Spatiotemporal insights of phytoplankton dynamics in a northern, rural-urban lake using a 3D water quality model.

Lake St. Charles, located north of Quebec City, Canada, is a shallow fluvial lake with two distinct basins bridging rural and urban landscapes. Mainly used as a source of drinking water for 300,000 residents, the lake has faced a steady degradation in water quality due to urbanization and the discharge of domestic wastewater. This study introduces a 3D hydrodynamics and water quality model using the Environmental Fluid Dynamics Code to enhance our understanding of algal bloom dynamics in Lake St. Charles. More specifically, we ran simulations for eight years (i.e., a three-year period for calibration, 2015 to 2017; and a five-year period for validation, 2018 to 2022) to reproduce the complex circulation patterns and dynamics of water quality within the system. The simulation results for chlorophyll-a demonstrate seasonal fluctuations in phytoplankton biomass, closely aligning with in situ observations and achieving Relative Root Mean Square Error (RRMSE) values below 50%. (i) In spring, runoff from snowmelt brought phosphorus into the lake, triggering primary production. Diatom growth was initially predominant in the shallow southern basin, then spread to the deeper northern basin due to favorable environmental conditions, including flow- and wind-induced currents, warmer water temperatures and nutrient availability. (ii) In summer, warm water temperatures stimulated biological activity, leading to the growth of cyanobacteria at the expense of diatoms, as well as a drop in phosphorus. (iii) The cyanobacteria persisted into the fall but began to decline in mid-November. (iv) Winter conditions, including the presence of an ice cover, limited the input of phosphorus and minimized phytoplankton production, but diatoms were observed in low concentrations near Des Hurons River inflow. Overall, during the open-water period, the lake-maintained chlorophyll-a concentrations indicative of mesotrophic conditions, with occasional periods when the biomass increased above the eutrophic threshold. Temperature, nutrient levels, and the fluvial dynamics of the lake are the primary factors influencing phytoplankton formation and distribution in lake St. Charles.