Beyond concentration exceedances: Benthic macroinvertebrate β-diversity reveals ecological responses to metal pollution in a large river basin.

Heavy metal contamination is widespread in large river basins, yet its ecological consequences are often inferred mainly from concentration exceedances, leaving community-level biological responses insufficiently resolved. Given that β-diversity captures spatial community reorganization that may not be detected by local α-diversity, we examined whether taxonomic and functional β-diversity can reveal ecological responses to metal pollution beyond conventional chemical assessment. Surface water, sediments, and benthic macroinvertebrates were sampled at 23 sites in the Xiangjiang River basin, China, during the wet and dry seasons in 2023. Arsenic, cadmium, chromium, copper, manganese, nickel, and lead, together with physicochemical variables, nutrients, sediment particle size, and taxonomic and functional diversity metrics, were quantified. All metals were detected in nearly all water and sediment samples in both seasons. Arsenic, cadmium, and lead frequently exceeded guideline values for the protection of aquatic life, with maximum water-phase concentrations of 141, 2.90, and 52.2 μg/L, respectively. Among 59 recorded macroinvertebrate taxa, moderately pollution-tolerant taxa dominated. Biological Monitoring Working Party scores indicated moderate-to-bad ecological quality in the wet season, with improvement in the dry season. Taxonomic and functional α-diversity were generally higher in the dry season, whereas taxonomic and functional β-diversity were higher in the wet season. β-Diversity responded more strongly than α-diversity to environmental gradients (marginal R2: 0.73-0.87 vs. 0.25-0.59) and was significantly associated with turbidity, sediment texture, and water-phase heavy metals. Path analysis further suggested direct effects of these variables and indirect seasonal effects through turbidity and water-phase metals. These findings indicate that concentration exceedance screening alone does not fully characterize the community-level ecological consequences of metal pollution, highlighting the value of integrating chemical monitoring with multidimensional biomonitoring in large river basins.