Mixing processes in a reservoir corresponding to different water level operations caused spatial differences during two phytoplankton bloom events

Abstract

• Two algal bloom events were noticed for the difference in distribution area size. • Water level was the only environmental variable responding to the difference. • Lifting water level induced trapped flow which promoted bloom development. • Continuous conversion from kinetic energy to potential energy are responsible. • The findings provide guidance for reservoir operation design to control algal bloom. Reservoir operation may impact algal blooms by altering flow conditions, but relevant studies and a theoretical understanding of this subject are limited. In this study, two phytoplankton bloom events with different spatial coverage patterns in a reservoir were investigated. By examining environmental variables, a suitable nutrient content and water temperature were considered responsible for bloom initiation during both events, but the different water level operations could possibly explain the observed spatial difference. A two-dimensional hydrodynamic numerical model was then used to simulate the mixing processes within the reservoir during these two bloom events. The simulation results indicated that the rising water level during the bloom event with a larger coverage area caused closed circulation flow conditions above the intrusion layer, displacing surface water in front of the dam upstream until reaching inflowing river water, which then intruded to support water offtake and water level lifting operations, while the other event did not exhibit a similar flow pattern. The water age was sensitive to water level operation, which suggested that phytoplankton dispersal could be promoted by rising water level processes. Inflow densimetric Froude number analysis indicated that reservoir impoundment facilitated a conversion of the inflow kinetic energy into potential energy, thus promoting algal dispersal via circulation. This study provides insights into the impact of reservoir operation on phytoplankton communities, offering a reference for operational design to mitigate algal blooms in reservoirs.