Site-specific ontogenetic drivers of mercury concentrations in American alligators

Contaminant body burdens are determined by complex interactions between contaminant inputs into the environment, local ecological dynamics, and organismal ontogeny. Although a naturally occurring element, mercury (Hg) is a potent neurotoxin, commonly assessed in contaminant body burdens. Anthropogenic activity has affected its spatial distribution, resulting in regional "hotspots" with elevated Hg concentrations. Due to the propensity of methylated Hg to bioaccumulate within individuals over time and biomagnify across trophic levels, long-lived apex predators can carry substantial body burdens in affected ecosystems. However, the role of an organism's ontogeny and habitat in shaping individual Hg concentrations, especially within and across species, is not well understood. We assessed total Hg, carbon (δ13C), and nitrogen (δ15N) isotopic ratios in whole blood samples (n = 133) across three distinct habitats in the southeastern United States to investigate how size and dietary shifts in the American alligator (Alligator mississippiensis) influence Hg accumulation. Mercury concentrations were approximately eightfold higher in alligators inhabiting the Okefenokee Swamp, Georgia (mean = 0.62 mg/kg) compared with those inhabiting coastal habitats (Jekyll Island, Georgia and Yawkey Wildlife Center, South Carolina). Whereas individual size and nitrogen isotope signatures generally displayed positive relationships with Hg concentrations, model selection approaches revealed these relationships varied across populations, likely in response to site-specific differences in environmental Hg concentrations and life history attributes of the alligators. Collectively, our findings demonstrate that although Hg concentrations in A. mississippiensis are highly influenced by differences between sites, diet and body size can sometimes additionally affect individual variation within populations, suggesting that organismal ontogeny interacts with site-specific contamination and ecological factors to affect Hg body burdens.