Multi-assemblage biotic integrity index customized for different hydrological phases: Spatiotemporal aquatic health monitoring of a subtropical coastal urban river

Abstract Urban rivers in subtropical coastal regions suffer from intensive anthropogenic disturbances and strong seasonal hydrological fluctuations. However, most biological integrity assessments rely on single biotic groups and overlook hydrological seasonality, leading to unstable and one-sided evaluations. In this study, we developed hydrological period-specific Indices of Biotic Integrity (IBI) based on phytoplankton (P-IBI), zooplankton (Z-IBI), periphytic algae (A-IBI), benthic macroinvertebrates (B-IBI) and fish (F-IBI). A comprehensive multi-taxa assemblage IBI (m-IBI) was further integrated from the five single-group indices. Coupled with the Water Quality Index (WQI), we evaluated the the spatial and seasonal variation of aquatic ecosystem health in the Dongxixi River, Xiamen. Results revealed that the overall ecological status of the whole river basin remained sub-healthy throughout all three hydrological periods. Single-taxon IBIs exhibited obvious spatiotemporal heterogeneity, whereas m-IBI showed higher stability and consistent correlation with WQI. Core biological indicators driving ecosystem status shifted with hydrological conditions, reflecting seasonal succession and functional complementarity among multi-trophic aquatic communities. Correlations between individual IBIs and physicochemical factors differed markedly across seasons. As a synthetic metric integrating multi-level biological responses, m-IBI eliminates evaluation bias originating from single-taxon limitations and presents robust performance across all hydrological periods. This study emphasizes the importance of integrating hydrological seasonality and multi-assemblage communities into river health evaluation, and provides a practical framework for assessment and ecological restoration of subtropical coastal urban rivers.