Assessment of water ecological health in the South-to-North Water Diversion Middle Route main canal based on P-IBI

The South-to-North Water Diversion Middle Route Project is one of the largest inter-basin water diversion projects in the world. Since its operation, the water ecological system in the main canal of the middle route has been undergoing establishment and evolution, underscoring the urgent need for a robust scientific ecosystem assessment framework to evaluate its current characteristics. This study, based on the Phytoplankton Index of Biological Integrity (P-IBI), on the basis of interannual comparison, utilized data from the autumn low-flow period of 2018 to select five diagnostic indicators: the number of phytoplankton species, cyanobacteria density, total phytoplankton density, dominance of the top three dominant species, and density of water quality indicator species. A tailored water ecological health assessment system for the main canal of the middle route was constructed, with core P-IBI indicators for the 2024 low-flow period chosen to validate the system's rationality and to reveal interannual dynamics in water ecological conditions. Results indicate that when the low-flow period is used as the evaluation hydrological period, the proportion of interannual ecological health increased by 34 % from 2018 to 2024, demonstrating that the project maintains a relatively healthy ecological state overall, the corresponding P-IBI shows a decreasing trend along the canal. Against the backdrop of interannual water quality improvement, the algal community maintained high species richness even at low densities. This study confirms that P-IBI-based assessment of phytoplankton integrity can accurately reflect the health status of the middle route of the South-to-North Water Diversion Project and provides methodological support for ecological health assessments of artificial water diversion projects.