High‐Resolution Spatiotemporal Monitoring of Water Quality and Trophic Status in Bay St. Louis Using Sentinel‐2 NDCI Time Series on Google Earth Engine

ABSTRACT Monitoring chlorophyll‐a (Chl‐a) is critical for assessing eutrophication and detecting harmful algal blooms (HABs) in nutrient‐sensitive estuarine systems. This study investigated the spatiotemporal distribution of Chl‐a and trophic status in Bay St. Louis, a shallow estuary on the Mississippi coast, using the Normalized Difference Chlorophyll Index (NDCI) derived from Sentinel‐2 MSI imagery within the Google Earth Engine platform. Monthly composites were generated from March 2017 to June 2025, and a trophic classification scheme was applied to quantify algal conditions across five transects representing diverse hydrological zones. The results reveal strong seasonal dynamics, with NDCI values peaking during the summer months, particularly in July and August. The 2019 freshwater‐driven HAB event was effectively captured, with elevated NDCI persisting for approximately 60 days. Spatially, transect 1 (adjacent to the Mississippi Sound) and transects 3 and 5 (influenced by riverine inputs and restricted mixing) exhibited the highest NDCI values. These areas frequently exceeded eutrophic thresholds, indicating persistent vulnerability to nutrient enrichment and reduced flushing capacity. Statistical analysis using the Kruskal–Wallis test confirmed significant differences in NDCI values across transects ( p < 0.001). Classification results showed that 32% of all monthly transect‐level observations were classified as “high eutrophic” (NDCI > 0.1), and 7.8% fell into “algal bloom” (NDCI > 0.4) or “severe bloom” categories. The study demonstrates that NDCI provides a sensitive and scalable means of monitoring Chl‐a variability in optically complex, shallow estuaries where traditional blue‐green algorithms underperform. This is the first long‐term, high‐resolution assessment of Chl‐a in Bay St. Louis using remote sensing. The findings contribute to understanding bloom dynamics in Gulf Coast estuaries and provide a replicable framework for water‐quality surveillance.