The construction of a reservoir dam alters the environment within its basin, including composition of the fish community, fish biodiversity, and the river ecosystem itself. This study was conducted in the Sanbanxi Reservoir and used eDNA metabarcoding technology comprising eDNA capture and extraction, PCR amplification, sequencing and database comparison analysis, and other environmental DNA metabarcoding standardized analysis processes to characterize the composition and diversity of fish communities and assess their current status. A total of 48 species of fish were detected. Previously, 68 species of fish were screened and identified in this reservoir based on the reports of Dai and Gu. The results for fish community composition showed that species of the order Cypriniformes are still the most dominant in the Reservoir with 38 species of cyprinids, accounting for 90.81% of all OTUs. Carp were no longer the dominant species, and Spinibarbus denticulatus, Homalopteridae, Cobitidae, and Sisoridae were not detected, with the exception of Misgurnus anguillicaudatus (Cobitidae). These families have the common characteristic of being adapted to survive in fast-water, sandstone substrate habitats. The results also show that two of the sampling sites, sbx03 and sbx10, significantly differed from other sampling sites due to their geographical environment. The impact of the construction of reservoirs on freshwater fish communities is extreme, since the transformation from a lotic to a lentic habitat contributes to habitat destruction and constrains fish in movement. The change in the aquatic environment before and after the storage of water in the Sanbanxi Reservoir has reduced the number of fish species found in the reservoir, and species characteristically found in fast moving, rapids habitats are virtually absent. The profound change in the aquatic environment from that of a lotic to a lentic habitat leads to changes in the composition of fish populations in the reservoir and to a certain extent a reduction in the ecological stability and species diversity within the reservoir. Therefore, the protection of fish diversity in the reservoir is of great significance to the stability of the ecosystem.

An accurate assessment of fish stocks is crucial for maintaining the health and stability of aquatic ecosystems. To understand the characteristics of fish diversity in the Qingshui River, this study utilized high-throughput sequencing data obtained through environmental DNA macrobarcoding technology (eDNA). The research explored differences in fish diversity and community characteristics in the Qingshui River by analyzing fish community structure, α diversity, β diversity, and the redundancy of environmental factors. This analysis aims to provide data support for water ecological monitoring and management. The results were as follows: (1) A total of 91 species of fishes from 18 families and seven orders were detected in the eDNA survey, and the dominant species was Cypriniformes, accounting for 72.5%. (2) There were significant differences in α diversity analysis in the Qingshui River (p < 0.05). (3) The fish community structure was similar in β diversity analysis. (4) In the redundancy analysis of environmental factors, pH, DO, TN, TP, ${\mathrm{N}\mathrm{O}}_2^{-}$ and ${\mathrm{M}\mathrm{n}\mathrm{O}}_4^{-}$ have the greatest impact on the Qingshui River Basin. eDNA technology has the advantages of high efficiency and low harm and can be used for water ecological monitoring and management. It provides a scientific basis for fish protection and a valuable reference for fish monitoring in the future.

1. As the primary producers of aquatic ecosystems, phytoplankton act as sensitive bioindicators that reflect environmental fluctuations through shifts in their community composition (Reynolds 2006)...