Decadal Land-Use Change and Water Quality Degradation Reshape the Functional Structure of Fish Assemblages in Guangxi Rivers, China

Long-term changes in land-use and water quality degradation are key pressures on riverine ecosystems, yet their effects on the functional structure of fish assemblages remain insufficiently quantified. We conducted a decadal assessment of fish assemblages in four rivers experiencing rapid land-use transformation in Guangxi, China—the Zuojiang (ZY), Youjiang (YJ), Yujiang (YuJ), and Guijiang (GJ) rivers. Species-level functional traits describing body shape, vertical habitat position, feeding guild, trophic level, and maximum total length were used to calculate multiple functional diversity indices related to environmental variables using redundancy analysis and random forest models. Over the decade from 2013 to 2023, impervious surface and bare land increased significantly, while forest, shrubland, and grassland declined, coinciding with marked deterioration in water quality, including higher chlorophyll-a and lower dissolved oxygen concentrations (p < 0.05). Fish abundance and species richness decreased, and formerly dominant benthic cyprinids and loaches were replaced by tolerant and invasive taxa such as Oreochromis niloticus and Tilapia zillii. NMDS and PERMANOVA indicated a significant shift in community composition between 2013 and 2023 (R2 = 0.14, p < 0.001), with fish assemblages shifting toward small-bodied, pelagic, omnivorous, and laterally compressed species. Functional richness declined, whereas functional evenness and functional dispersion increased, indicating functional simplification and homogenization. Multivariate and machine learning models consistently identified increasing impervious cover and declining dissolved oxygen as the strongest predictors of these changes. Our results demonstrate that land-use intensification and water quality deterioration are associated with reduced functional integrity in Guangxi rivers and highlight the value of trait-based indicators for monitoring and managing biodiversity in rapidly urbanizing catchments.