Dietary mercury exposure, bioaccumulation, and effects in larvae of the southern leopard frog, Rana sphenocephala
Unrine, Jason Michael
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Dietary mercury exposure and bioaccumulation in larvae of the southern leopard frog Rana sphenocephala) was characterized from field and laboratory data and the potential for adverse effects on growth, survival, and development were demonstrated at environmentally realistic dietary mercury concentrations. Aufwuchs (the accumulation of periphyton and associated organisms as well as dead and abiotic material on submerged surfaces) is an important dietary source for larval amphibians. Aufwuchs can accumulate concentrations of mercury as high as 1,600 ng Hg/g DW in aquatic habitats with no significant on-site geologic or anthropogenic sources of mercury. Diet is probably the most significant source of mercury exposure in amphibians because total mercury concentrations in aufwuchs can exceed aqueous mercury concentrations by a factor of 106. Inorganic mercury is the predominate species in aufwuchs and percent of total mercury present as methylmercury decreases with increasing total mercury concentration. Although total mercury concentrations in the diet may be high it appears from both field and laboratory evidence that bioavailability of both methylmercury and inorganic mercury is low and diminishes with increasing exposure concentration. Adverse effects on survival, growth, and development were observed in frog larvae exposed for the entire larval period to experimental diets containing mercury contaminated aufwuchs that had concentrations (1,500 -3,300 ng/g DW) of mercury reflective of the highest concentrations observed in aufwuchs from sites with no significant local anthropogenic or geologic sources of mercury. Mercury in guts of southern leopard frog larvae inhabiting Carolina bay wetlands on the coastal plain of South Carolina occurred at concentrations (1,500 ng Hg/g DW) nearly as high as those that have been observed in aufwuchs (1,600 ng/g DW). Mercury concentrations in larvae from some of the wetlands exceeded dietary concentrations at which adverse effects were observed in the laboratory. The overall conclusion of this research was that larval amphibians may be exposed to and bioaccumulate concentrations of mercury from the diet in sites with little or no significant local anthropogenic or geologic sources of mercury. Mercury accumulated by larvae in these sites may be sufficient to cause adverse effects that may ultimately decrease the number of juveniles recruited to the terrestrial environment with potential for reproduction. This suggests that global mercury pollution may have a potential role in amphibian declines.