The contents of labile sulfides in the bottom sediments of the central part of the Sea of Azov: Impact on benthic communities

At 17 stations in the central part of the Sea of Azov, the contents of labile (acid-soluble) sulfides were determined in the upper layer (0-3 cm) of the bottom sediments. At 14 of these stations, the contents of sulfides in the oozy silts were over 300 mg S/dm(3) of wet sediment; at seven of the stations, the contents of sulfide were as high as 420-720 mg S/dm(3) of wet ooze, or 0.1-0.2% of dry weight. At the other three stations, where neutral matter such as shells and sand prevailed in the samples, the contents of sulfides varied from 80 to 110 mg S/dm(3). At these stations, a high density and species diversity of the benthic fauna was retained. At other stations with labile sulfide contents over 200-300 mg S/dm(3), the benthos biomass decreased by one or two orders of magnitude. At most of them, it was below 3 g/m(2) and the small gastropod Hydrobia tolerant to sulfides dominated. The data obtained show that, in the central part of the Sea of Azov, reduced sediments with high contents of labile sulfides migrate towards the bottom surface, which conforms to the high intensity of the hydrogen sulfide formation process caused by the bacterial sulfate reduction. The study considers the environmental effects of the sulfide contamination of the upper layer of the bottom sediments in the Sea of Azov as a key factor causing the recurrent hypoxy in the near-bottom layers of the water, the suffocation occurrence, and the progressive depletion of the benthic and pelagic fauna.