Contribution for lake phytoplankton community succession under multi-source artificial hydrological regulation

In aquatic ecosystems, phytoplankton, as key primary producers, are extremely sensitive to environmental changes. Artificial hydrological regulation, as a water resource management strategy, affects the water quality and ecological environment of the receiving water area. Particularly under multi-source replenishment conditions, the spatiotemporal nonlinear response of water quality poses a challenge to quantify the contribution of multi-source replenishment to the phytoplankton community succession in large lakes. For example, Baiyangdian Lake (BL) has implemented multi-source artificial hydrological regulation measures, including the diversion of the Yellow River into the BL (Xiaobai River), the South-to-North Water Diversion Project (Baigouyin River) and the replenishment of upstream reservoirs (Xiaoyi River, Bao River and Fu River). Physicochemical parameters and phytoplankton community data were surveyed from 2017 to 2020 in July (summer) and October (autumn). The ecological impacts of artificial hydrological regulation were investigated by quantifying the correlations between physicochemical parameters and phytoplankton communities and by quantifying the contribution of artificial hydrological regulation to key factors. The results show that under the artificial hydrological regulation, the phytoplankton community was still dominated by Chlorophyta and Cyanophyta, but the emergence of Xanthophyta and Chrysophyta indicated that the cleanliness of the water column was improved, and the biodiversity indicators showed a fluctuating upward trend, which suggested that the succession trend was favorable. Ammonia nitrogen (NH₃-N), total phosphorus (TP), and dissolved oxygen (DO) are the key factors affecting phytoplankton community succession in BL. The replenishment sources contribution index (RSCI) was used to characterize the spatiotemporal response of the key factors, and it was found that Fu River has the highest contribution of more than 30%, followed by Xiaoyi River with a contribution of more than 25%. The spatial distribution of RSCI shows that the area with high RSCI values (>0.9) of Fu River accounts for more than 50% of the total area, the area with high RSCI values (>0.9) of Xiaoyi River accounts for more than 35%. Under the comprehensive influence of the RSCI of each replenishment source, the phytoplankton biodiversity index of BL increased from north to south. This study provides basic support for BL in realizing the optimized artificial hydrological regulation strategy and enhancing the ecological restoration benefits in the receiving water area.