A new analytical method for determination of the nitrogen isotopic composition of methionine: Its application to aquatic ecosystems with mixed resources

Compound‐specific nitrogen isotope (δ15N) analysis of amino acids is a powerful tool for estimating the trophic positions (TPs) of animals. The TP of an animal can be represented as a linear function of the isotopic difference between glutamic acid (δ15NGlu) and phenylalanine (δ15NPhe). However, the method using δ15NGlu and δ15NPhe cannot be applied to animals in mixed food webs where basal resources are derived from both terrestrial and aquatic primary producers, because the mean value of δ15NPhe relative to δ15NGlu differs greatly between terrestrial plants (+8.4‰) and aquatic algae (−3.4‰). To resolve this problem, the δ15N of methionine (δ15NMet) is useful. Because the C–N bond of methionine is not cleaved in its initial metabolic step, theoretically there should be little diversity in δ15NMet relative to δ15NGlu among primary producers and a small trophic discrimination factor for methionine in animal metabolism. We developed a dual‐column‐coupled GC‐C‐IRMS method to determine δ15NMet. Data collected from controlled feeding experiments and wild samples demonstrated that the isotopic difference between methionine and phenylalanine in terrestrial food webs (ΔMet−Phe = −16.5 ± 0.5‰) is clearly distinguishable from that in aquatic food webs (ΔMet−Phe = −5.0 ± 0.5‰). This approach allowed us to determine ecologically reasonable TP values for carnivores in a stream food web, which were substantially underestimated with the conventional method. This method has potential utility in assessing TP for animals that rely on varying proportions of both terrestrial‐ and aquatic‐derived resources, with no requirement to characterize δ15N in their basal resources.ISSN:1541-585