Effects of Sluice Interception on Water Quality and Spirogyra in the Typical Irrigation Ditches of Jianghan Plain, China

To elucidate water quality evolution and algal responses in sluice-controlled ditches, this study combined in situ monitoring (July–October 2025) in the Chong Lake Watershed of Jianghan Plain (China) with controlled experiments at Changjiang River Scientific Research Institute. This study provided the first evidence of how sluice-induced hydrodynamic changes affect water quality and Spirogyra outbreaks in Jianghan Plain irrigation ditches. In situ monitoring showed that sluice interception significantly altered hydrodynamics, reducing dissolved oxygen (DO) by 18% and increasing chlorophyll-a and total phosphorus by 32% and 12%, respectively, compared to control ditches. Simulation experiments confirmed these trends: under sluice control, suspended solids and DO decreased by 30% and 19%, while ammonia nitrogen and phosphate increased by 8% and 13%; nitrate nitrogen dropped by 20%. Spirogyra dominated both systems but shifted from attached filaments in controls to floating clumps in sluice-controlled ditches, with biomass rising 94%. Pearson correlation linked Spirogyra biomass negatively to DO and positively to ammonia and phosphate. Sluice interception promotes eutrophication and Spirogyra blooms by reducing DO and particulates, which inhibits nitrification and releases soluble phosphate. A flow velocity of 0.05 m·s−1 effectively suppresses such outbreaks.