Understanding stratification and turnover dynamics of a tropical lake using extensive field observations and 3D hydrodynamic simulations

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The stratification and turnover dynamics of a tropical lake were evaluated using field observations and 3D hydrodynamic simulations. Located in the Philippines, Sampaloc Lake is a 104-ha and 27-m deep volcanic crater lake with enclosed watershed, which is at risk of the impacts of intensive aquaculture, rapid urbanization and climate change. Temperature, dissolved oxygen (DO) and chlorophyll-a (Chl-a) were measured at seven sampling stations using a multiprobe. Kruskal-Wallis test revealed that the three parameters are not significantly different among stations, indicating that one sampling station can represent the water quality of the whole lake. Schmidt's Stability Index (SSI) and thermocline strength, together with DO and Chl-a gradients decreased from October 2022 (stratified) to January 2023 (turnover). After successfully verifying the 3D numerical model, sensitivity analyses of water temperature to varying weather, together with particle tracking simulations, were implemented to determine the timing of isothermal state, upwelling, partial mixing, and full turnover. Compared to air temperature, variations in wind speed have more pronounced effects on the delay or progression of isothermal conditions in the lake based on SSI, Lake Number and Wedderburn Number. Isothermal conditions do not necessarily coincide with the timing of full turnover, with the latter being delayed by two days than the former, on average. Results revealed that full turnover can occur several weeks earlier with the decrease in AT and increase in WS. This study can advance the understanding of thermal and turnover dynamics of stratified tropical lakes, leading to better management of the water quality of these water bodies.