Taxonomic and functional diversity of benthic invertebrates along a human impact gradient

In the context of increasing human pressures on freshwater systems, assessing the diversity of riverine biotic communities is essential for sustaining functional ecosystems. This study aimed to identify the main drivers of taxonomic and functional diversity in zoobenthic invertebrate communities across a strong gradient of anthropogenic pressure on freshwater systems. Benthic invertebrates were investigated at 23 sampling sites across three river basins (Someșul Cald, Someșul Rece, and Someșul Mic) in September 2020, within a densely populated region of Transylvania, Romania. Environmental parameters measured during sampling were used to explain community patterns; taxonomic diversity was calculated from zoobenthic densities, while six traits were used to assess functional diversity: body size, mode of living, life cycle, general tolerance, dietary specialization, and functional feeding group. Zoobenthic communities comprised 19 major taxonomic groups, including worms, mollusks, water mites, crustaceans, and insects, encompassing both sensitive and tolerant taxa. Diversity and water quality indices were significantly lower in the Someșul Mic basin, which includes areas strongly affected by human settlements. Human impact score contributed significantly to the explained variation in diversity patterns, alongside natural factors such as altitude, conductivity, sediment composition, flow velocity, and pH. Environmental filtering operated across all river sections. However, while the Someșul Cald and Someșul Rece communities showed high taxonomic diversity and functional redundancy consistent with an insurance effect, the Someșul Mic assemblages exhibited reduced diversity and redundancy, indicating increased functional vulnerability. This integrated taxonomic–functional perspective provides a useful framework for freshwater bioassessment, conservation, and restoration, particularly in densely populated regions.