Transboundary algal migration-induced systemic environmental risk in the Yangtze River Basin, China

The horizontal transport of algal biomass or populations across administrative or ecosystem boundaries via hydrological connectivity poses systemic environmental risks in the Yangtze River Basin, despite recent improvements in river or lake water quality. Here, we investigated the dynamics of algal biomass along the Yangtze River mainstream through high-resolution monitoring spanning from February 2023 to December 2024. Results demonstrated a consistent increase in average chlorophyll-a concentrations from 4 µg/L to 8 µg/L between the Three Gorges Dam and Lake Dongting outlet. Random Forest and SHAP analysis identified water temperature as the strongest driver of algal biomass variation (36.6%), followed by total nitrogen (25.9%) and total phosphorus (23.6%). Interestingly, ship traffic generated previously overlooked thermal pollution, with navigational channels exhibiting ∼1 °C higher temperatures than adjacent shorelines. As a result, bidirectional hydrological connections between the Yangtze River and connected lakes facilitate transboundary algal migration, undermining lake-specific restoration efforts and exacerbating basin-wide ecological instability. These findings necessitate a paradigm shift from localized to integrated, basin-scale management. Furthermore, a framework that synergistically addresses algal migration, thermal pollution, and nutrient dynamics is urgently needed to mitigate systemic environmental risks in the Yangtze River Basin.