Summary
The basis of using isotopic measurements to study trophic structure lies in documenting a regular and consistent pattern of isotopic enrichment with increasing trophic level. Nitrogen isotope ratios (δ 15N) in consumers become enriched in 15N by 3-4 % relative to their food, enabling elucidation of trophic position. Carbon signatures (δ 13C) remain relatively stable with trophic enrichment thus enabling carbon sources at the base of the food web to be linked with higher order consumers. However, when food webs are compared between multiple systems, isotopic variation amongst primary producers can cause misclassification of trophic positions in higher order consumers. Therefore, spatial variation in isotopic signatures amongst primary producers must first be quantified prior to any multiple system analysis. We chose several common, long lived, benthic first order consumers (Cucumaria georgiana, Laturnula elliptica, Paramoera walkeri, Perkinsiana sp. and Sterechinus neumayeri) to act as surrogates for primary producers and quantified the amount of isotopic variation at the base of the marine benthic food chain around Casey Station, East Antarctica. We also present preliminary results on the wider benthic food web delineated by stable isotope analysis.