Trophic structure of the marine food web at the Prince Edward Islands (Southern Ocean) determined by δ13C and δ15N 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 predictable enrichment in delta C-13 and delta N-15 with trophic position, only the zooplankton community displayed distinct trophic levels. The benthic and the kelp-associated communities showed trophic continua. Both pelagic (zooplankton) and benthic inter-island communities ultimately derived their diets from phytoplankton (associated with surface-water POM). However, planktonic grazers fed primarily on allochthonous nano- and pico-plankton, while inter-island benthos seemed to rely mostly on microphytoplankton derived from autochthonous blooms. In contrast, kelp-associated animals derived a high proportion of their diet from kelp. The nearshore benthic community had an intermediate position between kelp-associated and inter-island communities. While POM was of primary importance, kelp-derived organic matter accounted on average for >30% of the nearshore animals' diet. Findings of the present study indicate for the first time that autochthonous sources of organic matter (e.g, kelp-derived and microphytoplankton blooms), are important components of the diets of all but the zooplankton community at the sub-Antarctic Prince Edward Islands.