Speciation of mercury in aquatic systems depends, in large part, on the pH and concentrations of dissolved organic carbon (DOC), sulfide and dissolved oxygen (DO) in surface and porewaters. Spatial and temporal monitoring of these constituents, which are critical to the understanding of processes controlling mercury bioavailability, were conducted at an oligotrophic and a eutrophic site in the South Florida Everglades. Close-interval vertical measurements of sulfide and DO concentrations were made using water-quality multi-probes, a DO microprobe, and a portable spectrophotometer. Samples were also collected for DOC, sulfate and chloride determinations which were carried out in the lab by appropriate methods.
Sulfide concentrations in the water column at the eutrophic site were close to detection limit (0.001 mg/l) during the diel study in July 1997. Porewater concentrations were at least three orders of magnitude higher than surface water and reached a maximum level at night (14.2 mg/l). At the oligotrophic site during a diel study in January 1998, the bottom water (5 cm above the sediment/water interface) had roughly the same sulfide concentrations as porewaters during the day (about 0.08 mg/l). At night, however, bottom water sulfide concentrations increased to 0.13 mg/l while porewater concentrations increased only slightly. Based on DO profiles, the oligotrophic site was stratified while the eutrophic site was not. These results demonstrate significant differences between the eutrophic and oligotrophic sites with respect to production and exchange of sulfide and oxygen between the sediments and water column. In most instances, sulfide and oxygen were simultaneously present in the surface- and porewaters indicating a state of disequilibrium. The diel fluctuations of sulfide appear to be influenced by photosynthesis and availability of sulfate.
