Abstract: Compared with traditional taxonomy methods, environmental DNA macrobarcoding technology can detect biodiversity in ecosystems more comprehensively and efficiently. To clarify the responses of biodiversity and community structure at multiple trophic levels to changes in altitude gradient and environmental factors, we collected water and sediment samples from 11 sampling points in the upper reaches of the Yuhe River and its tributary, the Xiaoyu River in June 2021. Biodiversity and community structure of fish, phytoplankton, and zoobenthos were investigated using environmental DNA metabarcoding. The results showed that a total of 16 fish species in 16 genera were detected in the upper reaches of the Yuhe River and the Xiaoyu River, along with 128 phytoplankton species in 140 genera, and 47 species in 175 genera of benthic fauna. Among them, Cypriniformes was the dominant fish species, Bacillariophyta was the dominant phytoplankton species, and Arthropods was the dominant zoobenthos. There were significant differences in alpha diversity between the upper and lower reaches of the Xiaoyu River and the upper reaches of the Yuhe River. The community structure of fish and phytoplankton exhibited prominent spatial heterogeneity. Altitude significantly negatively correlated with environmental factors, including water temperature, pH, electric conductivity (EC), salinity (SAL), and total dissolved solids (TDS). Results of the DistLM analysis and correlation heat map analysis showed that organisms with lower trophic levels were more sensitive to changes in altitude and environmental factors. Among them, altitude, water temperature, pH, EC, SAL, TDS, total phosphorus (TP), and nitrite nitrogen (NO₂^--N) significantly impacted phytoplankton and zoobenthos. The main influencing factors for fish were altitude, water temperature, pH, and total nitrogen (TN). For species with the highest abundance at each sampling point, environmental factors (i.e. water temperature, suspended solids (SS), pH, SAL, and TDS) and altitude exerted both direct and indirect impacts on organisms of different trophic levels. Our results provide a reference for biodiversity conservation and water ecological health assessment in the Qinling Mountains.

Key words: Yuhe River, altitude gradient, environmental factor, biodiversity, community structure, trophic level, environmental DNA metabarcoding.