Coupled hydrodynamic and water quality simulation of algal bloom in the Three Gorges Reservoir, China

Patterns of spatial and temporal variability of algal bloom in the Three Gorges Reservoir (TGR) of China were investigated using Semi-implicit Eulerian-Lagrangian Finite Element (SELFE) model, an unstructured grid, 3-dimensional, hydrodynamic model fully coupled with an extended version of Water Quality Analysis Program (WASP) and the Carbon and Nutrients DIagenesis (CANDI) model. The coupled model was driven by the discharge, operational water level of the TGR, nutrients and phytoplankton from the upstream, climate condition included the solar radiation, heat fluxes and precipitation observed in the hydrometric and meteorological stations. The simulations were conducted to study the dynamic process of the algal blooms occurred in September 2007 and June 2008, and the modeling results agreed well with the field data included nutrients, chlorophyll-a concentrations in the TGR. The results indicated that the nutrients and chlorophyll-a concentrations showed obvious difference in the mainstream and the tributaries affected by the diurnal evolution of the phytoplankton during the algal bloom period. The location of algal bloom in the tributary Xiangxi River where the maximum chlorophyll-a concentration was greater than 30 μg/L was about 20 km from the river mouth, which coincided with the position of backwater tail controlled by the operation of the TGR. Meanwhile, the nutrients concentration near the river bed reached nearly 10 times of the concentration near the water surface after considering the nutrients released from the re-suspended sediment and the biogeochemical process in the river bed sediment. Finally, the climate change affected the aquatic ecosystem in the TGR was investigated and the countermeasure to inhibit the algal bloom in the tributary was suggested through the scenario modeling.