Diversity in trophic interactions of green sea turtles Chelonia mydas on a relatively pristine coastal foraging ground

Adult green sea turtles Chelonia mydas are often the largest-bodied herbivores in their communities and may play an important role in structuring seagrass and macroalgal communities. Recent studies, however, suggest that green turtles might be more omnivorous than previously thought. We used animal-borne video and nitrogen and carbon stable isotopic analysis of skin to elucidate diets of green turtles in the relatively pristine seagrass ecosystem of Shark Bay, Australia. Stable isotope values suggested that despite the presence of abundant seagrass resources, turtles assimilated most of their energy from a combination of macroalgae and gelatinous macroplankton (cnidarians and ctenophores). Video data suggested that macroplankton might be the most commonly consumed food source. Also surprising was the considerable variation in δ13C values, suggesting long-term dietary specialization by individual turtles. Overall, green turtle foraging under natural conditions may be less stereotyped than previously thought, and diets of green turtles inhabiting apparently similar ecosystems (e.g. seagrass-dominated ecosystems) may vary considerably across geographical regions. The apparently high degree of individual specialization in diets suggests that conservation efforts should account not only for the potential importance of non-benthic food sources for green turtle populations, but also for the possibility that subsets of the population may play different ecological roles and may be differentially vulnerable to anthropogenic impacts.