The pelagic foodweb in the upwelling ecosystem of Galicia (NW Spain) during spring: natural abundance of stable carbon and nitrogen isotopes

The structure of the pelagic foodweb in the upwelling ecosystem of Galicia (NW Spain) was studied by means of the enrichment in 13 C and 15 N isotopes in plankton (four size classes from 20 to >1000 μm), sardine (primary consumer) and dolphin (top consumer). Samples were collected in March 1998, during the spawning period of sardines at the beginning of the upwelling season. Isotopic enrichment across plankton size classes was lower than either enrichment between plankton and sardine or enrichment between sardine and dolphin. This difference may be attributed to the presence of chain-forming phytoplankton in some plankton size classes but also to a large degree of omnivory among zooplankton. Also, there was a size-dependent effect on 15 N enrichment of sardine. Sardines <18 cm had a variable isotopic enrichment but generally higher than those of larger sardines, revealing omnivorous but mostly zoophagous diets. In contrast, sardines ≥18 cm showed a linear decrease of 15 N enrichment with length, as a consequence of the increase in the efficiency of filter-feeding and in the amount of phytoplankton ingested. According to 15 N enrichment, we concluded that probably there are no more than four trophic levels in the studied ecosystem. Nevertheless, our results revealed that the pelagic food web in this upwelling area is more complex than expected due to the generalized omnivory in all organisms. Furthermore, the obtained relationships between δ 15 N and the size of organisms provide the basis for a quantitative analysis of changes in the trophic structure of this ecosystem.