Can selective withdrawal control algal blooms in reservoirs? The underlying hydrodynamic mechanism

Selective withdrawal is considered an effective and convenient method to mitigate thermal pollution and environmental problems in reservoirs and downstream rivers. However, the potential effects of selective with-drawal on harmful algal blooms in reservoirs remain uncertain, threatening sustainable reservoir management. This study developed and calibrated a three-dimensional (3D) hydro-ecological model for the Zipingpu reservoir to numerically investigate how selective withdrawal scenarios (including surface, middle, and bottom with-drawal) affect algal growth and hydrodynamic environments. The surface withdrawal scenario could inhibit algal growth efficiently, causing Chlorophyll a (Chl a) concentration below 10 mu g L-1 consistently. The Chl a concentration decreased from 27.0 mu g L-1 for bottom withdrawal and 21.2 mu g L-1 for middle withdrawal to 3.3 mu g L-1 for surface withdrawal in the bloom period. The nutrient, light, and temperature limitations among the three withdrawal scenarios showed slight differences and were not the main reason for the bloom differences. Compared with the bottom and middle withdrawal scenarios, the surface withdrawal scenario weakened strat-ification and decreased surface water temperature, suggesting inhibitory effects on algal blooms. The bottom and middle withdrawal scenarios significantly reduced surface flow velocity and made the flow direction against the water intake, indicating a favorable hydrodynamic environment for algal growth. The surface withdrawal sce-nario reduced hydraulic residence time, thereby restraining algal growth and accumulation in the reservoir. Implementing surface withdrawal might be an efficient and promising method to prevent algal blooms in res-ervoirs. These results not only highlight the potential of selective withdrawal for algal bloom control but also reveal that accelerating surface flow velocity is the key mechanism.

Automated summary

Selective withdrawal is an effective and promising method to mitigate thermal pollution and environmental problems in reservoirs and downstream rivers. However, the potential effects of selective withdrawal on harmful algal blooms in reservoirs are uncertain. This study developed a hydro-ecological model to investigate how different withdrawal scenarios affect algal growth and hydrodynamic environments in Zipingpu reservoir. The results showed that surface withdrawal could efficiently inhibit algal growth and decrease algal bloom concentration in the reservoir.