Hydrogen isotope assimilation and discrimination in green turtles

Although hydrogen isotopes (delta H-2) are commonly used as tracers of animal movement, minimal research has investigated the use of delta H-2 as a proxy to quantify resource and habitat use. While carbon and nitrogen are ultimately derived from a single source (food), the proportion of hydrogen in consumer tissues originates from two distinct sources: body water and food. Before hydrogen isotopes can be effectively used as a resource and habitat tracer, we need estimates of (net) discrimination factors (Delta H-2(Net)) that account for the physiologically mediated differences in the delta H-2 values of animal tissues relative to that of the food and water sources they use to synthesize tissues. Here, we estimated Delta H-2(Net) in captive green turtles (Chelonia mydas) by measuring the delta H-2 values of tissues (epidermis and blood components) and dietary macromolecules collected in two controlled feeding experiments. Tissue delta H-2 and Delta H-2(Net) values varied systematically among tissues, with epidermis having higher delta H-2 and Delta H-2(Net) values than blood components, which mirrors patterns between keratinaceous tissues (feathers, hair) and blood in birds and mammals. Serum/plasma of adult female green turtles had significantly lower delta H-2 values compared with juveniles, likely due to increased lipid mobilization associated with reproduction. This is the first study to quantify Delta H-2(Net) values in a marine ectotherm, and we anticipate that our results will further refine the use of delta H-2 analysis to better understand animal resource and habitat use in marine ecosystems, especially coastal areas fueled by a combination of marine (e.g. micro/macroalgae and seagrass) and terrestrial (e.g. mangroves) primary production.