Seasonal water storage of large reservoirs exacerbates eutrophication risk in the fluctuating backwater zone: A case study of Three Gorges Reservoir, China
Xian Liu, Wei Huang, Zixuan Qi, Cangbai Li, Guanhui Cheng, Yanpeng Cai
Abstract
The construction and operation of large-scale hydraulic projects improve water resource utilization efficiency, but negatively impact ecology and environment, especially exacerbating the eutrophication risk in the fluctuating backwater zones (FBZs). However, Due to its dual characteristics of both rivers and reservoirs, eutrophication patterns in FBZs have not been explored comprehensively. This study focuses on the FBZ formed by the Three Gorges Dam (TGD), the world’s largest hydropower plant. By integrating the multi-source monitoring data with Normalized Difference Chlorophyll Index (NDCI) inversed by remote sensing data, interpretable machine learning methods (XGBoost and SHAP) were applied to quantify the contributions of key eutrophication drivers. The results showed that as the water storage level rose to 175 m (2010–2016), the total phosphorus (TP) and total nitrogen (TN) concentrations in the FBZ increased by 10.4 % and 5.9 % respectively, compared to the natural river period (2004–2005). While TP concentrations have decreased in recent years (2021–2023), the pollution risk from TN has not been mitigated. Eutrophication drivers exhibit notable variability across different temporal scales, with pollutant emissions resulting from intense human activities being the primary drivers of elevated TN and TP concentrations on an annual scale. Seasonal variations in rainfall and temperature result in temporal heterogeneity in nutrient concentrations on a monthly scale. SHapley Additive exPlanation (SHAP) analysis reveals that the TGR’s water storage volume and runoff load in the FBZ account for 45.9 % of the total influence on algal blooms. This result highlights that reduced flow velocity, driven by upstream and downstream dam impoundments, is the dominant factor behind algal bloom outbreaks in the FBZ. This study analyses the complex influences that trigger algal blooms in the FBZ which differ from rivers and lakes， and that could provide important references for managing eutrophication in global FBZs.