Estimating underwater light attenuation from space – A spectral approach for case 2 marine waters

Water clarity is a key indicator of marine ecosystem health, responding to eutrophication and influencing the distribution of phototrophic life on the seafloor. Traditional ship-based measurements of underwater light profiles and diffuse light attenuation coefficients (K_d) remain fundamental for environmental monitoring, including in Danish waters. However, satellite-based estimates of water clarity are often challenged by optically complex (case 2) waters and limited spectral coverage. This study presents a novel empirical spectral modelling approach across the range of photosynthetically active radiation (PAR) from 400 to 700 nm that Model-derived K_d^PAR estimates were validated against in situ measurements from Danish marine waters (2018-2023) and compared with two simpler non-spectral satellite-based approaches. Results showed strong agreement between modelled and in situK_d^PAR (n = 1458, R = 0.63), outperforming existing satellite-based methods. The new K_d^PAR product effectively captured spatial and temporal variability in complex case 2 waters, though discrepancies were noted in optically shallow areas, heterogeneous water columns, and seasonal phytoplankton shifts. These findings highlight the potential of operational satellite-derived IOP products as a robust proxy for water clarity, providing a valuable supplement to limited in situ data. The improved spatial and temporal coverage supports enhanced environmental monitoring, aligning with EU Water Framework and Marine Strategy Framework Directives.