Abstract:

To analyze the health status of river ecosystems in the Naqu River Basin and reveal the impacts of cascade hydropower development, phytoplankton samples were collected from 17 survey stations across the Naqu River Basin in June 2025. Water environmental parameters were measured. The phytoplankton species were identified, and their cell density and biomass were calculated. The distribution characteristics of phytoplankton communities were analyzed, and the phytoplankton index of biotic integrity (P-IBI) evaluation system was constructed and applied to assess the ecological health status of the habitat in the Naqu River Basin. A total of 7 phyla and 82 species (genera) of phytoplankton were identified in the study area. Bacillariophyta accounted for over 60% of the total species, followed by Chlorophyta and Cyanophyta. Average phytoplankton density and biomass were higher in the upper reaches than in the lower reaches. Four stations in the upper reaches of the main stem and one station in a tributary were classified as “healthy” to “sub-healthy”, two stations in two midstream tributaries were rated as “moderate”, while ten stations in the middle and lower reaches of the main stem and downstream tributaries were categorized as “poor”. In the Naqu River Basin, sections classified as “healthy” or “sub-healthy” are mainly located in the upper reaches of the main stream, where human activities have minimal impact. Altitude, alkalinity, pH, and dissolved oxygen are the main environmental factors related to the ecological health status of rivers in the Naqu River Basin.

Key words: cascade hydropower, Naqu River Basin, phytoplankton, habitat, ecological health assessment