Variation among diets in discrimination of δ13 C and δ15 N in the amphipod Allorchestes compressa

Discrimination of stable isotopes of carbon (delta C-13) and nitrogen (delta N-15) 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 delta C-13 in A. compressa stabilised at values lower than those of the diets, which contrasts to the general assumption that consumer-diet discrimination of 613 C ranges from 0 to + 1 parts per thousand. Amphipods fed on seagrass yielded the lowest delta C-13 values, which were 9 to 10 parts per thousand lower than their diet, while amphipods fed on macroalgae had values 2 to 4 parts per thousand lower than their diet. In addition, contrary to the general assumption that consumer-diet discrimination of delta N-15 ranges from +3 to +5 parts per thousand, discrimination of delta N-15 was as low as -1 and + 1 when A. compressa was fed on brown and red algae, respectively, but as high as + 3 parts per thousand 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. (C) 2007 Elsevier B.V. All rights reserved.