Macrozoobenthic diversity of eutrophication hotspots under anthropogenic pressures in the southern Black Sea: Interactions between environmental factors and community structure.

The Black Sea is among the marine systems most affected by eutrophication, but benthic responses along its southern shelf remain poorly documented compared with the intensively studied northwestern shelf. Soft-bottom assemblages at ten stations off the Sakarya and Yeşilırmak river mouths, Samsun Port and the Samsun wastewater treatment plant were examined in July 2019 and January 2020, together with physicochemical conditions and anthropogenic pressures. In total, 118 species and 13,729 individuals were recorded; Polychaeta accounted for 72.8% of individuals, with dominancy of Aricidea claudiae, Prionospio maciolekae and Heteromastus filiformis. The stations formed four clusters, each containing both seasons, so differences between stations exceeded those between seasons. Canonical correspondence analysis on seven non-collinear variables, retained after collinearity screening and forward selection, was significant (F = 2.28, P = 0.002); depth, phosphate and coarse sand showed the strongest conditional effects. Ecological quality was assessed with two biotic indices. TUBI and M-AMBI*(n) ranked the stations concordantly (rs = 0.64, P = 0.002), both placing the outermost river-mouth station highest and the Sakarya and wastewater stations lowest, although M-AMBI*(n) assigned higher classes at several stations. Four alien species occurred at low density, leaving all stations at high ALEX status. Community structure was associated with a coupled depth-salinity-enrichment gradient rather than with nutrient enrichment alone.