Changes in the macrobenthic communities of the Gudauta Oyster Bank

Twenty-nine benthic invertebrate species were registered for the Gudauta Bank in the depth range of 8–32 m in 1990. The shallow-water biocenose was defined for the depths of less than 10 m, the biocenoses of Anadara inaequivalvis and A. inaequivalvis-Upogebia pusilla occupied the depths of 10–30 m; and the biocenosis of Pitar rudis was found for the depths of more than 30 m. The highest biodiversity and abundance of benthos was registered for the Anadara biocenoses, while the lowest, for the shallow-water ones. It was found that significant changes in the benthic communities took place in the forty years after the predator gastropod Rapana venosa was introduced into the Black Sea. The oysters became extinct on Gudauta Bank, and the species composition of the benthic community has been significantly depleted. Alien species of Anadara and Rapana became the most abundant by biomass, comprising more than 80% of the total biomass of the benthos. The population density of Rapana constituted 12 ind./m 2 and was the highest ever registered for the studied areas of the Black Sea. Insufficient nutrition preconditioned the small size of the mollusks.