Large-scale patterns of green turtle trophic ecology in the eastern Pacific Ocean

Trophic position and niche width are fundamental components of a species’ ecology, reflecting resource use, and influencing key demographic parameters such as somatic growth, maturation, and survival. Concepts about a species’ trophic niche space have important implications for local management and habitat protection, and can shed light about resilience to changing climate for species occurring over broad spatial scales. For elusive marine animals such as sea turtles, trophic niche is challenging to study, and researchers often rely on other metrics, such as isotopic niche, as a proxy. Here, stable isotope analysis (δ¹³C and δ¹⁵N values) was conducted on bulk skin tissue of 718 green turtles (Chelonia mydas) distributed among 16 foraging areas in the eastern Pacific from the USA to Chile, a range spanning ~10,000 km. Compound-specific nitrogen isotope analysis of amino acids (CSIA-AA) was applied to 21 turtles among seven sites. Isotopic niche space was determined via Bayesian ellipse area (BEA) and convex hull area (CHA) analyses of bulk isotope values, which were also used along with amino acid δ¹⁵N values to determine trophic position (TP). Substantial variability in bulk tissue δ¹³C and δ¹⁵N values was found within and among sites, and amino acid δ¹⁵N values confirmed this was largely due to spatial differences in baseline nitrogen isotopic compositions, but also to a lesser extent from TP differences among the green turtle foraging populations. Isotope niche space varied among sites, influenced by the diversity of prey types and relative input of terrestrial- vs. marine-derived nutrients; BEAs were the most suitable measurement of isotopic niche space due to the larger influence of outlying values with the CHA approach. Amino acid isotope-derived TP estimates that accounted for local habitat conditions (e.g., mixed seagrass/macroalgae diet) performed the best among several approaches; TP ranged from 2.3 to 3.6, which indicates an omnivorous diet for most populations. In addition to providing additional spatial resolution for δ¹³C and δ¹⁵N isoscapes in the eastern Pacific, especially in coastal habitats, this study further establishes CSIA-AA as an effective tool to study the trophic ecology of sea turtles across a variety of food webs and habitats.