Current status of holo- and meroplankton of the Sea of Azov during the formation of the ice cover

The Sea of Azov is an inland freezing marine water basin. Winter season is considered to be one of the most important seasons for understanding patterns of functioning and formation of productivity of the ecosystem of the Sea of Azov. However, holo- and meroplankton during the formation of ice cover in the sea have not been studied enough. In recent years, several alien species, including Arctic species of polychaete worms, which in their development have the stage of pelagic larvae, have naturalized in the Sea of Azov. The aim of the work is to study the taxonomic composition and numerical abundance of winter holo- and meroplankton of the Sea of Azov in December 2018. Zooplankton sampling was conducted in the bays of the Sea of Azov, viz., Taganrog and Temryuk during the formation of seasonal ice cover. Zooplankton samples were collected from December 3 to 14 at temperatures from 0 to +3 °C at 14 stations, 9 of which were performed in the Taganrog Bay (the port area of Yeisk) in three replications, and 5 of which – in the Temryuk Bay (each sample – in one replication). Zooplankton was sampled throughout the water column at depths of 4–8 meter using a big-sized Juday net with an opening diameter of 37 cm (mesh size was 120 μm) by total catch. The material was fixed by 2–4 % neutral formaldehyde and treated in the laboratory by the conventional procedure. Calculations of biomass were made using the tables of the average mass of organisms. The results showed that under similar temperature conditions the density of holo- and meroplankton organisms in the Taganrog Bay was four times higher than in the Temryuk Bay. Winter subglacial zooplankton was represented by two groups of organisms – native eurythermic forms of holoplankton and polychaetes larvae. As before, calanoid copepod composition was dominated by euryhaline Ponto-Caspian species Eurytemora affinis (Poppe, 1880). However, the species composition of the winter meroplankton of the Sea of Azov changed significantly in comparison with that of the period up to 2014. Unusual high density (118–119.9 thousand ind.·m−3) of polychaete larvae of Marenzelleria genus, the recent invader in the Sea of Azov, was registered in the Taganrog Bay at a low water temperature of 0…+1.2 °C. The peak of zooplankton numerical density (128.9–136.7 thousand ind.·m−3) was observed in winter season for the first time. Winter subglacial maximum of abundance of the polychaetes larvae of Marenzelleria sp. was 4–6 times higher than the abundance of meroplankton, previously noted in June, the most productive month of the year. Naturalization of polychaete worms Marenzelleria sp. can lead to a radical restructuring of the Sea of Azov ecosystem and to an increase of its productivity. Further studies of the phenology of these polychaetes larval stages in this basin are needed.