Bioaccumulation and biomagnification of mercury and selenium in the Sarasota Bay ecosystem

The bioaccumulation and biomagnification of Hg and Se were investigated in Sarasota Bay, Florida, USA, to characterize the Hg exposure risks to wild bottlenose dolphins in the bay. Concentrations of total mercury (THg), monomethylmercury (MMHg), and total selenium (TSe) were monitored in the bay, the latter of which might reduce Hg toxicity. The food web structure and dolphins' trophic level-specific consumption rates were evaluated using stable isotope ratios of carbon (δ(13) C) and nitrogen (δ(15) N). Regressions developed for Hg biomagnification in the food chain were log10 CTHg (nanograms per gram) =0.27 × δ(15) N (‰) - 0.42, R(2)  =0.87, for THg and log10 CMMHg  =0.33 × δ(15) N (‰) - 1.0, R(2)  =0.93, for MMHg. Unlike Hg, nearly constant TSe concentrations were observed at 248 ± 179 ng g(-1) in the food web, and the TSe-to-THg molar ratio was predicted by log10 (CTSe /CTHg ) = -0.10 × δ(15) N (‰) +2.8, R(2)  =0.60. The THg-uptake rates of Sarasota bottlenose dolphins are estimated to vary between 2.1 and 4.9 µg kg(-1) d(-1) ; however, the estimated TSe-uptake rates (15.1 µg kg(-1) d(-1) ) were higher than those for THg, and the Hg-exposure risks of the Sarasota Bay resident bottlenose dolphins are considered to be low. Approaches employed in the present study can be extended to other environments to characterize Hg contamination in aquatic systems and Hg exposure risks in top predators.