The Role of Abiotic Environmental Factors in the Vertical Distribution of Macrozoobenthos at the Northeastern Black Sea Coast

In this study, an attempt is made to evaluate the impact of environmental factors on the vertical distribution of the macrozoobenthos at the northeastern shelf of the Black Sea. The samples of macrozoobenthos were taken by an Ocean grab at the depths of 10–70 m in 2014–2017 along four transects between the Abrau Peninsula and the settlement of Shepsi. The particle size distributions of the bottom sediments and the near–bottom water temperature were also measured. Two major groups of stations were distinguished according to the quantitative structure and composition of the macrozoobenthos—shallow (10–40 m), corresponding to variable temperature conditions, and deep (50–70 m), corresponding to constant temperature conditions. Two subgroups were distinguished in the shallow zone coinciding with the upper mixed layer (10–15 m) and thermocline (20–40 m). The first subgroup was characterized by sandy sediments inhabited by the community of the suspension feeder bivalve, Chamelea gallina; silty sands at the stations of the second subgroup contained a community dominated by several Veneridae species. Silty sediments were spread all over the deep-water zone; the pelophylic bivalves Parvicardium simile and Modiolula phaseolina were dominant. The ratio of silt to sand fractions in the bottom sediments and the near-bottom water temperature were critical factors in distinguishing between the groups of stations. The subgroups were distinguished according to the percentage of aleurite and pebble. The coefficient of sediment sortedness, which reflects the stability of the hydrodynamics and the rates of sedimentation, also influences the differences (mostly due to Bivalvia). The total variation explained by depth almost overlapped with the variation accounted for by other factors. The major role in distinguishing between the groups belonged to the two taxa constituting most of the biomass (Bivalvia) and the density and species diversity (Polychaeta).