Climate change affects litter decomposition in the benthic and hyporheic zones of stream mesocosms

Abstract Leaf litter decomposition is crucial for the functional ecology of streams, governing nutrient cycling, energy flows and food web configurations. Climate change affects stream ecosystem properties by altering temperature, sediment composition and flow regimes, hence potentially disturbing the natural dynamics of litter decomposition. This study examines the individual and combined impacts of warming, fine sediment and low‐flow on microbial, shredder‐mediated and total leaf litter decomposition in benthic and hyporheic zones, employing a large‐scale mesocosm experiment. Decomposition rates were consistently higher in the benthic zone than in the hyporheic zone (+36%) due to greater oxygen availability and shredder activity, as demonstrated by a significant positive association between total decomposition and shredder density in the benthic zone. However, the hyporheic zone exhibited greater sensitivity to stressors, particularly fine sediment, which impeded vertical exchange and reduced oxygen transport, causing significant declines in microbial, shredder‐mediated and total decomposition rates. Low‐flow emerged as a critical stressor with ambivalent effects: it reduced macroinvertebrate densities and decomposition rates in the benthic zone, but simultaneously increased shredder density in the hyporheic zone. This migration led to enhanced shredder‐mediated decomposition in the hyporheic zone under low‐flow conditions, highlighting its refuge function for macroinvertebrates during droughts. These effects were negated in fine sediment treatments, where physical clogging and anoxic conditions significantly reduced microbial and total decomposition rates in the hyporheic zone (19% and 41%, respectively). Notably, warming had a dual role: it slightly enhanced microbial decomposition in the benthic zone, but led to a negative synergistic trend when combined with low‐flow. Overall, interactions amongst stressors were most pronounced in the hyporheic zone when fine sediment was combined with one or both other stressors, likely due to amplified anoxia and reduced accessibility for decomposer communities. This study underscores the complex interplay between multiple stressors affecting stream ecosystem functionality and the linkage between benthic and hyporheic zones, with fine sediment presenting the most pervasive threat by impairing hyporheic exchange processes and habitability. The results emphasize the urgent need for targeted management strategies to mitigate sedimentation and sustain critical ecosystem services under changing climatic conditions. Read the free Plain Language Summary for this article on the Journal blog.