Temporal prediction of algal parameters in Three Gorges Reservoir based on highly time-resolved monitoring and long short-term memory network

Many dammed rivers throughout the world have experienced frequent harmful algal blooms (HABs) in the context of climate change and anthropogenic activities. Accurate forecasting of algal parameters (i.e., algal cell density and microcystin concentration) has great practical significance for taking precautions against HABs risks. Long short-term memory (LSTM) networks have recently shown potential in predicting water quality parameters. However, there is still little known about the robustness of the LSTM in forecasting highly time-resolved measurement of algal parameters. This study developed a hybrid deep-learning architecture (XG-LSTM) composed of one XGBoost module and two parallel LSTM models to predict algal cell density and microcystin concentration in the Three Gorges Reservoir (TGR). The proposed model was validated by in situ multi-sensor-system monitoring data at four bloom-impacted tributaries in the TGR. Each modelling process utilized the antecedent information of the algal parameters and the corresponding environmental variables as inputs for forecasting the algal parameters for the coming hours and days. As expected, the presented model achieved better performance than those without special feature extraction procedures, providing that the use of selected environmental parameters can improve LSTM performance. In addition, the hybrid XG-LSTM model successfully captured the time-series patterns of both algal cell density and microcystin concentration compared with other data-driven models, further suggesting the reliable utilization of this model in early warnings of bloom toxicity. Thus, the results presented demonstrate the potential of deep learning technology for real-time prediction of algal parameters in the TGR, and possibly for rapid detection of developing HABs in other aquatic ecosystems.