Decoupling between physicochemical recovery and ecological integration in reclaimed water ecological buffer zones

Ecological buffer zones (EBs) are designed to reduce water quality disparities between reclaimed water and receiving waters, but whether physicochemical and biological recovery occur over similar spatial scales remains unclear. In Xiamen, China, surface water was sampled along a river-type ecological buffer zone (R-EB) and a lake-type ecological buffer zone (L-EB), from their reclaimed water inflows to their downstream endpoints. The fate of 350 emerging contaminants (ECs) across 9 categories was monitored, with 170 detected and their combined concentrations ranging from 0.700 to 713 ng/L. Pharmaceuticals, organophosphate esters (OPEs), phthalate esters (PAEs), and pesticides accounted for approximately 80% of total EC concentrations, which decreased by more than 35% within 50 m of the inflows. In contrast, community recovery inferred from environmental DNA (eDNA) metabarcoding extended nearly 4,000 m downstream. This recovery was accompanied by taxonomic turnover, including increased Cyclotella read representation and a hydrophyte shift from Canna to Commelina. Lower-trophic assemblages showed the strongest associations with EC gradients, suggesting greater sensitivity. Most nutrient-related parameters, including COD, NH₃-N, and TP, generally met the surface water standard (Class V, GB 3838-2002, China) in R-EB and L-EB. However, TN consistently exceeded the standard (TN, 2 mg/L), ranging from 4.63 to 23.18 mg/L, posing a persistent eutrophication risk that could hinder ecological integration. Physicochemical recovery occurred rapidly near the inflows, whereas ecological integration extended much farther downstream. These findings provide an ecological basis for optimizing EB design and reclaimed water management through source control of TN in reclaimed water effluent and ecological measures that enhance nitrogen assimilation and removal.