Spectral variations of the remote sensing reflectance during coccolithophore blooms in the Western Black Sea

Among the many calcifying marine organisms, coccolithophores are the major producer of particulate inorganic carbon (PIC). Calcium carbonate plates covering coccolithophores, called coccoliths and released during blooms, are responsible for a large increase of the water reflectance. Aiming at investigating the spectral features of the remote sensing reflectance RRS(λ) of marine waters during coccolithophore blooms, this study exploits the radiometric data of Ocean Color sites of the Aerosol Robotic Network (AERONET-OC) in the Western Black Sea (Galata and Gloria/Section-7) by focusing on bloom events that occurred in 2017 and 2020. The analysis, besides showing elevated RRS(λ) in the blue-green spectral region in the presence of coccoliths, confirms a shift toward the blue of the RRS(λ) spectra as the bloom declines and coccoliths accumulate at the surface. Results also document a decreased capability of determining the bloom state in the presence of optically complex waters such as those associated with river runoff. Finally, the comparison of satellite versus AERONET-OC radiometric data for the extreme conditions created by the presence of coccolithophores, indicates agreements between RRS(λ) not significantly different from those previously determined for various satellite data products in the absence of appreciable concentrations of coccoliths.