Local flushing time and its relationship to reservoir eutrophication in Xiangxi Bay, China

Abstract Zhao Q, Mao J, Cai H, Dai H. 2023. Local flushing time and its relationship to reservoir eutrophication in Xiangxi Bay, China. Lake Reserv Manage. 39:72–87. Reservoir impoundment often accelerates the process of eutrophication. Consequently, algal blooms frequently occur in weakly flushed, eutrophic tributary embayments. As a reliable and straightforward criterion for characterizing the hydrodynamic influence on water quality, local flushing time was investigated in a long, narrow bay formed where a tributary enters the Three Gorges Reservoir in China. We determined the local flushing time using a calibrated 3-dimensional hydrodynamic and transport model. The spatial and temporal variations of the local flushing time were studied under different operational conditions characterized by water level fluctuations. The local flushing time showed responses to both upstream inflow and the Three Gorges Reservoir mainstream backwater. The spatial–temporal distributions of local flushing time are significantly affected by density currents that frequently occur in the bay because of the operation of the Three Gorges Reservoir. Positive correlations were found between local flushing time and total nitrogen, and dissolved silicon concentrations. The spatial heterogeneity of local flushing time demonstrates that there is a specific zone with a longer local flushing time (i.e., low flushing rates) in the upper middle portion of the bay. High-risk areas for algal blooms coincide with the quasi-stagnant flow zone. These findings could help to develop strategies to manage algal blooms in weakly flushed eutrophic bays.