A meta-analysis of amino acid δ15N trophic enrichment factors in fishes relative to nutritional and ecological drivers

The use of amino acid (AA) nitrogen stable isotopes (delta N-15) from consumer tissues aims to provide precise estimates of trophic position (TP), but the drivers of AA isotope fractionation remain unclear. In particular, the main factors driving the variability in TEFAA among taxonomic groups and trophic levels remain largely unexplained, which challenges the application of universal values for TEFs. While the relationship between protein content and quality and TEFs has been examined, studies have yielded inconsistent results, and the role of protein and lipid nutritional requirements as well as feeding regime have not been considered. Likewise, drivers that influence physiological and nutritional processes have not been examined relative to TEFAA variation. We conducted a meta-analysis of controlled feeding experiments within a single group, teleosts fishes, to evaluate the relationship between five nutritional factors (protein and lipid content, protein and lipid content relative to nutritional requirements, and feeding regime) and three ecological drivers (diet type, life stage, and habitat type) on TEFAA. We considered a broad range of protein levels (8-71%) in diets and found no relationship between source TEFAAs and percent protein relative to nutritional requirements, whereas lipid content relative to nutrient requirements, feeding regime and habitat type partially explain the variability in TEFs of Lys, but not for Phe and Met TEFs. The variability for the latter was representative of robust source AAs. Among trophic AAs, Asp, Ile, Pro, and Leu TEFs were significantly higher in species from brackish than marine habitats possibly due to osmoregulation involvement. TEFGlu was sensitive to protein content and feeding regime within teleosts, but relatively constant when comparing TEFs among teleosts, non-teleosts, and all taxa. Our results indicate that TEFAA is less variable within a single taxon than among multiple taxa and that such variation is not negligible. Our results indicate that delta N-15(AA) values could provide better TP estimates if using taxon-specific values, and highlights the need to explain the mechanisms of AA fractionation and quantify the variability in TEFs used during error propagation for TP estimates.

Automated summary

This study conducted a meta-analysis of controlled feeding experiments within teleost fishes to evaluate the relationship between nutritional factors and ecological drivers on AA isotope fractionation. The results suggest that source TEFAA values could provide better trophic position estimates if using taxon-specific values, and emphasize the need to further study the mechanisms of AA fractionation and quantify the variability in TEFs used for trophic position estimates.