Research article: small-scale spatial variation of δ13C and δ15N isotopes in Antarctic carbon sources and consumers

Regional food web studies that fail to account for small-scale isotopic variability can lead to a mismatch between an organism’s inferred and true trophic position. Misinterpretation of trophic status may result, substantially limiting spatial and temporal comparability of food web studies. We sampled several carbon sources and consumers in a nested design to assess the variability of food web members across small spatial scales (100 s of m to several km) in regions around the Windmill Islands and Vestfold Hills in East Antarctica. For carbon sources, δ13C in sea ice POM was particularly variable between locations (km apart) and between sites (100 s of m apart) with replicate samples varying by up to 16‰. Macroalgae δ13C was less variable (replicate samples ranging up to 6.9‰ for the red alga Iridaea cordata), yet still differed between locations. Sediment POM and pelagic POM were the least variable, displaying minimal differences between locations or sites for δ13C and δ15N. Three out of eight consumers were significantly different between locations for δ13C, and five out of eight for δ15N, with the fish Trematomus bernacchii the most variable for both δ13C and δ15N. At smaller scales, the amphipod Paramorea walkeri showed significant variation between sites in δ13C but not in δ15N. We attribute small-scale variability to the dynamic physical environment for carbon sources in coastal systems and a close coupling of diet to habitat for consumers. We highlight the need to account for small-scale spatial variation in sampling designs for regional food web studies.