Using MERIS/Envisat data to assess the Secchi depth — A case study from Lithuanian Baltic Sea waters

In this study water transparency measured in situ with Secchi disk was compared with maximal remote sensing signal depth Z90_max derived from MERIS/Envisat full resolution (300 m) data in the Lithuanian optically Case 2 coastal waters of the Baltic Sea. Eight MERIS full resolution Level 1b images from late spring and summer of 2010 were processed using three neural network based processors for optically Case 2 or coastal and inland waters - C2R, Eutrophic, Boreal. In the results we found that Eutrophic processor gave the best validation result (R²=80%) when salinity was >; 6.2 PSU, whereas Boreal was better in the plume area, where salinity <;6.2 PSU (R²=40%). The redundancy analysis indicated that salinity and temperature were the most important environmental factors explaining the deviation between estimates by processors and in situ measured Secchi depth. Thus, the plume zone of the freshwater from hypereutrophic lagoon was the most sensitive region for the validation. Consequently, the investigation confirmed Z90_max as a proxy for the Secchi depth and the sufficiency of satellite based remote sensing technique to monitor the changes of water transparency in the Lithuanian coastal waters.