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 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.