Abstract: In the aim of investigating the inhibitory effect of Three Gorges Reservoir (TGR) operation in flood season on water bloom in its tributaries, we monitored the algal community and environmental parameters of water body in Xiaojiang River, a representative tributary of TGR, before, during, and after TGR operation from June 24^th to July 3^rd, 2022. We employed the Kruskal-Wallis rank test to analyze the changes in water environment, the Pearson correlation and stepwise regression analysis to examine the relationship between algae and environmental factors, and RDA to assess the contribution of different environmental factors to the variation of algae community structure. Additionally, we constructed a structural equation model to analyze the influence of reservoir operation on water bloom. The results demonstrated a significant decrease in algal biomass characterized by chlorophyll a concentration (from 58.33 μg/L to 6.41 μg/L) after the operation in the permanent backwater area where severe bloom occurred. In the whole backwater area, water temperature, pH value, and TN concentration decreased, while TP concentration and NO^-₃-N concentration increased (P<0.01). Stepwise regression analysis revealed that algal biomass was primarily influenced by total nitrogen (TN), nitrate (NO^-₃-N), phosphate (PO^3-₄-P), water temperature, electrical conductivity, and chemical oxygen demand (COD), explaining 79% of the biomass variation. The structural equation model indicated that water level rise led to a reduction in COD concentration and an evident decrease of chlorophyll a concentration. This study conclusively shows that proper reservoir operation can effectively inhibit water bloom in the tributary of the Three Gorges Reservoir. These findings provide a basis and a solution for reservoir eco-friendly management practices.

Key words: reservoir operation in flood season, water bloom, Kruskal-Wallis rank test, RDA, Xiaojiang River