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Assessment of MERIS ocean color products using in situ data collected in the Northwestern Black Sea

In this study the performance of Medium Resolution Imaging Spectrometer (MERIS) geophysical products were evaluated using in situ measurements in northwestern part of the Black Sea. The in situ chlorophyll a (Chl-a), total suspended matter (TSM) and Scechi disk depth (SDD) were collected from two Institute of Oceanology, BAS cruises during spring season of 2006 and 2009. A strict comparison method between two data sets was adopted resulting in 27 and 33 match ups for TSM and Chl a respectively. The method comprised the arithmetic average of valid MERIS values calculated over the 3 × 3 - pixels square found within time difference of ± 3 h from the in situ measurement. Additionally, classification of the in situ data was performed, in order to assess accuracy of MERIS algal_2 and TSM products in respect to the sea water with different optical properties. The in situ data were subdivided into three groups to represent clear, moderate turbid and turbid waters by applying K - mean clustering method on the measured concentrations of Chl-a, TSM and transparency. Statistical analysis of MERIS derived TSM and Chl-a and in situ data were carried out for the different water types. In the case of TSM concentrations, was found a reasonably good agreement between two data sets in clear and moderate turbid waters. The estimated R2 are greater than or equal to 0.66 and log_RMS differences for clear and moderate turbid waters ranging between 0.09 g/m3 and 0.19 g/m3. The similar correlations were estimated for Chl-a concentrations, but the value for log_RMS in turbid waters is higher to those obtained for TSM. For both variables, the best results of comparison were found for the moderate turbid waters, that corresponding to high coefficients of determination (R2>0.75) and small log_RMS differences of 0.1 mg/m3 for Chl-a and 0.09 g/m3 for TSM. The present study suggested that MERIS Case 2 products are reasonably accurate in moderate turbid shelf waters, but not yet reliable in turbid coastal waters of the northwestern Black Sea.