Variation among diets in discrimination of δ13C and δ15N in the amphipod Allorchestes compressa

Discrimination of stable isotopes of carbon (δ13C) and nitrogen (δ15N) was examined for the amphipod Allorchestes compressa Dana using controlled laboratory experiments. Amphipods were fed exclusively on single diets (fresh or decomposed macroalgae or seagrass) for three weeks. Macrophyte type (i.e. seagrass, brown algae or red algae) had a greater influence on the stable isotope ratios of A. compressa than the state of decomposition of the macrophyte material. The experiments revealed that δ13C in A. compressa stabilised at values lower than those of the diets, which contrasts to the general assumption that consumer-diet discrimination of δ13C ranges from 0 to + 1‰. Amphipods fed on seagrass yielded the lowest δ13C values, which were 9 to 10‰ lower than their diet, while amphipods fed on macroalgae had values 2 to 4‰ lower than their diet. In addition, contrary to the general assumption that consumer-diet discrimination of δ15N ranges from + 3 to + 5‰, discrimination of δ15N was as low as − 1 and + 1 when A. compressa was fed on brown and red algae, respectively, but as high as + 3‰ when fed on seagrass. The results show that discrimination of stable isotopes of carbon and nitrogen can vary considerably depending on the food source, demonstrating that validation of assumptions about discrimination are critical for interpreting stable isotope data from field studies.