BENTHIC FAUNA OF THE SOUTH-WESTERN PART OF THE SEA OF AZOV: TRANSFORMATION OF MACROZOOBENTHOS BIOCOENOSIS AT THE BEGINNING OF THE XXI CENTURY

Abstract

Cerastoderma biocoenosis has been one of the main biocoenoses that occupied the most extensive areas and prevailed in quantitative development in the benthos of the Sea of Azov for more than 80 years, since the first quantitative description were made there in 1930s. However, the penetration of the bivalve Anadara kagoshimensis (a recent invader to the Black Sea basin) to the Sea of Azov in 1989 determined the beginning of subsequent biocoenotic changes. Systematic benthic surveys were conducted in 2016–2017 during cruises #84, #86, #90, #96, #100 aboard R/V “Professor Vodyanitsky” on the extensive south-western section of the Sea of Azov. The results of the surveys revealed the formation of a single Anadara biocoenosis, in which the previously dominant native species Cerastoderma glaucum and Abra segmentum became subdominants. The present research continues the analysis of these data. Its purpose is to assess the features of transformation the Cerastoderma biocoenosis, which was the main native biocoenosis in the Azov Sea – to the biocoenosis of alien species Anadara kagoshimensis. The paper uses materials of three time-periods corresponding to different levels of salinity of the Sea of Azov: 1930s (9.3–11.1 %), the 1950s (11.6–12.1 %) and 2016–2017 (13.5–14.0 %). The results of the analysis showed a high level of similarity (60 %) in the composition of the cores of two native biocoenoses of Abra and Cerastoderma and the new Anadara biocoenosis. The transformation of the native biocoenoses on the southwestern section of the basin into the Anadara biocoenosis resulted both from the successful expansion of A. kagoshimensis and a decrease in the quantitative development of the aboriginal species complex under the conditions of increasing salinity of the basin. A compensatory increase in the quantitative development of zoobenthos was observed in 2016–2017 exceeding that of the 1930s due to A. kagoshimensis. From 1930s to 1950s the number of species in the Cerastoderma biocoenosis in the groups of Mollusca, Polychaeta and “Other species” did not change significantly, but in Crustacea group it decreased by more than 2 times. From 1950s (Cerastoderma biocoenosis) to 2016-2017s (Anadara biocoenosis) there was a nearly two-fold increase in the number of species in the groups of Mollusca, Polychaeta and “Other species” while the number of Crustacea species increased slightly.