Phytoplankton and macrophyte contributions to littoral food webs in the Galician upwelling estimated from stable isotopes

The natural abundance of stable carbon and nitrogen isotopes was analysed in plankton and intertidal benthos at coastal sites of Galicia (NW Spain) to determine the relative importance of phytoplankton versus benthic macrophytes, and marine versus anthropogenic nutrients, in supporting littoral food webs in this upwelling ecosystem. Phytoplankton was significantly less enriched in heavy isotopes than most macrophytes (seagrasses and 3 classes of macroalgae). Nitrogen isotopes showed significant enrichment of primary consumers (zooplankton, benthic suspension feeders and deposit feeders) relative to primary producers. The average enrichment between trophic levels, however, was low (Delta delta C-13 = +0.1 parts per thousand, Delta 8 delta N-15 = +1.4 parts per thousand), suggesting the prevalence of omnivory. The use of marine nitrogen was inferred from the low nitrogen isotope abundance values of phytoplankton and rhodophycea. In contrast, phaeophycea and chlorophycea displayed high nitrogen isotope enrichment, particularly at sheltered sites, suggesting the influence of anthropogenic nitrogen. The diets of benthic consumers reflected the local diversity of available sources, with a large influence of phytoplankton and marine nitrogen at open sea sites. Average contributions of phytoplankton to diet always exceeded 40% in the case of suspension feeders but were generally < 20% for deposit feeders. Estuarine macrophytes were generally < 10% of diet for both groups. Carbon isotopes indicated a significant contribution of macrophytes (77%) to sinking particles. In contrast, most of the nitrogen of sinking particles and consumers was provided by marine sources. These results confirm the dominant role of marine nitrogen and phytoplankton in supporting littoral food webs in the Galician upwelling ecosystem, despite local inputs of anthropogenic nutrients.