d15N and d13C Measurements of Antarctic Peninsula Fauna: Trophic Relationships and Assimilation of Benthic Seaweeds1

SYNOPSIS. Measurements of d13C, d15N, and C/N for a variety of Antarctic pen-insula fauna and flora were used to quantify the importance of benthic brown algae to resident organisms and determine food web relationships among this di-verse littoral fauna. d13C values ranged from216.8 ‰ for benthic algal herbivores (limpets) to 229.8 ‰ for the krill, Euphausia superba; the average pooled value for brown macroalgae, including their attached filamentous diatoms, was220.6‰. There was no correlation between biomass d13C or d15N with C/N content, and consequently both d13C and d15N values were useful in evaluating trophic relation-ships. d15N values of the fauna ranged from 3.1 to 12.5‰, with lowest values re-corded in suspension feeders (e.g., bryozoans) and highest values in Adelie pen-guins (12.5‰) collected in 1989. The comparatively lower d15N value for a Chin-strap penguin (6.9‰) collected in 1997 is attributed to the different dietary food sources consumed by these species as reflected in their respective d13C values. Sig-nificant amounts of benthic macroalgal carbon is incorporated into the tissues of invertebrates and fishes that occupy up to four trophic levels. For many benthic and epibenthic species, including various crustaceans and molluscs, assimilation of benthic algal carbon through detrital pathways ranges from 30 to 100%. Conse-quently, the trophic importance of benthic brown algae may well extend to many pelagic organisms that are key prey species for birds, fishes, and marine mammals. These data support the hypothesis that benthic seaweeeds, together with their as-sociated epiphytic diatoms, provide an important carbon source that is readily incorporated into Antarctic peninsula food webs