A reach-scale bioassessment index for highly disturbed large rivers based on Species-Area (length) models

Ecological health assessment in large rivers remains challenging due to their tremendous natural environmental heterogeneity, the difficulty of obtaining representative samples, and the absence of near-natural reference conditions. Using the Yangtze River as a case study, we develop a reach-scale biological index (O/E_SL) for large rivers using fish and macroinvertebrate data. The index is developed in three steps. First, we classify reaches of the Yangtze River mainstem and tributaries using long-term stable spatial and geomorphic factors to control natural variability. Second, we expand the assessment scale from sites to reaches to enhance the spatial representativeness and robustness of the assessment results. Third, drawing on the species-area (length) relationship, we construct observed species (S_O)-length (L) quantile regression models for each reach type to objectively define reference conditions, estimate expected species richness (S_E), and calculate the O/E_SL index as the ratio of S_O to S_E. The O/E_SL index demonstrates robust sensitivity to hydromorphological alterations and reveals a concerning ecological status in the Yangtze River-only 13% of the 93 evaluated reaches are classified as healthy. Finally, we propose a basic methodological framework for ecological health assessment in highly disturbed large rivers.