Phytoplankton succession phenology trends in the backwaters of the three gorges reservoir in China and their drivers: Results from satellite observations

Information on long-term phytoplankton phenology trends is critical for clarifying the responses of aquatic ecosystems to environmental perturbations. However, the trends in the phenological metrics of phytoplankton blooms have not yet been comprehensively investigated in river systems with complex hydrodynamic conditions and typical phytoplankton community succession characteristics. Here, we investigated long-term phytoplankton phenology trends in the four typical backwaters of the Three Gorges Reservoir in China as well as the driving factors of these trends between 2009 and 2020 using in situ and satellite observations. The phenological metrics, including the initiation, succession, and termination dates of phytoplankton blooms, were extracted from sat-ellite imagery times series. We found that the phytoplankton bloom and green algal bloom durations have significantly increased, while the durations of cyanobacterial blooms have decreased since 2009. Specifically, the initiation (of cyanobacterial blooms) and first succession (from cyanobacterial blooms to green algal blooms) dates were advanced by 9.75 d and 23.25 d, respectively. However, the second succession (from green algal blooms to cyanobacterial blooms) and termination (of cyanobacterial blooms) dates were delayed by 24.25 d and 13.5 d, respectively. We revealed that air temperature and water level changes were likely responsible for these trends (mean relative contribution = 75.1 +/- 6.1 %). These findings provide new insights into regional climatic change and reservoir environmental management and can help researchers address harmful phytoplankton blooms and their associated adverse effects.