HR: 0800h
AN: B11B-0414    [Abstracts]
TI: Isotopic Methods for Determining the Relative Importance of Bioavailability Versus Trophic Position in Controlling Mercury Concentrations in Everglades Mosquitofish
AU: * Bemis, B E
EM: bemis@amarine.com
AF: Applied Marine Sciences, Inc., 4749 Bennett Drive, Suite L, Livermore, CA 94551, United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA 94025, United States
AB: The concentration of mercury in fish tissues is widely used as an indicator of the magnitude of mercury contamination in aquatic ecosystems. Eastern mosquitofish (Gambusia holbrookii) is an important sentinel species used for this purpose in the varied environments of the Florida Everglades, because mosquitofish are abundant, have a short lifespan, and migrate little. 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., trophic position) 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 and periphyton were collected during September 1996 from over one hundred sites throughout the Everglades and analyzed for mercury concentration. The USGS analyzed splits of the samples for nitrogen (d15N), carbon (d13C), and sulfur (d34S) isotopic composition, to investigate the causes of mercury variations. The d15N value of tissues is often 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. The d13C value can be useful for detecting differences in food web base (e.g., algal versus detrital), and thus the entry point of contaminants. Tissue d34S potentially indicates 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, which enters the food web base, mosquitofish sulfur isotopes should show positive correlations with SRB activity, methylmercury production, and mosquitofish mercury concentrations. The d15N, d13C, and d34S values of mosquitofish and periphyton are significantly correlated, 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. 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 significant, positive correlations with mosquitofish d34S and the d34S difference between mosquitofish and periphyton. This suggests that during the period studied, mosquitofish mercury concentrations in the Everglades were primarily influenced by the bioavailability of mercury, rather than by differences in trophic position. This study demonstrates that isotopic measurements, especially d34S, can be useful tools for determining causes of high mercury concentrations in fish populations.
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0432 Contaminant and organic biogeochemistry (0792)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0491 Food webs and trophodynamics (4817)
DE: 0497 Wetlands (1890)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting