The article is devoted to the study of biological characteristics of the north Caspian roach of the Daghestan coast of the Caspian Sea. Discussed are size and weight, fatness, passing the stage of gonad maturity, the absolute individual fecundity and feeding habits. The degree of occurrence of organisms in the food lumps with identification of dominant groups is determined.

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.

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Aim. Analysis of new data on populations of invasive ctenopfores Вeroe ovata and Mnemiopsis leidyi in the Caspian Sea and assessment of adaptation of B. ovata to new habitat conditions. Material and methods. The data was obtained in the Dagestan sector of the Middle Caspian Sea in September-November 2021. Ctenophores and zooplankton were collected by a big cone plankton net (BC) with a 0.5 m 2 opening and a Juday plankton net (0.1 m 2 opening). Small ctenophores, eggs and zooplankton content were determined in samples fixed with formaldehyde to a final concentration of 4%. Results. In September-October 2021, the Вeroe ovata were detected in the shelf waters of Dagestan. The total number of ctenophores reached 32 ind/m 2 , biomass -10 g/m 2 . Individuals up to 35 mm in size were recorded. The abundance of M. leidyi in areas where B. ovata was found 3-10 times less often than in areas where it was absent. The habitat of Вeroe ovata was limited to the southern part of the Dagestan shelf and a salinity range of 7.1-5.5 psu. Conclusion.In 2021, new data was obtained on the Вeroe ovata in the Caspian Sea. The detection of B. ovata in the Caspian Sea in 2021 indicates that the ctenophore overcame winter conditions of the Caspian Sea. The fact determines the further evolution of the Caspian Sea ecosystem. The parameters of the impact of the new alien on the population of M. leidyi were recorded. An analysis of B. ovata distribution in the waters of the Dagestan shelf depending on environmental conditions was carried out.

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.