Variability of higher trophic level stable isotope data in space and time – a case study in a marine ecosystem

Rationale 1In shelf and coastal ecosystems, planktonic and benthic trophic pathways differ in their carbon stable isotope ratios (δ13C values) and nitrogen stable isotope ratios (δ15N values) and they increase predictably with trophic level. Stable isotope data are therefore used as a tool to study food webs in shelf and coastal ecosystems, and to assess the diets and foraging behaviour of predators. However, spatial differences and temporal changes in prevailing environmental conditions and prey abundance may lead to considerable heterogeneity in stable isotope values measured in focal animal species. Methods Here we assess spatial and temporal variability of δ13C and δ15N values in tissue samples of fish, squid and crustacean species captured over three years during research cruises close to the Falkland Islands, Southwest Atlantic. Results Both in δ15N values and especially in δ13C values, intra-species differences were large and often exceeded inter-species differences. Spatial patterns were weak, albeit statistically significant. The distribution of δ13C values was related to latitude, while the δ15N values varied with longitude. The distance from the coast and depth of catch influenced both δ13C and δ15N values. However, the importance of temporal variability greatly exceeded that of spatial variability. In addition to a moderate overall seasonal effect, we found that species differed strongly in their specific seasonal changes. Conclusions Seasonal differences in the relative position of species or species groups in the C-N isotope space suggest changes in the utilisation of planktonic vs. benthic trophic pathways, indicating flexible foraging strategies in response to variable environmental conditions. These seasonal differences should be taken into account when analysing higher trophic level feeding ecology with stable isotope analysis. Copyright © 2015 John Wiley & Sons, Ltd.