Prolonged low flows and non‐native fish operate additively to alter insect emergence in mountain streams

Abstract Climate‐induced flow alteration is decreasing snowpack and advancing snowmelt, subjecting mountain streams to longer low‐flow periods. Yet, anticipating how stream ecosystems respond to prolonged low flows remains challenging because distinct trophic levels can respond differently, and non‐native predators can dampen or amplify responses. Here, we conducted a large‐scale mesocosm experiment to examine how early, prolonged low flows projected by the end of the century in California's Sierra Nevada may alter mountain stream food webs and emerging insect flux—a critical stream‐to‐land cross‐ecosystem linkage. Additionally, we tested whether Brown trout ( Salmo trutta ), a widespread non‐native top predator, would change food‐web responses to low‐flow conditions. We found that early low flows and non‐native fish had additive effects. Early low flows did not alter the overall rate of emerging insects, but they shifted community structure and reduced the prevalence of small‐sized individuals—possibly reflecting larger size at emergence and faster growth rates due to warming. In contrast, non‐native fish presence increased seasonally‐aggregated abundance of stream insects up to 12%, mainly by increasing abundance of Chironomidae and small‐sized Ephemeroptera and Trichoptera. In channels with fish, benthic algal biomass doubled, and scraper‐grazer and collector‐gatherer insects emerged 60% and 55% more, likely benefiting from trout predation on mesopredators. This experiment illustrates that prolonged low flows and invasions can profoundly alter mountain river food webs even when operating additively. We also show how mesocosm‐based research can help explain disruption of cross‐ecosystem linkages driven by global change.