Analysis of bio-optical properties in estuarine and coastal waters-study through in-situ and remote sensing observations.

Abstract

Estuarine and coastal waters are characterised by highly variable and optically complex conditions resulting from spatially and temporally heterogeneous distributions of optically ac tive constituents (OACs), notably phytoplankton pigments, total suspended matter (TSM), and chromophoric dissolved organic matter (CDOM). These aquatic environments, generally clas sified as Case II waters, challenge conventional remote sensing methodologies due to the non linear and spectrally overlapping contributions of absorption and scattering processes governed by these constituents. This thesis presents a comprehensive investigation of the bio-optical dynamics of estuarine and coastal ecosystems through an integrative approach combining multi-seasonal in-situ radiometric, spectrophotometric, and remote sensing observations with satellite-derived ocean colour products. The research specifically emphasizes the quantification and remote sensing of CDOM and its implications for radiative transfer processes and coastal ecosystem functioning. A semi-analytical retrieval algorithm was developed for the robust esti mation of CDOM absorption at 440 nm (a440 CDOM) across diverse optical regimes by isolating the CDOMsignal influencing the spectral reflectance and minimising the attenuation by the other OACs. Thedevelopedalgorithmwasoftheform, a440 the three wavelength index given as x = ( 1 R412 rs cdom = −0.01368x2+0.102x+0.02295, with − 1 ) ×R560 R490 rs rs . This algorithm was formulated and validated using an extensive global dataset of in-situ measurements comprising absorption and attenuation coefficients, water-leaving radiance, and CDOM spectral slopes, acquired from estuarine and nearshore coastal zones. Comparative evaluation against widely adopted empiri cal and semi-analytical algorithms demonstrated superior performance of the proposed model, yielding the highest coefficient of determination (r2 = 0.64) and the lowest error metrics (rmse = 0.1039, mae = 0.064, mape = 51.96%). Notably, its accuracy improved under conditions where CDOM exerted a dominant influence on the total absorption budget. The validation using satellite-derived data from MODIS-Aqua and Sentinel-3 OLCI further substantiated the algorithm’s applicability across a spectrum of Case II waters