Bioavailability of methylmercury to sacramento blackfish (Orthodon microlepidotus): Dissolved organic carbon effects

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Abstract
The effect of dissolved organic carbon (DOC) on methylmercury (MeHg) uptake across the gills of Sacramento blackfish (Orthodon microlepidotus) was investigated using the Hg‐203 radioisotope (half life = 46.9 d). The efficiency of fish gills in extracting MeHg (1.4 ng/L) from water was measured using a McKim‐type fish respirometer that separated exposure (inspired) water from expired water. Blackfish gill ventilation and oxygen consumption rates remained constant, while Me203Hg uptake was decreased significantly (p < 0.05) in the presence of DOC (2 and 5 mg C/L). Mean Me203Hg extraction efficiency, uptake rate constant, and blood to inspired water ratio decreased 78%, 73%, and 63%, respectively, with 2 mg C/L of DOC, and 85%, 82%, and 70% with 5 mg C/L DOC, compared to the Me203Hg reference treatment group. Because respiratory parameters remained unchanged, reductions in Me203Hg uptake indicate strong interactions between DOC and Me203Hg. Methyl203Hg levels in fish gills, kidney, and spleen from 2 and 5 mg C/L were significantly lower (p < 0.05) than those observed from the reference treatment group. These reductions in uptake (bioavailability) support the hypothesis that trans‐gill transport of Me203Hg is inhibited when it is complexed by DOC in the aqueous medium, decreasing Me203Hg uptake and accumulation in fish organs.