Seasonal dynamics of the sulphate reduction rate on the north-western Black Sea shelf

The microbial sulphate reduction rate (SRR) in the surface layer (0-30 cm) of shallow water sediments of the north-western Black Sea shelf has been measured using the 35S radioisotopic technique. It was shown that SRR is accompanied by changes of salt composition of porewaters and isotopic (d34S) composition of sulphate. The hydrogen sulphide-derived minerals are formed in the solid phase of sediments. An expected exponential dependence of the isotopic composition of reduced sulphur compounds on the SRR was observed. In near-shore sediments, terrestrial organic carbon (d13C = - 26) as well as marine phytoplankton organic carbon (d13C = - 22) are associated with the SR processes. In August 1995 and May 1997-1998, anoxic conditions in near-bottom waters of coastal areas were not detected. The average value of H2S production in nearshore sediments varied from 3·62 mmol S m-2 day-1 in spring to 8·89 mmol S m-2 day-1 in summer 1995; these fluxes were twice as high as those measured in spring 1997 and 1998