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Compound-specific nitrogen stable isotope analysis provides an approach for estimating animal trophic position that may overcome key issues associated with stable isotope analysis of bulk tissue. Yet compound-specific trophic discrimination factors have not been estimated for a broad range of habitats, taxa, and diets. We conducted a controlled-feeding experiment to characterize the variation in compound-specific ([TDF.sub.AA]0) and bulk ([TDF.sub.Bulk]) trophic discrimination factors of four freshwater fish species fed on three distinct diets. We also compared [TDF.sub.AA] of fish muscle and scale to evaluate the viability of scales for making food web inferences. Mean [+ or -] 1 SD [TDF.sub.Bulk] was 2.2[per thousand] [+ or -] 0.9[per thousand], and there were significant effects of species and diet trophic position on [TDF.sub.Bulk]. Mean [+ or -] 1 SD [TDF.sub.AA] was 6.9[per thousand] [+ or - ] 0.8[per thousand]. Although there was no effect of species on [TDF.sub.AA], there were significant differences in [TDF.sub.AA] across the three diets. [TDF.sub.AA] from fish scales were not significantly different from those of muscle. Our study illustrates the advantages of estimating trophic position using compound- specific stable isotopes and the need for continued investigation of factors resulting in variation in TDF values. L'analyse des isotopes stables d'azote de composes precis fournit une approche pour estimer la position trophique des animaux qui pourrait permettre de surmonter certains problemes cles associes a l'analyse des isotopes stables de tissus en vrac. Cependant, les facteurs de discrimination trophique de composes precis n'ont pas ete estimes pour un vaste eventail d'habitats, de taxons et de regimes alimentaires. Nous avons mene une experience d'alimentation controlee afin de caracteriser les variations des facteurs de discrimination trophique pour des composes precis ([TDF.sub.AA]) et des echantillons en vrac ([TDF.sub.Bulk]) pour quatre especes de poissons d'eau douce nourries selon trois regimes alimentaires distincts. Nous avons egalement compare les [TDF.sub.AA] de muscles et d'ecailles de poisson afin d'evaluer l'utilite des ecailles pour faire des inferences concernant le reseau trophique. Le [TDF.sub.Bulk] moyen [+ or -] 1 ET etait de 2,2 [per thousand] [+ or -] 0,9 [per thousand], et il y avait des effets significatifs de l'espece et de la position trophique du regime alimentaire sur les valeurs de [TDF.sub.Bulk]. Le [TDF.sub.AA] moyen [+ or -] 1 ET etait de 6,9 [per thousand] [+ or -] 0,8 [per thousand]. S'il n'y avait aucun effet de l'espece sur les valeurs de [TDF.sub.AA], des variations significatives des [TDF.sub.AA] entre les trois regimes alimentaires etaient presentes. Les [TDF.sub.AA] des ecailles de poisson n'etaient pas significativement differents de ceux des muscles. L'etude illustre les avantages d'estimer la position trophique a partir d'isotopes stables de composes precis et la necessite de poursuivre l'etude des facteurs de variation des TDF. [Traduit par la Redaction], Introduction The estimation of animal trophic position using stable nitrogen isotope analysis is an important approach in food web ecology (Cabana and Rasmussen 1996; Vander Zanden and Rasmussen 1999; Post [...]

The differential discrimination of nitrogen isotopes (15N/14N) within amino acids in consumers and their diets has been routinely used to estimate organismal tropic position ( TP). Analogous isotopic discrimination can occur within plants, particularly in organs lacking chloroplasts. Such discrimination likely arises from the catabolic deamination of amino acids, resulting in a numerical elevation of estimated TP, within newly synthesized biomass. To investigate this phenomenon, we examined the 15N/14N of amino acids (δ15 NAA) in spring leaves and flowers from eight deciduous and two annual plants. These plants were classified on the basis of their time of bloom, plants that bloomed when their leaves were absent (Type I) versus plants that bloomed while leaves were already present (Type II). Based on the δ15 NAA values from leaves, both plant types occupied comparable and ecologically realistic mean TPs (=1.0 ± 0.1, mean ± 1σ). However, the estimated TPs of flowers varied significantly (Type I: 2.2 ± 0.2; Type II: 1.0 ± 0.1). We hypothesize that these results can be interpreted by the following sequence of events: (1) Type I floral biomass is synthesized in absence of active photosynthesis; (2) the catabolic deamination of amino acids in particular, leaves behind 15N in the residual pool of amino acids; and (3) the incorporation of these 15N-enriched amino acids within the biomass of Type I flowers results in the numerical elevation of the TPs. In contrast, the actively photosynthesizing Type II leaves energetically sustain the synthesis of Type II flower biomass, precluding any reliance on catabolic deamination of amino acids. Amino acids within Type II flowers are therefore isotopically comparable to the Type II leaves. These findings demonstrate the idiosyncratic nature of the δ15 NAA values within autotrophic organs and have implications for interpreting trophic hierarchies using primary producers and their consumers. [ABSTRACT FROM AUTHOR]

The differential discrimination of nitrogen isotopes ( 15 N/ 14 N) within amino acids in consumers and their diets has been routinely used to estimate organismal tropic position (TP). Analogous isotopic discrimination can occur within plants, particularly in organs lacking chloroplasts. Such discrimination likely arises from the catabolic deamination of amino acids, resulting in a numerical elevation of estimated TP, within newly synthesized biomass. To investigate this phenomenon, we examined the 15 N/ 14 N of amino acids (δ 15 N AA ) in spring leaves and flowers from eight deciduous and two annual plants. These plants were classified on the basis of their time of bloom, plants that bloomed when their leaves were absent (Type I) versus plants that bloomed while leaves were already present (Type II). Based on the δ 15 N AA values from leaves, both plant types occupied comparable and ecologically realistic mean TPs (=1.0 ± 0.1, mean ± 1σ). However, the estimated TPs of flowers varied significantly (Type I: 2.2 ± 0.2; Type II: 1.0 ± 0.1). We hypothesize that these results can be interpreted by the following sequence of events: (1) Type I floral biomass is synthesized in absence of active photosynthesis; (2) the catabolic deamination of amino acids in particular, leaves behind 15 N in the residual pool of amino acids; and (3) the incorporation of these 15 N-enriched amino acids within the biomass of Type I flowers results in the numerical elevation of the TPs. In contrast, the actively photosynthesizing Type II leaves energetically sustain the synthesis of Type II flower biomass, precluding any reliance on catabolic deamination of amino acids. Amino acids within Type II flowers are therefore isotopically comparable to the Type II leaves. These findings demonstrate the idiosyncratic nature of the δ 15 N AA values within autotrophic organs and have implications for interpreting trophic hierarchies using primary producers and their consumers.