Analysis of seasonal distribution of chromophoric dissolved organic matter in turbid estuaries applying a novel semi-analytical algorithm

Abstract

The study presents a novel semi-analytical algorithm to improve accuracy of chromophoric dissolved organic matter (CDOM) retrieval from remote sensing reflectance ($R_{\mathrm{rs}}$) in optically complex waters. The bio-optical data used in the study were collected from monsoonal estuaries and coastal waters along the eastern Arabian Sea, and supplemented with repositories of Global Ocean Carbon Algorithm Database and NASA bio-Optical Marine Algorithm Dataset. To retrieve CDOM absorption at 440 nm ($a_{\mathrm{cdom}}^{440}$), a three-wavelength index of the form, $x=\left(\frac{1}{R_{\mathrm{rs}}^{{\lambda }_1}}-\frac{1}{R_{\mathrm{rs}}^{{\lambda }_2}}\right)\times R_{\mathrm{rs}}^{{\lambda }_3}$ was developed based on the fundamental relation between $R_{\mathrm{rs}}$ and inherent optical properties (absorption and backscattering). This index was regressed and fine-tuned with randomly chosen in-situ data representing different optically complex regions. The wavelengths ${\lambda }_1$, ${\lambda }_2$, and ${\lambda }_3$, are 412 nm, 490 nm and 560 nm. The resultant algorithm is $a_{\mathrm{cdom}}^{440}=-0.01368x^2+0.102x+0.02295$. The validation between in-situ measured and satellite-derived (Ocean and Land Colour Instrument and Moderate Resolution Imaging Spectroradiometer sensors) $a_{\mathrm{cdom}}^{440}$ values were statistically assessed for the global as well as optically diverse regional subsets of the Gulf of Mexico, Chesapeake-Delaware Bay (CDB) and Mandovi-Zuari estuaries (MZE). Their performance over the in-situ ($r^2$ > 0.6; $\mathrm{mape}$ < 45 %) as well as satellite-retrieved reflectance ($r^2$ = 0.72–0.89, and $\mathrm{rmse}$ = 0.124–0.2686 m⁻¹) surpassed retrieval by widely-used empirical, machine learning, and semi-analytical models (In-situ:- $r^2$ = 0.05–0.52; $\mathrm{mape}$ = 53–1167 % & satellite-retrieved: $r^2$ = 0.09–0.86; $\mathrm{rmse}$ = 0.1124–1.065 m⁻¹). Application to the MZE and CDB regions revealed spatial and seasonal variability in CDOM, reflecting their surface physical and biogeochemical forcing.