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Rationale: Concerns exist over observed shifts in value and variance of nitrogen isotopes following physicochemical extraction of lipids from organic matter. The mechanisms behind these apparent changes in bulk tissue δ15N values are not fully understood yet have major implications for analytical costs and integrity of data interpretations. Methods: Changes in proximate analysis, amino acid composition, C:N ratios, bulk tissue and amino acid δ13C and δ15N values, and resulting isotope‐based food web metrics were compared between lipid‐intact and lipid‐extracted muscle tissue of fishes spanning <1% to >20% muscle fat content to identify mechanisms of nitrogen isotope fractionation associated with physicochemical lipid extraction. Results: Bulk δ13C and δ15N values increased and %N, C:N ratios and crude protein content decreased following lipid extraction. Resulting bulk isotope niche spacing and overlap varied significantly between lipid‐intact and lipid‐extracted tissues. While amino acid composition significantly changed during lipid extraction, particularly for lipid‐associated amino acids (e.g., Glu, Lys, Ser), individual amino acid δ13C and δ15N values, and their associated compound‐specific isotope analysis of amino acids (CSIA‐AA)‐based food web metrics, did not. Conclusions: Physicochemical lipid extraction caused significant tissue composition changes (e.g., leaching of amino acids and 15N‐deplete nitrogenous waste) that affected δ13C and δ15N values and tissue %C and %N beyond simply removing lipids. However, lipid extraction did not alter individual amino acid δ13C or δ15N values or their associated CSIA‐AA‐based food web metrics. [ABSTRACT FROM AUTHOR]

Fish ecologists have used geochemical values in otoliths to examine habitat use, migration, and population connectivity for decades. However, it remains difficult to determine an unambiguous dietary [δ.sup.13]C signature from bulk analysis of otolith. Studies to date have focused on the aragonite component of otoliths with less attention paid to the organic fraction. We describe the application of compound-specific stable isotope analysis (SIA) to analyze amino acid (AA) [δ.sup.13]C values from small amounts ( Depuis des decennies, les ecologistes des poissons emploient les valeurs geochimiques des otolithes pour etudier l'utilisation de l'habitat, la migration et la connectivite entre les populations. Il reste, cependant, difficile de determiner une signature [δ.sup.13]C alimentaire non ambigue par analyse globale de l'otolithe. Jusqu'a maintenant, les etudes se sont interessees a la composante aragonite des otolithes et ont porte moins d'attention a la fraction organique. Nous decrivons l'utilisation de l'analyse des isotopes stables (SIA) de composes specifiques pour l'analyse des valeurs de [δ.sup.13]C des acides amines (AA) sur de petites quantites ( [Traduit par la Redaction], Introduction The use of geochemical values in animal tissues as tags to track movement patterns of animals across isotope gradients in the environment (isoscapes) has become increasingly popular in terrestrial [...]

Compound‐specific stable isotope analysis of individual amino acids (CSIA‐AA) has emerged as a transformative approach to estimate consumer trophic positions (TPCSIA) that are internally indexed to primary producer nitrogen isotope baselines. Central to accurate TPCSIA estimation is an understanding of beta (β) values—the differences between trophic and source AA δ15N values in the primary producers at the base of a consumers' food web. Growing evidence suggests higher taxonomic and tissue‐specific β value variability than typically appreciated.This meta‐analysis fulfils a pressing need to comprehensively evaluate relevant sources of β value variability and its contribution to TPCSIA uncertainty. We first synthesized all published primary producer AA δ15N data to investigate ecologically relevant sources of variability (e.g. taxonomy, tissue type, habitat type, mode of photosynthesis). We then reviewed the biogeochemical mechanisms underpinning AA δ15N and β value variability. Lastly, we evaluated the sensitivity of TPCSIA estimates to uncertainty in mean βGlx‐Phe values and Glx‐Phe trophic discrimination factors (TDFGlx‐Phe).We show that variation in βGlx‐Phe values is two times greater than previously considered, with degree of vascularization, not habitat type (terrestrial vs. aquatic), providing the greatest source of variability (vascular autotroph = −6.6 ± 3.4‰; non‐vascular autotroph = +3.3 ± 1.8‰). Within vascular plants, tissue type secondarily contributed to βGlx‐Phe value variability, but we found no clear distinction among C3, C4 and CAM plant βGlx‐Phe values. Notably, we found that vascular plant βGlx‐Lys values (+2.5 ± 1.6‰) are considerably less variable than βGlx‐Phe values, making Lys a useful AA tracer of primary production sources in terrestrial systems. Our multi‐trophic level sensitivity analyses demonstrate that TPCSIA estimates are highly sensitive to changes in both βGlx‐Phe and TDFGlx‐Phe values but that the relative influence of β values dissipates at higher trophic levels.Our results highlight that primary producer β values are integral to accurate trophic position estimation. We outline four key recommendations for identifying, constraining and accounting for β value variability to improve TPCSIA estimation accuracy and precision moving forward. We must ultimately expand libraries of primary producer AA δ15N values to better understand the mechanistic drivers of β value variation. [ABSTRACT FROM AUTHOR]