Vertical mixing and horizontal transport unravel phytoplankton blooms in a large riverine reservoir

A precise understanding of the mechanisms causing phytoplankton blooms in reservoirs is still lacking, especially in large riverine reservoirs. To better understand these blooms, the role of the complex hydrodynamics caused by dam operation must be quantified. Here we examine how synergistic hydrodynamic processes, rather than individual metrics, trigger blooms in Xiangxi Bay, a typical tributary bay of the Three Gorges Reservoir, China. We used a 3D ecological-hydrodynamic model, which integrated hydrodynamics with the abiotic factors that limit phytoplankton growth to simulate one whole year (2010). By implementing a scaling criterion, we quantified the contribution of local phytoplankton growth and hydrodynamic processes, including advection transport and vertical mixing, on bloom dynamics. Results indicated vertical mixing was the main process inhibiting blooms in colder months (from October to February) but horizontal advection, which flushed and diluted blooms, was dominant in warmer months (from May to July) when stratification was intense and nutrients were replete. Accordingly, blooms occurred when both vertical mixing and horizontal advection were low. We suggested a potential dam operation strategy to mitigate blooms during stratification, which involves withdrawing the warm surface water from upstream reservoirs to increase horizontal flows in the surface layer. Extending the application of critical turbulence model, our study shows how vertical mixing and horizontal advection rate interact with phytoplankton growth rate to drive blooms in highly dynamic riverine systems.