A multi-task deep neural network reveals inflowing river impacts for predictive lake management.

Lake ecosystems, vital freshwater resources, are increasingly threatened by pollution from riverine inputs, making the management of these loads critical for preventing ecological degradation. Predicting the combined effects of multiple rivers on lake water quality is a significant challenge; traditional mechanistic models are computationally intensive and data-dependent, while conventional machine learning methods often fail to capture the system's multifaceted nature. This complexity creates a critical need for an integrated predictive tool for effective environmental management. Here we show a multi-task deep neural network (MTDNN) that can accurately and simultaneously predict four key water quality indicators-permanganate index, total phosphorus, total nitrogen, and algal density-at multiple locations within a complex lake system using data from its inflowing rivers. Our model, applied to Dianchi Lake in China, improves predictive precision by up to 56.3 % compared to established mechanistic and single-task deep learning models. Furthermore, the model pinpoints the specific contributions of each river and identifies water temperature and wastewater effluent as dominant, site-specific drivers of pollution. Scenario-based forecasting demonstrates that using reclaimed water for lake replenishment is a viable strategy that does not cause deterioration. This MTDNN framework offers a powerful and transferable tool for data-driven lake management, enabling targeted interventions and sustainable water resource protection.