Concentrations of mercury (Hg), methylmercury (MMHg), arsenic (As), selenium (Se), and cadmium (Cd) were measured in atmospheric deposition, stream water, and biota in two streams in western Maryland. Overall, concentrations were slightly higher in the water of the lower pH Herrington Creek tributary (HRCT). Bioaccumulation factors were also higher for HRCT compared to Blacklick Run (BLK). MMHg concentrations in biota increased with trophic level and essentially all the Hg was as MMHg in predatory insects and insectivorous/carnivorous fish. Thus, the overall trophic status of the organism was indicated by the %MMHg in its tissues. Levels of As, Se, Cd, and Hg, however, decreased with increasing trophic level. Adsorption of As to the exoskeleton of invertebrates appears to be an important accumulation mechanism. MMHg was distributed evenly throughout crayfish and fish organs, whereas As, Se, Cd, and Hg were found in higher concentrations in detoxifying organs. Concentrations in biota in this study were somewhat elevated compared to other rural sites, but were less than those of point source-contaminated sites. Overall, as atmospheric inputs to the two watersheds were similar, the results of this study show the importance of water chemistry in determining the bioaccumulation of the metals and metalloids into insects. Subsequent transfer to higher trophic levels is related to both the ability of the organisms to depurate and the mode of accumulation, either directly from water or from food.

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In order to better understand the role of the gills in the accumulation of contaminants, branchial uptake and bioaccumulation of inorganic (HgCl 2 ) and monomethylmercury (CH 3 HgCl) were quantified in the shore crab Carcinus maenas, using living animals and an in vitro perfused gill preparation exposed to 50 µg l -1 of either chemical forms in the external medium. In addition, localization of accumulated mercury was studied using the histochemical autometallographic technique by both light and electron microscopy. Gill tissue strongly accumulated either inorganic or methylated mercury at similar levels in vitro and in vivo. For both chemical forms of the metal, only about 1% of the total mercury input was recovered in the effluent fluid from in vitro perfused gills and could thus be considered available to distribute inside the animal via the circulatory system. Inorganic Hg was histochemically found to be accumulated at 2 markedly different locations in the gills: at the cuticular surface in direct contact with the contaminated medium and at high local levels in the central vacuole of gill nephrocytes. Although also present at these 2 locations, methylmercury was distributed more diffusely and more evenly in all cells. These results suggest that the high affinity of gill tissue for both forms of mercury may confer on this organ the role of an external barrier strongly limiting the invasion of the metal toward other compartments of the body.

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