A remote sensing algorithmic framework for characterizing chromophoric dissolved organic matter in lagoon-estuarine...

Chromophoric Dissolved Organic Matter (CDOM) significantly influences coastal biogeochemistry, yet its variability remains understudied in dynamic estuarine systems. This study presents a remote sensing algorithmic framework for estimating CDOM optical properties in coastal lagoon-estuarine systems with strong seasonal and spatial variability. As a proof of concept, the proposed optical framework is developed using Sentinel-2 derived spectral remote sensing reflectance and one-year (2018-2019) of in situ spectral CDOM absorption in Chilika Lagoon, Asia's largest brackish water lagoon. The framework enables the retrieval of a comprehensive suite of CDOM optical properties, including absorption coefficients in the ultraviolet-blue spectral region, CDOM spectral slopes, and the spectral slope ratio. A novel metric, the Normalized Spectral Slope Ratio (NSSR), is proposed to integrate spectral slope and absorption magnitude into a single, robust indicator of CDOM variability and source characterization. Applying this framework to Sentinel-2 MSI images, we observed a 36 % increase in CDOM absorption post-Super Cyclone Amphan, particularly in the river-influenced northern sector of the lagoon. This increase resulted from the surge in freshwater discharge to the lagoon driven by extreme cyclone-induced precipitation approximately 10 times higher than the pre-cyclone levels (average ~ 7.15 mm). The 33 % decrease in NSSR and 47 % increase in CDOM absorption indicated an influx of high molecular weight terrestrial CDOM, significantly altering the lagoon's biogeochemical balance. These findings demonstrate the framework's effectiveness in providing a comprehensive understanding of CDOM dynamics and source characterization in coastal lagoon-estuarine systems and highlight the sensitivity of estuarine ecosystems to extreme meteorological events.