SEDIMENT SULFUR DYNAMICS RELATED TO BIOMASS-DENSITY PATTERNS IN ZOSTERA-MARINA (EELGRASS) BEDS

Rates of sulfate reduction and the cycling of sulfur were measured in estuarine sediments vegetated with Zostera marina L. (eelgrass), and i n 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 line ar correlation between shoot density and depth-integrated sulfate redu ction rates, and rates were 5-fold higher at the dense station (59.1 m mol m(-2) d(-1)) than at the bare site (12.2 mmol m(-2) d(-1)). The ac cumulation of particulate organic matter was low in the vegetated sedi ments, and there was no correlation between the organic content and mi crobial activity. The accumulation of dissolved organic carbon (DOC) w as higher in the vegetated sediments, whereas pools of short chain fat ty acids were low (< 5 mu M) at all stations. There was a positive rel ationship between the depth-integrated DOC pool and sulfate reduction rates along the shoot density gradient, indicating a direct plant effe ct probably from a production of labile organic matter within the eelg rass bed. Sulfate reduction rates were primarily enhanced in the roote d zone. The cycling of sulfur compounds was rapid, as there was an acc umulation of dissolved sulfides in the pore waters throughout the exam ined 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, i ndicating a rapid reoxidation of sulfides at the location.