Seasonal and Long-Term Phytoplankton Dynamics in the Middle Caspian According to Satellite Data and In Situ Observations in the First Decades of the 21st Century

Abstract

Seasonal and long-term variability of phytoplankton in the Middle Caspian was studied based on remote sensing data of the sea by SeaWiFS and MODIS-Aqua scanners in 1998–2021 and the results of field observations in 2004–2021. The seasonal variability of chlorophyll “a” (CHL) calculated from satellite data using a regional algorithm indicated that the autumn and winter seasons were the main phytoplankton production periods of the year. In summer, a period of stagnation was observed in phytoplankton growth in the surface layer. However, according to satellite data in the first months of each year, winter blooms were observed recurrently in the Middle Caspian Sea, as confirmed by the results of field observations in 2004–2021. The phytoplankton biomass during the winter vegetation period reached 4.5–5.0 g/m3. In the first decade of the century (2004–2006), winter blooms were almost entirely (as much as 96%) formed through the mass growth of the alien diatom Cerataulina pelagica. In the modern period (2021), the winter bloom was formed both by three endemic diatom species Thalassionema nitzschioides, Cyclotella comta and Dactyliosolen fragilissimus and by two alien species Pseudo-nitzschia seriata and Cerataulina pelagica. In spring, the diatom Cyclotella caspia and the dinoflagellate Prorocentrum micans, both endemics, dominated. In summer, the phytoplankton biomass was composed of the mass growth of small flagellates and dinoflagellates in the seasonal thermocline layer, which current remote sensing methods cannot record. The diatoms’ contribution to the community’s total biomass in summer did not exceed 3%. In the autumn phytoplankton, the main role was played by a diatom component represented by alien species, mainly Chaetoceros peruvianus.