Taxonomic and functional responses of benthic diatoms to river modification by small hydropower plants

Small hydropower plants are increasingly recognised as important drivers of hydromorphological alteration in river ecosystems, yet their effects on benthic diatoms remain insufficiently understood. We investigated the taxonomic and functional responses of benthic diatom assemblages to two operational-channel hydropower plants along the Hernád River (Hungary). Twenty-five benthic samples were collected from the main channel, operational channels, and hydromorphologically modified river sections. Taxonomic composition differed markedly among habitat types, whereas functional trait composition showed weaker differentiation, indicating functional redundancy despite pronounced taxonomic turnover. Species richness, effective Shannon diversity, and evenness did not differ significantly among habitat categories; however, functional richness and functional divergence revealed significant differences, highlighting functional reorganisation associated with hydromorphological modification. Significant distance–decay relationships were detected within both operational channels and modified river sections, suggesting that spatial structuring persisted despite river fragmentation. Fifteen threatened taxa and several non-native species were recorded, but neither habitat-specific segregation of threatened taxa nor invasion hotspots were identified. Our results demonstrate that small hydropower plants can substantially modify the taxonomic and functional organisation of benthic diatom assemblages without necessarily reducing local taxonomic diversity. The observed responses suggest community reorganisation rather than biodiversity loss and highlight the importance of maintaining longitudinal connectivity in regulated rivers.