INORGANIC SULFUR TURNOVER IN OLIGOHALINE ESTUARINE SEDIMENTS

Inorganic sulfur turnover was examined in oligohaline (salinity < 2 g kg-1) Chesapeake Bay sediments during the summer. Cores incubated for < 3 hr exhibited higher sulfate reduction (SR) rates (13-58 mmol m-2 d -1) than those incubated for 3-8 hr (3-8 mmol m-2 d-1). SR rates (dete rmined with (SO42-)-S-35) increased with depth over the top few cm to a maximum at 5 cm, just beneath the boundary between brown and black s ediment. SR rates decreased below 5 cm, probably due to sulfate limita tion (sulfate < 25 muM). Kinetic experiments yielded an apparent half- saturating sulfate concentration (K(s)) of 34 muM, almost-equal-to 20- fold lower than that determined for sediments from the mesohaline regi on of the estuary. Sulfate loss from water overlying intact cores, pre dicted on the basis of measured SR rates, was not observed over a 28-h r incubation period. Reduction of (SO42-)-S-35 during diffusion experi ments with intact core segments from 0-4 and 5-9 cm horizons was less than predicted by non-steady state diagenetic models based on (SO42-)- S-35 reduction in whole core injection experiments. The results indica te that net sulfate flux into sediments was an order of magnitude lowe r than the gross sulfur turnover rate. Solid phase reduced inorganic s ulfur concentrations were only 2-3 times less than those in sediments from the mesohaline region of the Bay, despite the fact that oligohali ne bottom water sulfate concentrations were 10-fold lower. Our results demonstrate the potential for rapid SR in low salinity estuarine sedi ments, which are inhabited by sulfate-reducing bacteria with a high af finity for sulfate, and in which sulfide oxidation processes replenish the pore water sulfate pool on a time scale of hours.