| Food items contaminated with mercury pose potentially deleterious health risks to a wide variety of biota. Mercury enters the food web at the microbial level and is methylated and then biomagnified to higher trophic levels via ingestion. As upper-level predators brown pelicans (Pelecanus occidentalis) and laughing gulls (Larus atricilla) represent species at possible risk for neurological impacts of mercury accumulation. To assess the potential risk of mercury accumulation in an estuarine ecosystem I analyzed stomach contents, tissue mercury concentrataions and stable isotopes of these two bird species and their potential prey items (fishes, invertebrates) from Mobile Bay, Alabama, an estuary located in the southwestern portion of the state. Results suggest biomagnification of mercury from lower to higher trophic levels in this system. Mean Hg values for fishes ranged from 0.014 - 0.313 mg/kg, while bird samples ranged from 0.063 - 0.331 mg/kg. Birds accumulated mercury into their tissues, with the greatest accumulation in egg whites of brown pelicans (mean = 0.331 mg/kg) followed by adult brown pelican muscle tissues (mean = 0.205 mg/kg). Fishes exhibited mercury magnification with increasing body length (R2 = 0.417, p < 0.001), supporting previous findings of size as an indicator of mercury contamination in these species. The majority of mercury appears to originate from marine sources accounting for organisms with more enriched sulfur and carbon stable isotope concentrations also having accumulated more mercury into their tissues. Further monitoring of Hg levels in fishes and birds in this region could better elucidate spatial and temporal trends in Hg movement and magnification. |
