Effects of artificial mixing on phytoplankton in a warm stratified drinking water reservoir: Characterization, mechanism, and implication

To improve water quality, water-lifting aerators (WLAs) are usually installed in reservoirs for artificial mixing. In this study, using taxonomic groups methods of phytoplankton, the characteristics and mechanisms of their response to artificial mixing were investigated during a two-year monitoring period in Xikeng Reservoir (XKR) in southern China, and compared the differences in response in the context of higher and lower phytoplankton abundance. The results showed that artificial mixing caused a significant decrease in phytoplankton abundance in the surface water column, but a short-term increase followed by a decrease in phytoplankton in the middle and bottom, and ultimately a homogenization of vertical phytoplankton with complete mixing of the water column. The phytoplankton showed a shift from Cyanobacteria to Bacillariophyta in artificial mixing process, while morphological characteristics of phytoplankton shifted from the larger Volume (V) to the smaller V at the same time. Besides, artificial mixing may be more likely to result in structural variation in lower phytoplankton abundance than in higher abundance. The mixing depth (Z_mix) and light availability (Z_eu/Z_mix; the euphotic depth (Z_eu)) were the key drivers of phytoplankton succession caused by artificial mixing in XKR, rather than water temperature (WT) and nutrients. This study also provided a successful example of effective control of phytoplankton overgrowth in a reservoir under higher WT and nutrient conditions, which had important implications for ecological managers and researchers in reservoirs or lakes.