Consequences of employing amino acid vs. bulk-tissue, stable isotope analysis: a laboratory trophic position experiment

An important metric of environmental health is food web structure because it reflects species richness, natural history diversity, and resource availability. While bulk-tissue stable isotope analysis has proven valuable for food web studies, field conditions may severely restrict its use and data can be quite variable. Amino acid stable isotope analysis potentially reduces this variability, in part by eliminating the need for signatures near the trophic base because a single top consumer contains both the primary producer signature (constant phenylalanine signature) and information reflecting number of trophic transfers (a progressively increasing delta N-15 signature of glutamic acid). To evaluate the ecological sensitivity and cost/benefits of the techniques, we conducted a laboratory food chain experiment with four trophic levels. Water fleas (Daphnia magna) were cultured on a diet of powdered algae and then fed daily to guppies (Poecilia reticulata) for three months. These invertivorous fishes were then consumed by piscivororus bluegill sunfishes (Lepomis macrochirus) for a subsequent three months. All members of the food web were analyzed for N-15 values and degree of fractionation using both bulk-tissue and amino acid stable isotope techniques. Our experiment demonstrated that the amino acid technique more accurately identified the true trophic position (TP) and food chain length (FCL = maximum TP) with significantly less variability around mean values for each consumer trophic level. Moreover, use of amino acids requires significantly fewer replicates to identify TP. We discuss here the relative advantages and disadvantages of both approaches for determining TP and FCL and recommend that investigators switch as soon as possible to the amino acid isotope technique for determining FCL.