Flow variability and macroinvertebrates jointly regulate stream periphyton and metabolism: Insights from experimental stream mesocosms

Climate change profoundly alters riverine flow regimes and community composition, affecting key ecosystem functions. We used an experimental mesocosm approach to examine how gradual flow velocity reduction (Experiment 1) and flushing events (Experiment 2) influence periphyton community composition and metabolism, with and without a macroinvertebrate assemblage. We prepared eight stream mesocosms with pre‐grown periphyton, half including macroinvertebrates. Six mesocosms gradually transitioned from high (0.25 m s −1 ) to low flow velocity (0.05 m s −1 ), followed by three flushing events of increasing frequency (i.e., reducing time between events) and same intensity, raising from 0.05 to 0.25 m s −1 for 6 h before returning to base flow. Two control mesocosms (one with and one without macroinvertebrates) remained at constant flow (0.1 m s −1 ) throughout the experiment. We measured algal biovolume, taxonomic composition, and metabolic rates (gross primary production; ecosystem respiration; net ecosystem production) over time. Macroinvertebrates altered community composition and reduced algal biovolume, with stronger effects at low flow. Flow reduction had scale‐dependent effects: at the chamber scale it lowered periphyton gross primary production and net ecosystem production, while at the whole‐mesocosm scale it decreased ecosystem respiration more than production, increasing net ecosystem production. Flushing events decreased algal biovolume, but enhanced periphyton autotrophy, though this effect weakened with repeated disturbance. Macroinvertebrate assemblages, while reducing total algal biovolume, enhanced the resistance of metabolic responses to flushing. Together, these results show that hydrological variability and macroinvertebrate presence jointly regulate periphyton structure and function and provide mechanistic insight into the processes controlling carbon cycling in running waters.