Trophic relationships in a tropical stream food web assessed by stable isotope analysis

1. Stable isotope analysis, coupled with dietary data from the literature, was used to investigate trophic patterns of freshwater fauna in a tropical stream food web (Guadeloupe, French West Indies). 2. Primary producers (biofilm, algae and plant detritus of terrestrial origin) showed distinct delta C-13 signatures, which allowed for a powerful discrimination of carbon sources. Both autochthonous (C-13-enriched signatures) and allochthonous (C-13-depleted signatures) resources enter the food web. The migrating behaviour of fishes and shrimps between marine and freshwater during their life cycles can be followed by carbon isotopes. Here, shrimp delta C-13 signatures were shown to shift from -16 parts per thousand (for juveniles under marine influence) to -24.7 parts per thousand (for adults in freshwater habitats). For resident species, delta C-13 values partly reflected the species' habitat preferences along the river continuum: species living in river mouths were C-13-enriched in comparison with those collected upstream. 3. Nitrogen isotopic ratios were also discriminating and defined three main trophic guilds among consumers. The delta N-15 values of herbivores/detritivores were 5.0-8.4 parts per thousand, omnivores 8.8-10.2 parts per thousand and carnivores 11-12.7 parts per thousand. 4. Mixing model equations were employed to calculate the possible range of contribution made by respective food sources to the diet of each species. The results revealed the importance of omnivorous species and the dependence of riverine biota on terrestrial subsidies, such as leaf detritus and fruits. Finally, the abundance of shrimps and their feeding habits placed in relief their key role in tropical freshwater food webs. Isotopic analysis provides a useful tool for assessing animal feeding patterns.