Variability in the Light Absorption Coefficients of phytoplankton, non-algal particles and colored dissolved organic matter in European Coastal Waters: A Two-Year Dataset in the Context of Historical Observations

Abstract. Coastal ecosystems play a major role in global biogeochemical cycles, which are challenging for ocean color satellite observations due to independent variability in the dynamics of phytoplankton, non-algal particles (NAP), and colored dissolved organic matter (CDOM). Field measurements and match-ups with satellite data are required to assess the validity of multi-sensor satellite products and optimize (or develop new) bio-optical inversion algorithms. For this purpose, the water reflectance, light absorption and backscattering properties of coastal waters must be measured together with the concentration, composition and size of the optically-active water constituents. In this study, we report the light absorption properties of phytoplankton, NAP and CDOM measured over two years along European coastal waters. Our results show distinct contributions of CDOM from the Atlantic to Mediterranean waters, with CDOM dominating the non-water absorption budget in most regions between 380 and 560 nm. Concerning phytoplankton absorption, the B95 model (Bricaud et al., 1995), linking chlorophyll concentration to phytoplankton absorption, reproduces the overall spectral shape but exhibits regional biases, with both over and underestimation, depending on the local phytoplankton community size structure, physiological photo-acclimation states, and specific taxonomic compositions. These results highlight limitations in the current parameterizations under contrasting coastal optical regimes. The mass-specific absorption coefficient of NAP (a_NAP^∗​) reflects variability in particle assemblages and river inputs, with a systematic significant contribution in the near-infrared (NIR) part of the spectrum. Overall, our results provide improved constraints on regional optical variability and contribute to the refinement of bio-optical models and ocean color algorithms in complex coastal environments.