Phytoplankton community assembly and health assessment in the middle-lower Jialing River via high-throughput sequencing

This study evaluates the aquatic ecosystem health of the Jialing River (Nanchong section), a key Yangtze tributary, through phytoplankton-based assessment, providing scientific support for its ecological conservation and rehabilitation. In May 2023, phytoplankton samples from 22 sites in the Jialing River system were analyzed via high-throughput sequencing. Multivariate analyses (Spearman/NMDS/RDA/SEM) revealed diversity patterns and drivers, enabling ecological quality assessment of the mainstem and Xichong River tributary. Community assembly mechanisms were further elucidated. Results: (1) Phytoplankton composition analysis revealed 341 species from 196 genera and 7 phyla in the Jialing River system. Bacillariophyta, Chlorophyta, and Cyanophyta were dominant phyla. The mainstem was dominated by Synechococcus and Chlorella, while Synechococcus, Aulacoseira, Cyclotella, and Gomphosphaeria were predominant in the Xichong River tributary. (2) Significant differences (P < 0.05) in phytoplankton community structure and diversity were observed between the Jialing mainstem and Xichong River, with permanganate index (CODMn) and total phosphorus (TP) identified as key influencing factors. (3) The P-IBI assessment classified the mainstem as ''healthy'' and the Xichong River as ''moderate'' in ecological status. P-IBI showed significant correlations (P < 0.05) with TP, CODMn, NH3-N, and diversity indices (Simpson, Pielou, Shannon). (4) Stochastic processes dominated phytoplankton community assembly, indicating relatively good ecosystem health. Overall, the mid-lower mainstem exhibited better ecological conditions than tributaries, though poor tributary health remains a potential risk to the entire watershed.