Remote sensing facilitates the exploration of algal bloom dynamics and its hydrometeorological drivers in tributary bays of the Three Gorges Reservoir

Reservoir construction has led to frequent algal blooms in tributaries of backwater areas, threatening water quality, aquatic ecological safety, and public health. The mechanisms governing algal blooms in reservoir bays are complex, leading to spatiotemporal occurrence patterns that are difficult to accurately identify. In this study, Sentinel-2 imagery from 2019 to 2023 were used to characterize the spatiotemporal distribution of algal blooms in Xiangxi Bay (XXB) near the dam of the Three Gorges Reservoir (TGR). The Google Earth Engine (GEE) platform was used for remote sensing image selection, preprocessing, and spectral index calculation, while random forest (RF) model and polynomial regression were employed to develop chlorophyll-a (Chl-a) retrieval model. Box plots and the Mann–Whitney U test were used to compare bloom and non-bloom conditions across nine hydrometeorological variables from the perspective of external influences, while the internal bloom mechanisms were analyzed by integrating the critical depth theory of algal growth with the hydrodynamic mixing characteristics and bloom occurrence patterns in XXB. The results indicated that: (1) Sentinel-2 imagery effectively captures bloom dynamics in narrow tributary embayments, and the Normalized Difference Chlorophyll Index (NDCI) accurately retrieves Chl-a, with the retrieval model achieving an R 2 of 0.76; (2) In recent years, algal blooms in XXB have been more likely to occur in March, July, August, and September, particularly in August, with approximately 18% of the XXB experiencing blooms, predominantly at level II intensity. High-frequency bloom areas are mainly distributed in the mid-to upper reaches of the backwater zone; (3) During the flood season, mid-layer intrusion from the Yangtze River and upstream tributary inflow promote stable stratification in XXB, especially under low-rainfall conditions, thereby favoring algal bloom formation in the mid-to upper reaches. Enhancing hydrodynamic disturbance through physical mixing or reservoir inflow and water-level regulation, particularly under low 10-day rainfall (