Stable Isotope Discrimination by Consumers in a Tropical Mangrove Food Web: How Important Are Variations in C/N Ratio?

Abstract

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 Curuca Estuary, northern Brazil. Our results showed that the relative amount of the heavy isotope significantly increased with TL. Linear models of δ 15N vs TL were highly significant and explained 75 % of the total variability in food web δ 15N. Models for δ 15N yielded a much better fit than the models built for δ 13C, mainly due to a higher variability in source δ 13C and stronger trophic discrimination for 15N than for 13C. Enrichment in δ 15N was 2.3 ± 0.20 ‰ per TL (all data). The increase in δ 13C 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 13C-rich algae-based food web. Furthermore, the increase in δ 13C 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.