Water level fluctuations shape phytoplankton community in the Xiangxi Bay from Three Gorges Reservoir

Water level fluctuations (WLFs) constituted a dominant factor controlling the structure and function of freshwater ecosystems but the mechanism of WLFs on phytoplankton community structure was still unknown. We investigated the characteristics of phytoplankton community structure in Xiangxi Bay from January 2017 to December 2020. Results indicated water level (WL) of the Three Gorges Reservoir was divided into four distinct stages: the decreasing stage (DS), the low water level stage (LS), the storage stage (SS), and the high water level stage (HS). Notably, Cyanophyta predominated during the LS, with Microcystis sp. being the dominant species. Bacillariophyta was predominant in other three WL stages, with Melosira sp. and Cyclotella sp. as the dominant species. The highest biomass appeared in LS, whereas the lowest appeared in HS. Moreover, alpha diversity appeared to be lower in both HS and LS compared to DS and SS. Redundancy analysis showed WL as the key driver of phytoplankton community. Partial least squares path model analyses demonstrated that WL not only altered chemical factors (path coefficient = -0.62, P < 0.01), thereby leading to changes in phytoplankton biomass (path coefficient = 0.56, P < 0.01), but also changed the physical factors (path coefficient = -0.69, P < 0.01) and consequently had an impact on phytoplankton biomass (path coefficient = 0.33, P < 0.01). Furthermore, WL influenced phytoplankton diversity by altering chemical and physical factors. In conclusion, WL was an important factor influencing phytoplankton community, which implied that reservoir operation was the potential strategy to regulate phytoplankton communities.