Trophic position variability of European sardine by compound-specific stable isotope analyses

Abstract

The study of the trophic position (TP) of organisms is crucial to understand the trophodynamics of ecosystems. The European sardine from the western Mediterranean has been highly exploited during the last decades and has shown an important population decline. To determine the TP, we quantified seasonal nitrogen stable isotope (δ ¹⁵N) values of compound-specific stable isotope analysis of amino acids (CSIA-AA), as well as bulk δ ¹⁵N (SIA-bulk) values in muscle from three areas of the north-western Mediterranean. Our results highlight that, despite observing latitudinal changes in nitrogen SIA-bulk, the TP of sardine remains similar between sites. These results suggest that changes in nitrogen SIA-bulk are mainly driven by changes in the isotopic ratio of the baseline. Furthermore, the seasonal nitrogen variation in SIA-bulk is linked to differences in the isotopic baseline of the pelagic food web observed in the source AAs. Using CSIA-AA allowed us to decouple the variation in the baseline from the trophic variation producing robust TP estimates. Without using CSIA-AA, the interpretation of nitrogen SIA-bulk was hampered by the lack of a proper regional baseline of the primary producers.