Trophic support of species-rich tropical fish communities remains uncertain in small forest streams where dense and constant canopy cover strongly limits the solar radiation available to aquatic producers. We sampled >1300 fish from 80 species in 14 remote headwater streams during the dry season in French Guiana. Carbon and nitrogen stable isotope analyses and mixing models were run to estimate the trophic position (TP) of each species and its dependence on terrestrial organic carbon sources. Matching patterns were found between nitrogen stable isotope ratios and the literature on diets of species at the base and top of the food web. Nearly 60% of the species fed exclusively on animal prey (mean TP > 3), including piscivorous and parasitic fish species. Phylogenetic analysis revealed that TP were distributed broadly along the phylogeny and changed gradually within lineages. Carbon stable isotope ratios differed strongly between herbivores that fed on aquatic primary producers and those that fed on terrestrial plants along banksides. Terrestrial carbon was dominant in nearly 90% of species, and most of them depended more or less exclusively on terrestrial carbon. Ancestral state reconstructions indicated that only a few lineages showed local changes toward the consumption of algal carbon. We conclude that fish in neotropical headwater streams shaded by a dense tree canopy depend strongly on terrestrial subsidies. Changes in this aquatic-terrestrial linkage would have major consequences for the remarkable fish biodiversity in these streams.

Stable isotope analyses (d13C, d15N, and C/N ratio) of 1306 fish from 80 species collected in 14 remote headwater streams in French Guiana during the dry season , 2011-2012.