Assessing the potential of amino acid δ13C and δ15N analysis in terrestrial and freshwater ecosystems

Understanding the structure and dynamics of food webs requires accurate estimates of energy flow among organisms. Bulk tissue carbon (δ13C) and nitrogen (δ15N) isotope analysis is often used to this end; however, the limitations of this technique can outweigh the benefits. The isotope analysis of individual amino acids is being increasingly employed to trace energy flow and estimate consumer trophic level. Central to this compound‐specific approach are the concepts of essential amino acid (AAESS) δ13C fingerprinting and amino acid (AA) δ15N β‐values, both of which have been understudied and are poorly constrained in terrestrial and freshwater producers. We present AAESS δ13C data for 112 terrestrial and freshwater producers collected from two aridland habitats in the northern Chihuahuan Desert (New Mexico, USA) and AA δ15N data for a subset (n = 28) of these samples. We characterized AAESS δ13C fingerprints by performing linear discriminant analysis on the δ13C values of isoleucine, leucine, lysine, phenylalanine, threonine and valine for four producer groups—C3 plants, C4 plants, CAM plants and filamentous green algae. We explored potential biochemical mechanisms underlying these AAESS δ13C fingerprints by calculating differences between the δ13C values of AAESS products and their AA precursors. This allowed us to estimate and compare isotopic discrimination for specific AAESS synthesis pathways across producer groups. We found a near‐perfect separation of AAESS δ13C fingerprints among producer groups; all groups reclassified with >95% success within our multivariate framework. We also found varied isotopic discrimination for specific AAESS synthesis pathways among producer groups. Contrary to previous studies, we found no differences in β‐values between terrestrial C3 and C4 plants for any trophic‐source AA pairing. Furthermore, we found that Lys δ15N values were less variable and more closely related to bulk tissue δ15N values than Phe δ15N values in terrestrial and freshwater producers. Synthesis. We conclude that AAESS δ13C fingerprints are a higher resolution tracer for freshwater food webs, where instream algae have overlapping bulk tissue δ13C values with terrestrial C3 plants. Additionally, βGlx‐Lys and βPro‐Lys are the best for AA δ15N‐based consumer trophic‐level estimates in freshwater food webs containing both terrestrial and aquatic resources.