An extensive validation of ocean color products for the black sea

Ocean color remote sensing provides an efficient approach to monitor the state and changes in water bodies on Earth. The validation of an ocean color product is a key step in assessing its accuracy, directly influencing the confidence with which conclusions can be drawn. In this study, we compiled a comprehensive in situ optical and biogeochemical dataset to assess the performance of ocean color products provided by the Copernicus Marine Service (CMEMS), Copernicus Evolution – Research for harmonized and Transitional water Observation (CERTO), as well as products distributed by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) in the Black Sea. Results showed that both the CMEMS and CERTO provide accurate remote sensing reflectance in the Black Sea at visible bands with the highest accuracy at 555 nm or 560 nm, reaching a mean absolute percentage deviation (MAPD) lower than 14%, but higher errors were found in the short-blue (700 nm) bands. Chlorophyll-a concentration (Chl-a) products derived from two CMEMS datasets: multi-sensor (CMEMS MULTI) and Sentinel-3 OLCI (CMEMS OLCI), and two CERTO datasets: Sentinel-3 OLCI (CERTO OLCI) and Sentinel-2 MSI (CERTO MSI), agreed with in situ-measured values. However, the Chl-a products from EUMETSAT showed larger errors, with MAPDs higher than 150%. The total suspended matter (TSM) concentration products from the EUMETSAT OLCI, CERTO OLCI and CERTO MSI showed good agreements with the in situ data. The two diffuse attenuation coefficients for downwelling irradiance at 490 nm (Kd(490)) products from CMEMS showed a good accuracy with MAPDs lower than 20%. Recommendations based on the results are provided to further improve the ocean color products in the region, including merging ocean color algorithms developed from different services based on optical water type classifications.