Interannual Variability of Bio-Optical Parameters of the Caspian Sea within EBSAs Based on Ocean Color Data

The results of calculating trends in the temporal variability of bio-optical characteristics of Ecologically or Biologically Significant Marine Areas (EBSAs) in the Caspian Sea are presented. The analysis is based on data from the MODIS-Aqua (Moderate Resolution Imaging Spectroradiometer) ocean color satellite sensor for the period from 2002 to 2025, processed using regional bio-optical algorithms developed at Shirshov Institute of Oceanology, Russian Academy of Sciences. Sea surface temperature time series were also analyzed to assess climate change in each zone. A comparison of the calculated characteristics for the EBSAs with those for the subregions of the Caspian Sea revealed good agreement. Negative significant trends in satellite estimates of chlorophyll-a concentration (from -0.016 to -0.008 mg/m3 per year) are observed in EBSAs of the Southern Caspian Sea, whereas on the eastern coast (from Turkmen Aylagy to Kendirli Bay) and in the Northern Caspian, a predominantly positive trend is established with a maximum value of +0.027 mg/m3 per year in the EBSA N14. Due to changes in the Caspian Sea level, an increase in suspended matter concentration is predominantly observed – a positive significant trend is established in 8 EBSAs out of 16. A significantly positive trend in sea surface temperature was recorded only in one zone, related to the northeastern coast of the Caspian Sea (+0.24 °C per year).