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Aquatic Cycling of Hg in the Everglades

Bacterial Demethylation of MeHg in the South Florida Ecosystem

As part of the south Florida ACME program, the USGS conducted periodic field measurements of methylmercury (MeHg) degradation potential rates in sediment, between 1995 and 1998, at a suite of sites located in the Water Conservation Areas and Everglades National Park. Due to changes in methodology, only data since June 1996 is presented here. Intact sediment cores were collected by hand with polycarbonate core tubes (8 cm i.d. x 15-30 cm). The unconsolidated surface floc layer was treated as one discrete interval for June 1996, December 1996, and January 1998. Sediment below this zone was sectioned into 2 cm to 4 cm discrete depth intervals. Discrete depths were sampled for the whole core during April 1997, July 1997, and June 1998. For each interval, sediment from 3 to 6 cores was pooled for a homogenized composite sample. Sub-samples (3 cc) were amended with radiolabeled ¹⁴C-MeHg (0.5 to 10 nCi) and incubated anoxically for 0.2 to 2.7 days (at 18-25 ^oC) in 13 cc serum bottles. Incubations were arrested by adding 1 ml of 3 M NaOH. All incubations were initiated within 10 hrs of sample collection, except during April 1997, which were initiated three days after collection. All site/depth discrete sample sets included n=3 replicates and one autoclave killed control. The gaseous ¹⁴C- endproducts (¹⁴CH₄ and ¹⁴CO₂) were assayed by a CH₄ combustion/CO₂ trapping assay as described in Marvin-DiPasquale and Oremland (1998) Environ. Sci. Tech. 32:2556-2563.

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