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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Stable Isotope Discrimination by Consumers in a Tropical Mangrove Food Web: How Important Are Variations in C/N Ratio?
Auteurs :	Ralf Schwamborn, Auteur ; Tommaso Giarrizzo, Auteur
Type de document :	Article
Dans :	Estuaries and Coasts (38(3) MAY 2015)
Article en page(s) :	813-825
Résumé :	Stable isotopes are widely used as tracers in food webs and as indicators of the trophic levels (TL) of consumers. The objectives of this study were to verify a possible ecosystem-wide increase in the relative contribution of the heavy isotope with trophic level, for carbon and nitrogen, and to analyze the relationships of C/N ratios and stable isotope ratios of consumers in a tropical mangrove food web. Samples of primary producers and consumers (benthic invertebrates, zooplankton, 31 fish species, and other vertebrates) were collected in the Curu double dagger A Estuary, northern Brazil. Our results showed that the relative amount of the heavy isotope significantly increased with TL. Linear models of delta N-15 vs TL were highly significant and explained 75 % of the total variability in food web delta N-15. Models for delta N-15 yielded a much better fit than the models built for delta C-13, mainly due to a higher variability in source delta C-13 and stronger trophic discrimination for N-15 than for C-13. Enrichment in delta N-15 was 2.3 +/- 0.20 aEuro degrees per TL (all data). The increase in delta C-13 with TL, in spite of being significant, could not be used to estimate trophic enrichment, since it was affected by the concentration of predatory fish in a C-13-rich algae-based food web. Furthermore, the increase in delta C-13 with TL could be fully explained by the change in C/N ratio (i.e., lipid content) with TL. Our results demonstrate the importance of considering TL and C/N ratios in stable isotope analyses of food webs.

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