The American alligator (Alligator mississippiensis) is a widespread species within coastal ecosystems of the southeastern United States. In addition to exerting top-down trophic control, alligators exhibit local movements between freshwater and estuarine habitats, facilitating the transfer of nutrients and resources across both terrestrial and aquatic environments, as well as across the freshwater-to-marine continuum. This study aimed to assess the food resources of alligators from freshwater, brackish, and marine habitats along coastal South Carolina, using naturally abundant stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) to investigate dietary variability along a salinity gradient. Given that high concentrations of mercury are found in apex predators like alligators and that this metal is efficiently transferred through the food web, we incorporated it as prior information into the mixing model. At each four sites, three subsites were established based on the salinity gradient, where we collected blood and muscle samples from alligators and tissue samples from potential prey items such as crayfish, crabs, and fish from different foraging guilds, together with basal resources (periphyton, detritus, terrestrial and aquatic plants) to analyze stables isotopes and mercury concentrations. Considering the rising salinity levels in coastal ecosystems due to saltwater intrusion and global environmental changes, which disrupt connectivity and alter habitats, this study aims to better understand the effects of such changes on food-web dynamics. By quantifying alligator nutritional subsidies, this research will contribute to the conservation of both alligator populations and coastal ecosystems.