Heatwaves and gradual warming differentially alter freshwater fish biodiversity and functional structure

Mean temperature rises and heatwaves both affect biodiversity1-3, yet their relative importance and the underlying mechanisms driving their impacts on freshwater biodiversity remain unclear. Here, using fish survey datasets encompassing 424 freshwater fish species across 27,639 sampling events at 3,822 sites over 39 years (1980–2018) in the Northern Hemisphere, we assessed the effects of gradual warming and heatwaves on riverine fish biodiversity. Warming was associated with higher total species richness, despite a reduced proportion of native species. This effect was more pronounced for natives in colder regions and larger streams, and for non-natives under greater human pressure. In contrast, heatwaves accelerated temporal assemblage turnover. Heatwaves and warming also showed divergent associations with changes in fish physiological and morphological traits. Specifically, heatwaves exerted more severe effects on natives, reducing their warming tolerance and depleting high trophic level species. Our analysis further revealed a distinct threat of heatwaves to economically important fish stocks. Given that baseline warming and heatwaves differentially reshape the taxonomic and functional structure of freshwater communities, targeted mitigation strategies are needed to safeguard freshwater ecosystems.