Stable nitrogen isotope analysis of amino acids as a new tool to clarify complex parasite–host interactions within food webs

Traditional bulk isotopic analysis is a pivotal tool for mapping consumer–resource interactions in food webs but has largely failed to adequately describe parasite–host relationships. Thus, parasite–host interactions remain undescribed in food web frameworks despite these relationships increasing linkage density, connectance and ecosystem biomass. Compound-specific stable isotopes from amino acids provides a promising novel approach that may aid in mapping parasite–host relationships in food webs. Here we apply a combination of traditional bulk stable isotope analyses and compound-specific isotopic analysis of nitrogen in amino acids to examine resource use and trophic interactions of five parasites from three hosts from a marine coastal food web (Wadden Sea, European Atlantic). By comparing isotopic compositions of bulk and amino acid nitrogen, we aimed to characterize isotopic fractionation occurring between parasites and their hosts and to clarify parasite trophic positions. Our results indicate that parasitic trophic interactions were more accurately identified using compound-specific stable isotope analysis due to removal of underlying source isotopic variation for both parasites and hosts. The compound-specific method provided clearer trophic discrimination factors in comparison to bulk isotope methods. Amino acid compound specific isotope analysis has widely been applied to examine trophic position within food webs, but our analyses suggest that the method is particularly useful for clarifying the feeding strategies for parasitic species. Baseline isotopic information provided by source amino acids allows clear identification of the fractionation from parasite metabolism by integrating underlying isotopic variations from the host tissues. However, like for bulk isotope analysis, the application of a universal trophic discrimination factor to parasite–host relationships remains inappropriate for compound-specific stable isotope analysis. Despite this limitation, compound-specific stable isotope analysis is and will continue to be a valuable tool to increase our understanding of parasitic interactions in marine food webs.