PLANKTONIC AND BENTHIC COMMUNITIES OF THE SOUTHEASTERN PART OF THE BALTIC SEA IN THE SUMMER PERIOD 2018–2019

During the expeditions in July 2018 and 2019 data were obtained on the state of summer plankton and benthos over the entire Russian water area in the southeastern part of the Baltic Sea. The highest abundance of phytoplankton (in terms of chlorophyll “a”) is typical for the coastal region, where it reaches the eutrophic state, and decreases in the seaward zone to the mesotrophic state. Brackish water conditions determined a rather small number of species of zooplankton (36 taxa) and zoobenthos (40 taxa), including invading species (4 in zooplankton and 6 in benthos). Copepoda dominated in zooplankton in the deep-water zone, while the proportion of Cladocera and Rotifera increased in the coastal zone. The abundance and biomass of zooplankton corresponded to the maximum level observed in recent years in the SouthEastern Baltic. Zooplankton was more abundant from the coast to depths of 40 m, as a result, the best feeding conditions for planktophagous fish extended at this area. In benthos, Polychaeta and Bivalvia dominated in abundance, and Bivalvia dominated in biomass. The hydrological conditions in the Russian zone determine the development of zoobenthos in two regions separated by the Gdansk depression, which is characterized by benthic “desert” conditions. The largest region extends from the coast to depths of 70–80 m, and the second – in the deep northwestern part and is confined to the Gdansk-Gotland Sill. High abundance and biomass of zoobenthos are characteristic of the Gdansk basin down to depths of 60 m, where favorable conditions are formed for benthophage fish. The species composition and abundance of zoobenthos on the Gdansk-Gotland Sill much lower and is determined by the presence of North Sea waters inflows, in particular, in 2019 there was an increase in the abundance and distribution of benthos as a result of the inflow of North Sea waters.