Using amino acid stable isotopes to estimate trophic position

Estimating the trophic positions of animals in food webs is useful in addressing many fundamental ecological and ecotoxicological questions. These estimates are generally based on gut content and stable isotope analyses, which have become essential tools for ecotoxicologists. While these tools have proven their usefulness, they involve several pitfalls that can result in large errors in the estimation of animals’ trophic positions. Relatively recently, these tools were improved with the analysis of the compound-specific amino acid nitrogen stable isotopes. The approach is based on the fact that certain amino acids (e.g., glutamic acid) exhibit metabolic isotope fractionation and others (e.g., phenylalanine) are incorporated into the tissues of consumers in a conservative manner. Indeed, amino acids of the latter category exhibit only a small change in their nitrogen isotope composition, which is similar to that found in primary producers. Therefore, the trophic position of an animal species is estimated based on a comparison of the nitrogen isotope composition of these two groups of amino acids from a single organism. This lecture will try to show the potential of the compound-specific stable isotope analysis of amino acids to illustrate food-web structure and estimate trophic positions in wild animal populations. It will summarize recent advances from field and laboratory studies and present case studies taken from the aquatic environment. In addition, this lecture will discuss the benefits, current limitations and analytical challenges related to this approach, which continues to open new research routes in many fields of ecology, including ecotoxicology.