Nutritional stress by means of high C:N ratios in the diet and starvation affects nitrogen isotope ratios and trophic fractionation of omnivorous copepods

Abstract

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 = δ¹⁵N_consumer - δ¹⁵N_{food 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 δ¹⁵N_Bg values. Our field results showed that increasing values of the seston carbon to nitrogen ratio (C:N_SES), chlorophyll a (Chl a), and δ¹⁵N_SES were related to higher δ¹⁵N_Bg values. C:N_SES and Chl a were also positively related to ∆¹⁵N; yet, C:N_SES alone explained 70% of the variation. C:N_SES values correlated with the presence of mixotrophic algae and ciliates that are key food resources for B. gracilipes. In our laboratory starvation experiment, the δ¹⁵N_Bg values increased significantly, pointing to use of internal N sources; yet, the change associated with starvation was less pronounced than that related to C:N_SES 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.

Keywords: Food quality; Food webs; Seston; Trophic nitrogen fractionation; Zooplankton.