LAUSR

Nutritional stress, from feeding on low-quality diets or starvation, may cause changes in consumers’ nitrogen isotope ratios (δ15N = 15N/14N) and trophic fractionation (∆15N = δ15Nconsumer − δ15Nfood source), however, research has shown mixed results in the magnitude and the direction of the change. This is potentially more complex for omnivores whose diets span a wide range of food resources. We conducted seasonal field samplings in Patagonian lakes and analyzed the relationship between seston (SES) quality parameters and the δ15N and ∆15N of an omnivorous copepod, Boeckella gracilipes (Bg). We also performed a 7-day laboratory starvation experiment, an extreme form of nutritional stress, to investigate if lack of food led to changes in δ15NBg values. Our field results showed that increasing values of the seston carbon to nitrogen ratio (C:NSES), chlorophyll a (Chl a), and δ15NSES were related to higher δ15NBg values. C:NSES and Chl a were also positively related to ∆15N; yet, C:NSES alone explained 70% of the variation. C:NSES values correlated with the presence of mixotrophic algae and ciliates that are key food resources for B. gracilipes. In our laboratory starvation experiment, the δ15NBg values increased significantly, pointing to use of internal N sources; yet, the change associated with starvation was less pronounced than that related to C:NSES changes in the field, suggesting depletion of the substrate pool in the former. We found that ∆15N values of omnivorous species consuming a low-quality diet would be higher than that from a conspecific with a high-quality diet; though fasting animals would show intermediate values.