Phytoplankton of the middle Caspian Sea: analysis of changes in the structure of the community over the past decades

Aim.  Analysis  of  changes  in  quantitative  and  structural  indicators  of phytoplankton in the western and central part of the middle Caspian Sea over the past decades, including according to remote sensing data.Material  and  Methods.  The  data  was  obtained  in  2004–2008  and 2019–2022 at different seasons of the year at 40 stations in the central and western part of the middle Caspian Sea. Phytoplankton samples were taken  from  4–6 layers.  A  total  of  300 samples  of phytoplankton  were analyzed. Determination of species and counting of the number of cells was carried out under the “Ergaval” light microscope. WoRMS guided matters of nomenclature.Results. The spring phytoplankton is dominated by the species traditional for the Caspian Sea – Cyclotella caspia diatoms and Prorocetrum micans dinoflagellates. The maximum abundance of C. caspia (5.0 x 104 cell/l) was recorded at depths of 35–40 m. In summer, the maximum phytoplankton biomass (2.2 g/m3) was noted in the seasonal thermocline and was formed due to small flagellates and dinoflagellates. Phytoplankton biomass during winter blooms reached 4.5–5.0 g/m3 and was determined by the development of diatoms (up to 96–99%). Winter blooms were formed by the diatom species traditional for the sea, as well as by the Pseudo‐nitschia seriata and Cerataulina pelagica species.Conclusion. It is shown that in the middle Caspian Sea, the winter and autumn seasons are characterized by a highly productive status. In January–February, periodic blooms of diatoms are observed, as confirmed by satellite data and in situ observations. In summer, phytoplankton biomass is determined by the mass development of dinoflagellates in the seasonal thermocline layer, which has not been recorded by remote methods. In the autumn phytoplankton the main role is played by the diatom component, represented mainly by alien species.