Impact of the Construction and Operation of the Datengxia Water Conservancy Project on Fish Diversity

Understanding how dams restructure fish diversity is critical for biodiversity conservation and reservoir management. Here, we integrated species, taxonomic, and functional diversity within a unified framework to assess fish community responses to the Datengxia Water Conservancy Project impoundment on the Hongshui River (Pearl River basin). Based on surveys before (2013) and after (2020–2022) impoundment across upstream, downstream, and tributary reaches, we recorded 162 species, including four nationally protected, four endangered, two critically endangered, and 19 exotic taxa. Our three-dimensional assessment uncovered three key patterns: (1) a marked community shift toward limnophilic and slow-flowing species, with three new dominants (Ptychidio jordani, Culter alburnus, and Oreochromis mossambicus) emerging, while rheophilic and migratory forms contracted; (2) trait-selective diversity losses—locomotion-related functional richness (FRic) declined substantially (up to 42.1% at Wuxuan), whereas feeding-related FRic increased markedly (up to 55.1% at Laibin), and reproduction-related traits exhibited stable compositional profiles (spring-dominated spawning, demersal-adhesive eggs) with heterogeneous index responses across sections; taxonomic diversity revealed closer taxonomic relatedness and structural imbalance; (3) a consistent spatial gradient (tributary > downstream > upstream), with downstream assemblages exhibited lower spatial variation in diversity indices. These community shifts were observed alongside habitat modification (lotic habitat shrinkage and fragmentation) and coincided with exotic invasions and over-dominance of resident species. These patterns should be interpreted as descriptive findings given the observational design. This three-dimensional assessment framework provides complementary insights for evaluating dam-related ecological impacts, generating testable hypotheses for future research and conservation planning.