Effects of low-head dams on fish assemblages in subtropical streams: Context dependence on local habitat and landscape conditions

•Beta diversity was used to quantify the degree to which local fish assemblages were altered by low-head dams.•Beta diversity driven by low-head dams varied spatially across basins and reference types.•Beta diversity related to Euclidean distance of habitat characteristics and/or landscape conditions.•Functional diversity helped to better understand the effect of dams on fish assemblages.•We provided guidelines for future research on low-head dams affecting fish assemblages. Low-head dams are a ubiquitous human disturbance in headwater streams. The effects of low-head dams on stream fish assemblages have gathered increasing interest in the past 20 years; whereas, the effects remain controversial among the existing studies. In this study, we collected fishes from 58 impoundments created by low-head dams and 58 reference free-flowing segments below or above dams within headwater streams of four basins in Wannan Mountains, China. We used the beta diversity, including taxonomic and functional indices based on species composition and species traits, respectively, to quantify the degree to which fish assemblages were altered by low-head dams by comparing pairwise assemblages between impoundment and reference free-flowing segment for each dam surveyed. We aimed to assess whether taxonomic and functional beta diversities varied across the 58 low-head dams surveyed and how beta diversities related to local habitat, landscape conditions, and dam size measures. We found that both taxonomic and functional beta diversities showed quite high spatial variation, with the coefficient of variance of 54.47% and 76.74%, which were dominated by species turnover (68.8%) and functional nestedness (62.8%), respectively. Taxonomic beta diversity, functional beta diversity and functional turnover rate significantly varied across basins, and taxonomic turnover and nestedness rates significantly varied between reference types (i.e., free-flowing segments below vs. above dams). We also found that taxonomic beta diversity, functional beta diversity, and functional turnover rates significantly related to local habitat and/or landscape conditions. Environmental factors explained more spatial variances in functional diversity than taxonomic diversity. Our results suggest that the effects of low-head dams on fish assemblages vary spatially across different dams and relates to environmental factors. In addition, our results highlight the need for reliable approaches for assessing the effect