The rise and fall of mercury contamination in Everglades biota, 1890 – 2003: a retrospective study of wading bird museum specimens.
Peter C. Frederick1, Becky Hylton1, Julie A. Heath1 and Marilyn G. Spalding2. (1. Department of Wildlife Ecology and Conservation, University of Florida, Gainesville Fl. 2. Department of Pathobiology, School of Veterinary Medicine, University of Florida, Gainesville Fl.).
Surveys of wetland predatory animal tissues during the late 1980’s and 1990’s showed that the Everglades ecosystem was highly contaminated with mercury (Hg), resulting in health advisories for human fishers and increased mortality in many predatory animals. Ciconiiform birds have very high bioaccumulation factors for this contaminant, and their feathers have been shown to be reliable indicators of Hg dynamics in the wetland food web. Although several potential sources of Hg contamination have been identified, the timing and track of contamination is largely unknown in biota prior to the late 1980’s. To help identify the source, and to search for “precontamination” background levels, we compared methylmercury (MeHg) content in feathers of museum specimens (Great Blue Herons, White Ibises and Great Egrets) going back to 1900 with those from more recent collections (1985 – 2003). Over 99% of mercury in feathers was in the methylated form, suggesting that there had been little contamination of museum specimens with mercuric preservatives, Mean MeHg concentrations in feathers during the recent period ranged between 7.46 mg/kg (dry weight) for White Ibises and 21.03 mg/kg for Great Blue Herons. During the period 1900 – 1980, mean Hg concentrations ranged between 1.04 mg/kg (White Ibises) and 3.38 mg/kg (Great Blue Herons), and were significantly lower than the recent period for all species (Figure 1). The approximate onset of contamination in the feather dataset (between 1975 and 1980, see Figure 2) and the enormous recent reduction in mercury contamination during the late 1990’s (>75%) is consistent with trends in Hg emissions from local municipal sources. We also believe that because dietary mercury in wading bird prey can be derived quantitatively from feather tissue levels, the historical measurements may be useful for establishing target regulatory goals for background mercury in Everglades fishes. The pattern of mercury emissions and contamination during the 1990’s suggests a significant lag (4 – 7 yr) between changes in mercury inputs from local sources and responses in wading bird tissues.
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Figure 1. Comparison of mean methylmercury concentrations in feathers of piscivorous birds from the Everglades between 1900 –1980 (red) and 1985 – 2000.
Frederick, Peter C. Department of Wildlife Ecology and Conservation, P.O. Box 110430 University of Florida, Gainesville, Fl. 32611-0430. . Tel. (352) 846-0565.
Figure 2. Methylmercury concentrations in feathers of White Ibises between 1900 and 2000. Numbers on X-axis are years (eg, Jan 41 is 1941).