Integrative Assessment of Hong Kong Chironomidae (Diptera) Shows High Species Richness Linked to Spatial and Environmental Factors

Inland waters face escalating anthropogenic pressures, driving an unprecedented collapse in freshwater biodiversity. Enhanced knowledge of aquatic taxa is essential to reverse this decline. Chironomidae (non-biting midges), often the dominant zoobenthic group in freshwater ecosystems, remain poorly documented globally. Here, we provide the first integrative assessment of Chironomidae biodiversity in Hong Kong through a year-long survey of five streams. Integrative taxonomy expanded the known species in Hong Kong from 17 to 243, and yielded a reference library of 827 cytochrome c oxidase subunit I (COI) barcodes representing 225 species. Beta-diversity partitioning revealed that community dissimilarity was primarily driven by species turnover, which was strongly associated with environmental gradients but only weakly related to geographic distance. Variation partitioning revealed that environmental factors explained slightly more variation in community composition (9.0%) than spatial factors (6.7%). These patterns indicate that environmental filtering and mass effects play key roles in structuring Chironomidae metacommunities in Hong Kong, with dispersal limitation exerting little influence. Cross-database barcode matching analysis suggested that Hong Kong fauna is predominantly tropical-to-subtropical, with the strongest affinities to coastal East and Southeast Asia (e.g., eastern China, Thailand, Malaysia). Many species displayed wide geographic ranges, likely facilitated by high passive dispersal and broad ecological tolerances. This study delivers the first robust biodiversity baseline for Hong Kong Chironomidae and a well-curated DNA barcode library. These resources will benefit taxonomic refinement and eDNA-based biomonitoring, strengthening conservation of human-impacted freshwater ecosystems.