Spatiotemporal monitoring of harmful algal blooms using time-series Sentinel-2 MSI Level-2A imagery: a case study of El-Mex Bay, Alexandria, Egypt

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Introduction Harmful algal blooms (HABs) pose significant threats to aquatic ecosystems, fisheries, and public health worldwide. Materials and methods This study investigated bloom dynamics at two stations (El-Fanar and El-Mex) within the highly eutrophic El-Mex Bay, Alexandria, Egypt, by integrating in situ physicochemical and phytoplankton data with Sentinel-2 multispectral imagery processed through ArcGIS 10.5. Three spectral indices the Red Tide Index (RTI), Maximum Chlorophyll Index (MCI), and Normalized Difference Chlorophyll Index (NDCI) were computed and evaluated using a literature-referenced threshold method. Results and discussion Summer 2023 blooms were predominantly multi-specific, dominated by Prorocentrum cordatum, Skeletonema costatum, Thalassiosira rotula , and Heterosigma akashiwo . RTI and NDCI mean pixel coverage peaked in August, while MCI maxima occurred in July, consistent with chlorophyll accumulation. Principal component analysis (PCA) revealed that RTI and MCI explained near to 100% of spectral variance, confirming their utility as primary bloom indicators. Validation of red tiding from the extracted values of RTI proved that the two studied sites were highly eutrophic, with red tiding recorded in August. Water temperature and inorganic nutrient concentrations were the principal environmental drivers of bloom initiation and succession. Pearson correlation analysis showed that total phytoplankton abundance was positively correlated with water temperature, H. akashiwo abundance, and Chl. a, while temperature was negatively associated with species richness, diversity, and evenness. In contrast, S. costatum was positively related to nitrate, silicate, and diversity indices, indicating more diverse assemblages. These results identify temperature and nutrient availability as the main drivers of bloom development and succession. The integration of Sentinel-2 imagery, spectral indices, and ecological indicators provides an effective framework for near-real-time HAB monitoring in optically complex coastal waters.