Temporal changes in rainfall affect taxonomic and functional composition of stream fish assemblages in central Amazonia

Climate change is a major threat to biodiversity. Rainfall patterns over recent decades have changed and extreme events are intensifying. These are expected to affect the hydrological dynamics and structure of forest streams in the Amazon basin, which strongly depend on the local rainfall regime, with potential consequences for local fish assemblages. Here we used four repeated surveys of stream fish assemblages across 10 × 10 km area of a pristine forest reserve in central Amazonia over an 18‐year (2001–2018) period to investigate the temporal dynamics of fish taxonomic and functional composition in relation to changes in stream habitats and local climatic regime. We investigated changes in magnitude, direction, and congruence of fish assemblage trajectories over time. Total annual rainfall increased over time, with stormy days (i.e. daily rainfall above 20 mm) becoming more frequent during recent decades and a concomitant change of stream substrates from litter‐dominated to free sand. The overall taxonomic and functional composition of fish assemblages changed consistently over time, with stronger differences between the first and last surveys due to alterations in relative abundances of some of the most abundant species. Functional composition moved from open water species (compressed‐bodied species with less developed fins and terminal‐oriented mouths) to species adapted to lateral pools and shallow marginal areas (species with fusiform body shape, more developed fins, and superior‐oriented mouth) We provide evidence of a concerted directional change of fish assemblages associated with increased precipitation and a higher frequency of stormy days over the last 2 decades. Considering the short time interval and the subtleness of the climatic changes assessed in this study, as well as the historically assumed structural stability of Amazonian streams, these findings are surprising and indicate those stream fish assemblages may respond quickly to climate changes.