Omnivory is ubiquitous in aquatic and terrestrial ecosystems and may increase stability of food webs. Turtles are long-lived and among the most abundant organisms in lentic aquatic habitats. Many species are omnivores and thus may be especially important in regulating ecosystem dynamics in these systems. Turtles are also one of the most threatened vertebrate groups in the world; if they indeed play pivotal roles in lentic ecosystems, their declines may disrupt or even destabilize those ecosystems. In order to assess the role of turtles in an aquatic ecosystem, we examined sources of primary production, trophic structure, and ontogenetic diet shifts of five species of turtles, other herpetofauna, and fish in a large shallow lake in northern Florida. Stable isotope analyses (δ13C and δ15N) of 42 species of animals and five sources of primary productivity revealed that macroalgae and/or particulate organic matter were the foundation of the food web. Trophic structure of the herpetofauna and fish indicated a ...