Spatial variation in fish assemblages is explained by hydrological connectivity

Abstract Hydrological connectivity in river systems facilitates the movement of animals, nutrients, and energy across riverine landscapes but is threatened by climate change and water resource developments. These anthropogenic disturbances are expected to alter connectivity patterns in wet-dry tropical river systems that are driven by a highly variable seasonal flow regime. We studied the composition and functional diversity of fish assemblages across a large wet-dry tropical river system in northern Australia that is under consideration for new agricultural and water resource developments. We explored changes in the environment, species turnover, and relationships between environmental characteristics and fish species composition, quantifying how they were influenced by hydrological connectivity, to identify species and habitats at risk of impacts of reduced connectivity. Characteristics of the aquatic environment were significantly different between low connectivity, high connectivity, and off-channel sites, and these environmental characteristics were associated with changes in fish community composition. Fish species turnover occurred across the catchment, with unique assemblages in headwater sites. High turnover between adjacent sites of low connectivity suggested that reduced movement opportunity creates localized populations, and these differences in species presence/absence are likely to be exacerbated by reduced connectivity from water resource developments. Species turnover was also related to functional diversity, which itself varied across connectivity levels. Habitat specialists and species with functional traits that depend on hydrological connectivity, such as diadromous species, are most at risk from anthropogenic change. Protection of the flow regime and environmental characteristics that support these species is important for maintaining biodiversity.