CONTEMPORARY STATE OF BOTTOM BIOCENOSES OF THE GELENDHIK BAY OF THE BLACK SEA

Abstract

The state of macrozoobenthos and meiobenthos of the Gelendzhik Bay of the Black Sea was assessed in 2020–2021 based on three bottom grab surveys. Five types of biotopes were identified based on the particle size distribution of the bottom sediments, the redox potential of pore water, the content of organic carbon, and the type and presence of macrophytobenthos. A study of the oxygen content in the water column did not reveal hypoxia. Four types of macrozoobenthos communities were noted in the bay. The main dominants were bivalves and gastropods. In the central part of the bay, the communities were similar to typical Black Sea communities from the venus sand belt. The spatial structure of macrozoobenthos was associated with the type of biotope identified on the basis of abiotic characteristics and benthic vegetation. Macrozoobenthos was represented mainly by mobile burrowing infauna, belonging to the type of food filter feeders with a bivalve shell (bivalves). The meiobenthos of the bay was represented mainly by nematodes, which dominated both coastal areas and deeper waters. The spatial structure of the meiobenthos was determined by the granulometric composition of the bottom sediments, and the dependence on the type of vegetation and the redox potential of pore water was unreliable. In the coastal area, the reducing conditions were found in the surface layer of the bottom sediments, and also the low abundance and biomass of macrozoobenthos. The station with reducing conditions was distinguished by the dominance of larger (up to 5 cm) worm-like segmented polyphages and predators and the absence of juvenile forms of macrozoobenthos. At the station with reducing conditions, the lowest species diversity of meiobenthos was noted, and the nematode-copepod index reached 620:1, which is an order of magnitude higher than at other stations in the bay. A comparison of the current state of communities with historical data (1979, 1990) was made. It is shown that at the present stage the biodiversity of the bay has increased, while the abundance of macrozoobenthos has decreased. There were no stable areas of hypoxia or freezes in the bottom layer of waters.