Stable isotope and mercury analysis in the Mobile Bay, AL food web

Background/Question/Methods   Mercury (Hg) has been identified as a pollutant that poses a human health risk for people consuming contaminated seafood. The EPA has set a value of 0.1 µg/kg body weight/day for safe exposure to this neurotoxin. Mercury enters the food web at the microbial level and is methylated and then biomagnified, through ingestion, to higher trophic levels. Brown Pelicans (Pelecanus occidentalis) and Laughing Gulls (Larus atricilla) represent species at possible risk for neurological effects of accumulation of mercury due to their trophic position in the Mobile Bay food web. This study examined various members of the Mobile Bay food web (e.g., invertebrates, fish and birds) to track Hg movement. 15N and 13C stable isotopes in combination with mercury analysis allows for better understanding of complex trophic dynamics and the transfer of metals through organisms. The relationship between biomagnification of mercury and stable isotope signatures will generate a model for examining the transfer of this neurotoxic metal through food webs and ultimately into food for human consumption.

Results/Conclusions   Preliminary results suggest biomagnification of methylmercury moves from lower to higher trophic levels. Total mercury was measured in P. occidentalis and L. atricilla as well as in potentially important forage species, which included rangia clam (Rangia cuneata), spot (Leiostomus xanthrus), and Atlantic croaker (Micropogonias undulates). Average values were as follows: P. occidentalis egg 2.478 ppm (n=9), P. occidentalis chick 0.0649 ppm (n=8), and P. occidentalis adult 0.1235 ppm (n=10); L. atricilla egg 0.3197 ppm (n=5), L. atricilla chicks 0.0574 ppm (n=9), L. atricilla adults 0.1627 ppm (n=10); R. cuneata 0.0254 ppm (n=5); L. xanthrus 0.0144 ppm (n=7); and M. undulates 0.0176 ppm (n=5). These values show marked increases in Hg concentration with increasing trophic level, with the highest values occurring in eggs of both avian species. Stable isotope values and additional Hg measurements are currently being incorporated into this analysis to provide a better understanding of the Hg dynamics within this ecosystem. Preliminary results suggest that mercury concentrations have not reached neurotoxic levels in the organisms examined, but further analyses are warranted to verify these findings.