following a model selection process, the TELEMAC-3D hydrodynamic model and the AED2 biogeochemical module were selected as the core tools for this thesis. Meteorological and geospatial data were then collected, after which flexible computational meshes were generated for simulations. The next steps involved the setup of model configurations and the delineation of the littoral zone, which was based on the depth at which submerged aquatic vegetation can grow. The model was first applied to simulate the water temperature regime of Lake Erken over a one-year period. Although calibration and validation were not possible due to limited computational resources and time constraints, the model captured the overall thermal structure and the dynamics of mixing and stratification reasonably well, especially during spring and autumn mixing periods. However, the model generally underestimated water temperatures slightly, except at deeper depths during the summer stratification period, where it tended to overestimate them. Passive tracer and simplified methane simulations were subsequently carried out for different summer periods in 2023 to investigate littoral–pelagic exchange. The delineated littoral zone served as the site for tracer or methane input. The passive tracer simulations demonstrated how substances were transported from the littoral zone to the pelagic zone through advection and dispersion, driven by wind-induced hydrodynamics. The results indicated that varying wind conditions during the simulated periods influenced the nature of the exchange between these two zones. The results of the methane simulations showed a predominant net transport of methane from the littoral to the pelagic zone, with brief periods of reversal, highlighting the dynamic nature of methane redistribution driven by lake hydrodynamics. Additionally, the results underscored the significance of methane flux to the atmosphere in regulating the overall mass of methane within the lakes.