Assessing the impacts of reclaimed water replenishment on riverine aquatic biodiversity using environmental DNA...

Reclaimed water has been extensively utilized for ecological water replenishment in rivers; however, its effects on multitrophic aquatic biodiversity remain poorly understood. This study assessed the integrated effects of reclaimed water on multitrophic communities and identified key water quality drivers influencing aquatic biodiversity. The results showed higher nitrogen levels during both seasons and elevated pH values in winter in the downstream sections receiving reclaimed water relative to the upstream section. Utilizing an eDNA metabarcoding approach, we identified 96, 47, and 42 species within the phytoplankton, invertebrate, and fish communities in summer, and 58, 39, and 28 species in winter, respectively. Additionally, phytoplankton (ANOSIM: R = 0.473, P = 0.007) and fish (summer: R = 0.605, P = 0.007; winter: R = 0.770, P = 0.001) communities exhibited significant spatial heterogeneity across upstream, midstream, and downstream sections, which were closely associated with reclaimed water discharge at the midstream inlet. Furthermore, TP, NH₄⁺-N, pH, and COD were identified as key water quality drivers restructuring multitrophic communities. Notably, TP, NH₄⁺-N, and COD collectively influenced summer communities, whereas pH, TN, NH₄⁺-N, and NO₃-N were the primary drivers of winter communities. These findings underscore the need to control nitrogen inputs and pH fluctuations in reclaimed water management and suggest the complementary value of biodiversity assessments alongside water quality monitoring, pending validation with more robust sampling.