Marine environmental monitoring in the shelf zone of the Black Sea: Assessment of the current state of the pelagic ecosystem

The state of the shelf pelagic ecosystem has been assessed based on multidisciplinary monitoring performed in the northeastern Black Sea in 2005–2014. Seasonal and interannual variations in sea surface temperature (SST) and chlorophyll a (Chl-a) concentration have been analyzed along with the concentration of nutrients (silicate, nitrogen, and phosphate), biomass, and taxonomic compositions of phytoplankton, zooplankton, and gelatinous macroplankton. The linear trend shows an increase in the annual average SST by 0.9°C over the last decade. An increase in the winter SST is accompanied by a decrease in the concentration of silicates in spring (p < 0.05) and an increase in summer SST, by a decrease in Chl-a concentration and biomass of diatoms in the period of summer to fall (p < 0.05). A decrease in the phosphate concentration also has a negative effect on the development of diatoms (p < 0.01). The decrease in diatom biomass caused a decrease in herbivorous zooplankton biomass in the second half of the year (p = 0.05). Correlation analysis shows no significant dependence between the biomass of gelatinous top predators and mesozooplankton biomass. The assessed current state of the shelf pelagic ecosystem is regarded as stable; however, trends of a decrease in biomass and a change in the taxonomic composition of phytoplankton and zooplankton are observed during the last 2 years; the latter is likely to result from both direct and indirect effects of temperature increase in the upper sea layer.