Multiple stressors in river networks: local and downstream effects on freshwater macroinvertebrates

Abstract

River networks are complex ecosystems characterized by a continuous exchange of material and energy through longitudinal gradients. These ecosystems are threatened by various human‐induced stressors, which frequently co‐occur and may interact in complex ways, potentially triggering cascading effects throughout the river network. Aiming at assessing single and combined effects of flow intermittency and light pollution on macroinvertebrate communities, we performed a multiple stressors experiment in 18 flow‐through mesocosms. Each mesocosm was designed to mimic a simplified river network, with two upstream tributaries merging downstream, allowing us to assess both local and cascading effects. The experiment was performed in summer 2021 over seven weeks, applying the stressors either separately or co‐occurring in the upstream sections, following a randomized block design. Flow intermittency was simulated as the ponded phase of the drying process, whereas light pollution was applied with LED strips set to 10 lux. Drifting macroinvertebrates were sampled weekly during the treatment phase, and benthic macroinvertebrates were sampled at the end of the treatment phase. Both stressors, when applied individually, reduced benthos richness and abundance, whereas drift decreased with flow intermittency and increased with light pollution. When co‐occurring upstream, stressors showed the dominant effects of flow intermittency on the benthos and interactive effects on the drift. The effects of the single stressors and their interactions cascaded along the river network, with stronger downstream effects when stressors co‐occurred upstream. These findings show that the spatial distribution of multiple stressors along the river network can affect their resultant downstream effects, highlighting the importance of framing multiple‐stressors research in a spatial context. Considering the pressing needs of the growing human population, our results represent a step forward in anticipating the effects of cumulative stressors and in informing efficient conservation strategies for protecting freshwater ecosystems.