Phytoplankton Community Structure and Aquatic Ecological Health in the Tongyang River Basin (China) and Their Response to Nutrients Reduction Project.

The Tongyang River Basin, a rural tributary of Lake Chaohu with numerous polders, has been subjected to intensive anthropogenic degradation, leading to the implementation of a series of in situ nutrient reduction projects during 2023-2024. Based on spring samplings at 22 sites before (2021 and 2022) and after (2025) the projects, we characterized phytoplankton community structure and evaluated ecological health using the phytoplankton community index (Qr) and water quality index (WQI). Nonmetric multidimensional scaling (NMDS) revealed a significant postproject shift in overall community structure (permutational multivariate analysis of variance [PERMANOVA], R2 = 0.28, p < 0.001), with the postproject community exhibiting increased relative abundance of Bacillariophyta, Euglenophyta, and Chlorophyta, accompanied by a decline of Cyanobacteria. Redundancy analysis identified permanganate index, ammonium, orthophosphate, and turbidity as key drivers of community structure. Mean Qr increased significantly after project implementation (p < 0.05), with the top dominant functional groups shifting from low-scoring H1 (score = 1) to high-scoring MP (score = 3), reflecting reduced dissolved nutrient stress. In contrast, WQI declined slightly with DO and CODMn as the primary contributors (50.42% and 35.29%). This divergence highlights that Qr was more sensitive than WQI in detecting early ecological recovery, underscoring the value of biological indicators in restoration assessment.