Re-examining trophic dead ends: stable isotope values link gelatinous zooplankton to leatherback turtles in the California Current

Predator-prey interactions provide essential information for tracing energy flow through food webs and evaluating the structure and function of ecosystems. In pelagic environments, these interactions are often difficult to discern, which is problematic for identifying specific energy pathways that support populations of protected species. We examined the trophic ecology of an endangered population of leatherback turtles Dermochelys coriacea and their gelatinous prey in the California Current-Large Marine Ecosystem (CC-LME). We combined carbon and nitrogen bulk stable isotope analysis and compound-specific isotope analysis of amino acids (CSIA-AA) with Bayesian statistical approaches to examine the diets of leatherbacks and their prey (scyphozoans and thaliaceans) sampled in the CC-LME. Our objectives were to evaluate (1) temporal changes in leatherback trophic position, (2) the contribution of different gelatinous prey to leatherback diets, and (3) trophic structure of the leatherback food web by estimating trophic positions and isotopic niches of leatherbacks and their potential prey. Leatherback trophic positions did not change over time, although carbon isotope values suggest a temporary shift in leatherback habitat in 2005, coincident with anomalous upwelling conditions. Bayesian mixing models suggest that carnivorous sea nettles Chrysaora fuscescens were the largest contributor to leatherback diet, followed by filter-feeding thaliaceans. Isotope analyses provided useful and ecologically realistic estimates of trophic structure, where trophic positions were lowest for thaliaceans, intermediate for scyphozoans, and highest for leatherbacks. Overall, our findings provide information on leatherback foraging ecology over a 13 yr period and the trophic structure of gelatinous zooplankton that support the leatherback population in the CC-LME.