Phytoplankton response to artificial and natural mixing in a warm stratified drinking water reservoir: A simultaneous comparative study

Abstract

Artificial mixing is usually one of the means to improve water quality in reservoirs or lakes. In this study, XiKeng Reservoir (XKR), which was equipped with water-lifting aerators (WLAs) capable of artificial mixing, was selected in southern China. Using two phytoplankton classification methods (taxonomic and functional groups), high-frequency monitoring was synchronized for six months in the old and new reservoir areas to investigate the characteristics and mechanisms of phytoplankton response to artificial mixing and natural mixing, and to compare the differences between the two mixing processes. The results showed that both artificial and natural mixing significantly decreased phytoplankton abundance in the surface water layer. The phytoplankton abundance in the vertical dimension became homogenized with complete water column mixing. During the artificial mixing process, the phytoplankton shifted from Cyanobacteria to Bacillariophyta, and the functional groups shifted from M, SN, and S1 to P and D. The taxonomic groups shifted in line with artificial mixing during the natural mixing process, while the functional groups shifted from SN and S1 to P. While the mixing depth (Z_mix), light availability (Z_eu/Z_mix; the euphotic depth (Z_eu)), and relative water column stability (RWCS) were the main drivers of phytoplankton change due to artificial mixing at XKR, the natural mixing process was driven by RWCS and water temperature (WT). This study also provided a successful example of effective control of phytoplankton overgrowth in a warmly stratified drinking water reservoir, which will be valuable to water quality and ecological managers.