Decadal water quality dynamics in lakes along the Eastern Route of the South-to-North Water Diversion Project: Divergent trends and hydrometeorological drivers.

Understanding how hydraulic engineering interacts with water quality is essential for the sustainable management of large inter-basin water transfer systems. This study investigates decadal water quality trends and their dominant drivers in three key lakes-Lakes Gaoyou, Hongze, and Luoma-along the Eastern Route of the South-to-North Water Diversion Project (ER-SNWDP) from 2011 to 2023. Spatiotemporal variations in total nitrogen (TN), total phosphorus (TP), and chlorophyll-a (Chl-a) were analyzed using Ensemble Empirical Mode Decomposition, Generalized Additive Models, and Partial Least Squares Path Modeling to disentangle hydrometeorological and anthropogenic influences across multiple time scales. The results indicate progressive eutrophication in Lakes Gaoyou and Luoma, whereas Lake Hongze remained relatively stable. From 2011 to 2015, TLI decreased from 56.7 to 53.0 in Lake Hongze and from 50.4 to 44.5 in Lake Luoma, coinciding with reduced pollution pressure and enhanced freshwater inflow. Since 2015, however, all three lakes exhibited renewed deterioration. This is marked by a rebound in TLI to 51.9 in Lake Luoma and 56.4 in Lake Hongze by 2023, driven by distinct mechanisms: nutrient accumulation and pollution influx from the Huaihe River in Lake Gaoyou, inflow variability coupled with climatic fluctuations in Lake Luoma, and hydrological-temperature interactions in Lake Hongze. At monthly and seasonal scales, inflow nutrient loads, rainfall, and hydrological conditions dominated water quality variability, while at longer timescales, warming and hydrological variability became increasingly important, potentially enhancing internal nutrient mobilization. These findings demonstrate the critical role of hydrometeorological processes in regulating lake water quality under large-scale water transfer operations and highlight the need for lake-specific, adaptive management strategies emphasizing nutrient load reduction and flow regime optimization to enhance system resilience under climate change.