Optical mediation of chlorophyll-a trends and radiative heating in coastal waters

Long-term trends in satellite-derived chlorophyll- a (Chl- a ) in optically complex coastal waters may reflect changes in multiple constituents rather than phytoplankton biomass alone. Using satellite Chl- a observations from 1998-2023 and 10 years of in situ hyperspectral remote-sensing reflectance data, \(Rᵣₛ(λ )\) (sr \(⁻¹\) ), together with derived constituent-specific absorption coefficients from the eastern Arabian Sea, we identified coherent positive and negative trends and classified eight optical water types. Positive-trend types generally exhibited greater particulate absorption, whereas negative-trend types showed greater fractional coloured dissolved organic matter (CDOM) absorption at shorter wavelengths. Three-stream radiative-transfer modelling revealed corresponding differences in dimensionless subsurface spectral irradiance reflectance, \(R(z,λ )=E_u(z,λ )/[E_d(z,λ )+E_s(z,λ )]\) , euphotic depth, and radiative heating rate. Positive-trend types exhibited mean euphotic depths of 4.0-5.3 m, whereas negative-trend types portrayed shallower values of 3.1-3.6 m and more surface-intensified heating. CDOM-enhanced types exhibited approximately 1.18-fold greater depth-integrated radiative heating within their respective euphotic zones, despite reduced water-leaving reflectance at strongly absorbed wavelengths. These results demonstrated that constituent composition and spectral absorption mediated both the expression of long-term variability in satellite ocean colour and the vertical distribution and transmission of solar irradiance.