Macroinvertebrate responses to flow intermittence: when mesohabitat matters

Abstract River networks are highly dynamic environments, where local conditions range from lotic to lentic, promoting the co-occurrence of highly diverse biota. These environments are threatened by various human-induced stressors, among which water scarcity affects more than half of all running waters globally. While flow intermittence occurs naturally, its spatial and temporal extension is spreading under climate change and human pressures, endangering river biota. Here, we performed a mesocosm study aiming to investigate how flow reduction during drying events affects macroinvertebrate communities from different mesoscale habitats, such as riffles and pools. The experiment was performed in a replicated flow-through mesocosm system during summer 2021. We tested the effects of both intermittent and prolonged three-month-long flow reduction on macroinvertebrate communities from riffle and pool mesohabitats in terms of community composition, and resilience and resistance functional traits (e.g., resistance form, current preference, locomotion, dispersal strategy, reproduction drift propensity, etc.). Sampling was performed before, during, and after the exposure to flow reduction to assess both the direct effect of water scarcity and the post-drought recovery of macroinvertebrate communities. We found that communities from riffle habitats were more severely affected by flow reduction during drying events, showing a more severe decline in taxonomic richness and reduced abundance of desiccation-sensitive organisms under prolonged flow reduction treatments compared to intermittent ones. During flow reduction events, we did not observe a consistent taxa turnover toward drought-tolerant taxa, with only a few resistance trait modalities (e.g., organisms with tolerance for higher water temperature or interstitial ones) significantly associated with prolonged flow reduction. Moreover, the communities from riffle mesohabitats did not fully recover even one month after normal flow conditions were re-established, showing a low post-drought resilience. In pool mesohabitats, we did not detect significant effects of intermittent or prolonged flow reduction, with a community composition dominated by generalist taxa. These findings highlight the importance of accounting for mesohabitat-specific responses to drying when evaluating the ecological consequences of increasing flow intermittence and suggest that habitat heterogeneity plays a critical role in shaping the resistance and resilience of macroinvertebrate communities under water scarcity.