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 high degree of omnivory (TPs of 3–4), few specialists at high trophic levels (TP ≥ 4.0), and few strict primary consumers (TP = 2.0). Several predatory fish (Micropterus salmoides, Lepomis gulosus, and Pomoxis nigromaculatus) and herpetological predators (e.g., Alligator, Nerodia) were functionally trophic omnivores with trophic positions lowered by predation on primary and low trophic position secondary consumers. We found two major divisions in the trophic structure of turtles—an herbivore (Pseudemys floridana) and four omnivores (Apalone ferox, Chelydra serpentina, Sternotherus odoratus, and Trachemys scripta). There were at least three types of omnivores, species that are generalist feeders throughout their life (e.g., C. serpentina, S. odoratus), species that change their trophic position during ontogeny (e.g., A. ferox), and species with the same trophic position through ontogeny but resulting from different diet composition as juveniles and adults (e.g., T. scripta). Trophic position of some species indicated that even when turtles are primarily predatory, they also function as vegetative consumers and facultative scavengers (e.g., T. scripta, S. odoratus, A. ferox). Given that omnivory is a critical process that can stabilize food webs, there will certainly be dramatic shifts in how energy and nutrients flow through these lentic ecosystems if turtle populations decline.