Comparison of ecosystem modelling and isotopic approach as ecological tools to 2 investigate food webs in the NW Mediterranean Sea 3 4

23 24 Understanding how marine ecosystems are structured and how they function is a 25 scientific challenge. Nowadays, to learn about ecosystem trophic dynamics ecologists use 26 two complementary methodological approximations, mass-balance ecosystem models and 27 isotopic tools. However, despite the wide applications of these methodologies, the 28 comparison and combination of both are still scarce. The aim of this study was to examine 29 the trophic dynamics of representative species of the South Catalan marine ecosystem 30 (North-western Mediterranean) using the Ecopath ecosystem modelling tool and the stable 31 isotope approach. By using the two methodologies we depicted the trophic position (trophic 32 level and δN values) and trophic width (omnivory index and total isotopic area) of several 33 species of fish, cephalopods, cetaceans, seabirds and one sea turtle. Our results show a clear 34 correlation between the trophic levels estimated by the Ecopath model and the N values, 35 which validate the trophic position of several species in the study area and indicate that both 36 methodologies are useful to determine the trophic position of marine species in 37 Mediterranean marine food webs. In contrast, the two estimators of trophic width (the 38 omnivore index and the total isotopic area) were not strongly related, since the relationship 39 between modelling and stable isotope results was evident only for some species. Further 40 comparisons of trophic width calculated in other marine food webs may provide a better 41 understanding of our results and validate the accuracy of both methodological approaches to 42 calculate trophic width. 43 44