Research on the Gradient of Aquatic Ecological Integrity of Phytoplankton in Regional River Segments of Jiangsu Province and Its Driving Mechanism

To study the structure and distribution characteristics of the phytoplankton community in the Huaihe River Basin, Yangtze River Basin, and Taihu Lake Basin in Jiangsu Province, 126 sampling sites were set up in 35 rivers in the region, and samples were collected during the wet season (August–September) in 2023. Based on the monitoring results of phytoplankton, the study selected 20 candidate indicators and conducted range screening, discriminative ability analysis, and correlation analysis. Finally, seven core indicators were determined to construct the Phytoplankton Biological Integrity Index (P-IBI) evaluation system. The rating standards were determined by the ratio method, and the phytoplankton integrity of Jiangsu Province was evaluated. The differences were analyzed. As the results showed, the overall health status of rivers in Jiangsu Province was general. From the point of view of the basin scope, the Huaihe River Basin and the Yangtze River Basin were in sub-healthy state, and the Taihu Lake Basin was general. There were significant differences in the phytoplankton density community structure in the Yangtze River, Huaihe River, and Taihu Lake Basins. Phytoplankton integrity was positively correlated with total nitrogen and nitrate nitrogen, but not with other environmental factors. Nitrogen is the main factor affecting the integrity of river phytoplankton in Jiangsu Province.