Assessing multi-trophic aquatic community responses to reclaimed water discharge in an arid urban river system

Reclaimed water replenishment plays a crucial role in sustaining urban river systems in arid regions where natural water resources are scarce. However, its ecological impacts, particularly on microbial community diversity and structure, remain insufficiently understood, with significant knowledge gaps regarding how reclaimed water replenishment influences these dynamics. Here, we quantified changes in water quality and multi-trophic community structure along natural (NR), reclaimed (RR), and mixed (NRR) sections of the Xiaohei River, an arid urban river in northern China. The results indicate that reclaimed water discharge altered the physicochemical conditions of the river systems, particularly in terms of nutrient enrichment, with total nitrogen and total phosphorus concentrations increasing on average by 67% and 84%, respectively. After reclaimed water replenishment, significant shifts in community composition and diversity were observed, with the mean richness of algae and metazoans increasing by 20.2% and 29.7%, respectively, while that of bacteria and protozoa decreased by 19.6% and 4.1%. Nitrogen and phosphorus nutrients, together with hydrodynamic conditions, emerged as the primary drivers of biodiversity variation, with particularly strong associations observed for bacterial, algal, and protozoan communities. Chlorophyta and Rotifera displayed contrasting responses to reclaimed water replenishment, with Chlorophyta exhibiting a monotonic linear response and Rotifera showing a nonlinear (hump-shaped) relationship with physicochemical gradients, underscoring differential responses across trophic levels. This study improves understanding about the ecological effects of reclaimed water replenishment on aquatic ecosystems. It provides a scientific basis for nutrient control and ecological monitoring to support coordinated water allocation and ecological conservation in arid urban river systems.