Spatial Variability of Zooplankton and Hydrological Indicators of the Waters of the Southern and Central Baltic Sea in Late Summer of 2016

The late-summer zooplankton features of the structure and distribution in the South Baltic and the Baltic Proper (deeps 55–215 m) in 2016 were revealed, taking into account hydrological conditions. The taxonomic diversity of zooplankton was higher in the Baltic Proper (surface salinity ~6.5 psu), and the total quantitative indicators of zooplankton were lower than in the southern Baltic Sea (surface salinity ~7.5 psu), i.e., 18 and 14 species; 16 ± 11 and 36 ± 18 × 103 ind./m3, 208 ± 166 and 474 ± 182 mg/m3, respectively. In general, these values of zooplankton fall within the range of those typical for this season. The maximum abundance and biomass of zooplankton was recorded in the upper mixed layer up to the thermocline. The vertical distribution of particular species was determined by their ecophysiological features: warm-water species (Eubosmina maritima and Acartia tonsa) predominated in the warm surface layer, while cold-water stenohaline species (Pseudocalanus elongatus and Oithona similis) predominated in the layer below the halocline. As the water salinity decreased, brackish species were found: Keratella quadrata, Limnocalanus grimaldii grimaldii, as well as an increase in the share of Eurytemora affinis in abundance.