Stable nitrogen isotopic composition of amino acids reveals food web structure in stream ecosystems
The stable N isotopic composition of individual amino acids (SIAA) has recently been used to estimate trophic positions (TPs) of animals in several simple food chain systems. However, it is unknown whether the SIAA is applicable to more complex food web analysis. In this study we measured the SIAA of stream macroinvertebrates, fishes, and their potential food sources (periphyton and leaf litter of terrestrial C3 plants) collected from upper and lower sites in two streams having contrasting riparian landscapes. The stable N isotope ratios of glutamic acid and phenylalanine confirmed that for primary producers (periphyton and C3 litter) the TP was 1, and for primary consumers (e.g., mayfly and caddisfly larvae) it was 2. We built a two-source mixing model to estimate the relative contributions of aquatic and terrestrial sources to secondary and higher consumers (e.g., stonefly larva and fishes) prior to the TP calculation. The estimated TPs (2.3–3.5) roughly corresponded to their omnivorous and carnivorous feeding habits, respectively. We found that the SIAA method offers substantial advantages over traditional bulk method for food web analysis because it defines the food web structure based on the metabolic pathway of amino groups, and can be used to estimate food web structure under conditions where the bulk method cannot be used. Our result provides evidence that the SIAA method is applicable to the analysis of complex food webs, where heterogeneous resources are mixed.
KeywordsPeriphyton Terrestrial C3 litter Aquatic animals Compound-specific isotope analysis Two-source mixing model
We thank M. Itoh, K. Osaka, Y. Kohmatsu, H. Nakagawa, T. Egusa, S. Ishikawa, W. Hidaka, Y. Tamiya, and Y. Takaoka for their assistance in fieldwork. N. Tokuchi, N. Ohte, M. Kondoh, Y. Chikaraishi, N. Ohkouchi, R. O. Hall, and two anonymous reviewers provided valuable comments on an early draft of this manuscript. This research was supported by the Environment Research and Technology Development Fund (D-1102) of the Ministry of the Environment, Japan. Partial support was also provided by the River Fund (24-1215-022) in charge of the River Foundation and a Grant-in-Aid for Scientific Research (B) (no. 25291101). N. F. I. was supported by the Research Fellowship for Young Scientists (25-1021) of the Japan Society for the Promotion of Science.
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