Trophic structure of the marine food web at the Prince Edward Islands (Southern Ocean) determined by delta C-13 and delta N-15 analysis

The origins (autochthonous or allochthonous) and pathways of organic matter in various marine communities were investigated in the vicinity of the sub -Antarctic Prince Edward Islands using stable-isotope analysis. Four major assemblages, comprising zooplankton, kelp-associated species, inter-island and nearshore benthos, were considered. Despite consumers exhibiting a pred ictable enrichment in delta C-13 and delta N-15 with trophic position, only the zooplankton community displayed distinct trophic levels. The benthic a nd the kelp-associated communities showed trophic continua. Both pelagic (z ooplankton) and benthic inter-island communities ultimately derived their d iets from phytoplankton (associated with surface-water POM). However, plank tonic grazers fed primarily on allochthonous nano- and pico-plankton, while inter-island benthos seemed to rely mostly on microphytoplankton derived f rom autochthonous blooms. In contrast, kelp-associated animals derived a hi gh proportion of their diet from kelp. The nearshore benthic community had an intermediate position between kelp-associated and inter-island communiti es. While POM was of primary importance, kelp-derived organic matter accoun ted on average for >30% of the nearshore animals' diet. Findings of the pre sent study indicate for the first time that autochthonous sources of organi c matter (e.g, kelp-derived and microphytoplankton blooms), are important c omponents of the diets of all but the zooplankton community at the sub-Anta rctic Prince Edward Islands.