Functional and trophic diversity of tropical headwater stream communities inferred from carbon, nitrogen and hydrogen stable isotope ratios

Abstract Tropical freshwaters support an immense diversity of fishes and invertebrates but are understudied in comparison to temperate systems. This is especially true of headwater streams, as only a small number of studies has assessed the trophic dynamics underpinning food web structure in these streams. We used stable isotopes of carbon, nitrogen and hydrogen to determine the resource use of dominant invertebrate guild and fishes in seven headwater streams in Eastern and Western Thailand, and assessed the functional and trophic diversity of each community using isotope food web metrics. Benthic invertebrates (95% credibility interval: 37–85%) and fishes (39–79%) obtained most of their resources from autochthonous sources in each stream but allochthonous and autochthonous specialists were evident in each community. We observed an increase in isotopic diversity of fishes associated with increasing stream size, but this was primarily driven by increase in the range of isotope ratios of allochthonous and autochthonous food web endmembers rather than an increase in functional diversity. Maximum trophic position did increase with stream size. The snakehead, Channa gachua, was enriched in 2H relative to all other fishes, possibly reflecting facultative air breathing by this species. Fish communities in the headwater streams analysed filled a variety of trophic niches, predominantly fuelled by autochthonous primary production.