Résumé
The effect of upwelling conditions on the trophic level of four planktofagous fish (Gadiculus argenteus, Capros aper and juveniles of Micromesistius poutassou and Merluccius merluccius) on the continental shelf off Northwestern Spain was assessed by coupling stomach content analyses and isotopic data. The study was performed during two consecutive years characterized by contrasting oceanographic conditions during the summer season; the first year (2012) had an intermittent and weak upwelling season, while the second (2013) had an intense and extended upwelling period. Sampling was carried out in autumn, after the summer upwelling, along a transect parallel to the coast on the continental shelf (130-190 m depth), following the main upwelling gradient towards the Northeast. In addition to stomach content sampling and isotopic analyses of fish muscle, the isotopic signature of benthic detritus and sub- superficial phytoplankton were also identified to characterize the basal isotopic level. We hypothesize that differences in isotopic ratios under these contrasting upwelling intensities possibly originate from both changes in the isotopic ratios of basal levels and shifts in the feeding habits of planktophagous species. Basal ratios of carbon and nitrogen in benthic detritus showed neither interannual variability nor relation with the upwelling gradient. However, particulate organic matter (POM) had significantly higher δ15N and δ13C during the first year. Isotopic signature of the fish species was significantly different among years but followed different patterns in the four species. The ratio of 15N indicated that the four fish species considered had a trophic level of two, standing approximately 6 units over the basal level and probably predating on consumer species. Stomach sampling evidenced that C. aper has a more diverse diet based on mesozooplankton and suprabenthos, while euphausiids constituted the majority of diet in G. gadiculus and juvenile M. merluccius and M.poutassou in both years. Euphausiid species would act then rather as consumers than predators, as a preliminary analysis of their isotopic signature confirmed. While differences in isotopic ratios exist at the base of the food web, our analyses suggest that shifts in feeding habits may have a major role in determining species isotopic position.