HR: 0830h
AN: H51E-02    [Abstracts]
TI: Assessing the Influences of Mercury Bioaccumulation and Bioavailability in Everglades Food Webs
AU: * Kendall, C
EM: ckendall@usgs.gov
AF: USGS, Menlo Park, CA, United States
AU: Bemis, B E
EM: bemis@amarine.com
AF: USGS, Menlo Park, CA, United States
AB: Eastern mosquitofish are an important sentinel species used to monitor mercury contamination of the aquatic ecosystem in the Everglades. Like other freshwater fish, the primary route of mercury uptake into mosquitofish tissues is through diet as bioavailable methylmercury. Yet, it is unclear whether variations in mosquitofish mercury observed across the Everglades are due primarily to differences in bioaccumulation (i.e., increase of mercury at higher trophic levels) or abundance of methylmercury available to the food web base. We use isotopic methods to investigate the importance of these two controls on mosquitofish mercury at the landscape scale. As part of the USEPA REMAP project, mosquitofish, periphyton, and sediments were collected during September 1996 from over one hundred sites throughout the Everglades and analyzed for mercury concentration and many other parameters. The USGS analyzed splits of the samples for d15N, d13C, and d34S. Mosquitofish were analyzed as composites of 5-10 fish and periphyton and 0-5 cm sediment samples were analyzed in bulk. Tissue d15N is widely used to estimate the relative trophic positions of organisms in a food web, and should correlate positively with tissue mercury if bioaccumulation is an important control on mosquitofish mercury concentration. Tissue d34S values potentially indicate the extent of dissimilatory sulfate reduction in sediments, a process used by sulfate-reducing bacteria (SRB) during conversion of inorganic Hg(II) to bioavailable methylmercury. Because this process increases the d34S value of remaining sulfate (assimilated by the food web base), mosquitofish d34S should show positive correlations with SRB activity, methylmercury production, and mosquitofish mercury concentrations. Mosquitofish and periphyton isotopes are significantly correlated (d15N-Mosq vs. d15N-Peri, d34S-Mosq vs. d34S-Peri), indicating that a component of the bulk periphyton analyzed in this study is part of the mosquitofish food web. Mosquitofish mercury does not correlate significantly with tissue d15N or the d15N difference between mosquitofish and periphyton (D15N). Thus, differences in trophic level (and bioaccumulation) among the fish do not contribute a detectable influence on mercury variations in the samples studied. In contrast with the d15N results, mosquitofish mercury levels show strong positive correlations with mosquitofish d34S and D34S. This suggests that during the period studied, mosquitofish mercury concentrations in the Everglades ecosystem were primarily influenced by the bioavailability of mercury, rather than by varied trophic position. Our findings also suggest that the d34S of biota may provide a useful indicator of potential hotspots of mercury methylation.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1803 Anthropogenic effects
DE: 1806 Chemistry of fresh water
SC: Hydrology [H]
MN: 2005 Joint Assembly