Determination of precise nitrogen stable isotopic baselines from heterotrophic organism in coastal ocean environments using compound specific isotope analysis of amino acids

We sampled mussels (Mytilus edulis) along the coast of the Korean peninsula, and determined their trophic position (TP) using the nitrogen isotope ratio of amino acids (TPglu/phe). The TPglu/phe of mussels (2.0 to 2.5) is higher than TP of herbivores, suggesting that TP normalization is required to remove the trophic enrichment effect on basal nitrogen isotope ratio. We found similar trends in spatial variation between nitrogen isotopic baseline based on nitrogen isotope ratio of bulk tissue (delta N-15(base-bulk)) and that of phenylalanine (delta N-15(base-phe)) in mussels, reflecting different nitrogen sources among sampling sites. The present study suggests that delta N-15(base-bulk) and delta N-15(base-phe) are powerful tools to trace nitrogen isotopic baselines from primary consumers as well as higher TP organisms.