Hydrodynamic impacts on algal blooms in reservoirs and bloom mitigation using reservoir operation strategies: A review

Algal blooms that have substantial adverse effects are becoming increasingly prevalent in reservoirs, especially under global warming and eutrophication conditions. In the last decade, the essential role of hydrodynamic conditions on algal growth has been widely recognized. Inducing unfavorable hydrodynamic environments for algal growth through reservoir operations is considered an efficient, low-cost, and clean method for controlling algal blooms in reservoirs. However, systematic reviews of the hydrodynamic mechanisms on algal blooms and the application of reservoir operation strategies for bloom control have not been proposed yet, significantly hampering progress in the field and practical applications. To address this problem, the author first reviewed relevant literature and concluded that enhancing direct hydrodynamic effects (including flow velocity, water turbulence, and shear stress) could inhibit algal growth, while intensifying indirect hydrodynamic effects (including water retention, stratification, nutrient distribution, and light distribution) could promote algal blooms in reservoirs. The author then classified reservoir operation strategies into basic reservoir operation, selective withdrawal operation, and multi-objective optimal operation, and pointed out that the latter could be key to practical engineering applications. Moreover, hypolimnetic withdrawal might be a long-term prevention strategy for controlling algal blooms because it removes internal nutrient load and alters water stratification, while epilimnetic withdrawal, which flushes away algae in upper layers, may be an emergency measure. Finally, the author highlighted the challenges and opportunities associated with the governing hydrodynamic mechanisms for algal growth, the coupled effects of hydrodynamics and other environmental factors, nutrient cycling processes, underwater light distribution, key reference indexes, short-term reservoir operation strategies, withdrawal elevation optimization, and the internal mechanism driving the optimal operation for algal bloom control. This study identified the advances and gaps in the field and envisioned the potential directions for future research.