Rates of sulfate reduction and the cycling of sulfur were measured in estuarine sediments vegetated with Zostera marina L. (eelgrass), and in adjacent bare sediment, in summer during the intensive period of the growth season. Sulfate reduction rates were determined along a shoot density gradient (210 to 1026 shoots m-2). There was a positive linear correlation between shoot density and depth-integrated sulfate reduction rates, and rates were 5-fold higher at the dense station (59.1 mmol m-2 d-1) than at the bare site (12.2 mmol m-2 d-1). The accumulation of particulate organic matter was low in the vegetated sediments, and there was no correlation between the organic content and microbial activity. The accumulation of dissolved organic carbon (DOC) was higher in the vegetated sediments, whereas pools of short chain fatty acids were low (<5 μM) at all stations. There was a positive relationship between the depth-integrated DOC pool and sulfate reduction rates along the shoot density gradient, indicating a direct plant effect probably from a production of labile organic matter within the eelgrass bed. Sulfate reduction rates were primarily enhanced in the rooted zone. The cycling of sulfur compounds was rapid, as there was an accumulation of dissolved sulfides in the pore waters throughout the examined sediment layer (0 to 8 cm) in the vegetated sediments Burial of precipitated reduced sulfides increased with increasing shoot density, but the pools were low when related to the sulfate reduction rates, indicating a rapid reoxidation of sulfides at the location.
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