Rainstorm events trigger algal blooms in a large oligotrophic reservoir

Algal blooms occasionally occur in oligotrophic lakes or reservoirs; however, the mechanisms and conditions that trigger bloom formation remain unclear. Therefore, high-frequency (every 3 days) monitoring and high-resolution (100 sites) surveying of the algal biomass were performed during the summer of 2018-2020 to explore rainfall effects on algal growth and its associated mechanism in Xin'anjiang Reservoir, a large, deep, and oligotrophic reservoir in the subtropical region of China. The results showed that, in the short-term, rainfall inhibited algal growth in the river zone, while promoted algal growth in the lake zone. Path analysis (PLS-PM) and correlation analysis revealed that rainfall inhibited algal growth in the river zone by increasing discharge and decreasing light availability (turbidity) and water temperature, while promoted algal growth in the lake zone by increasing the nutrient concentrations, especially for phosphorus. In addition, we found that the stronger the rainfall is, the wider regions of the reservoir it impacted. Monthly observations at 100 sites showed substantial increases in both algal biomass (mean Chla concentration doubled) and nutrients concentrations in almost the whole reservoir after rainstorm events in Xin'anjiang Reservoir, indicating that rainstorm-induced nutrient pulsing loading could trigger algal blooms in oligotrophic reservoirs in the long-term. Extreme weather events are predicted to occur more frequently as the global climate changes in the future, and oligotrophic reservoirs are facing a greater risk of algal blooms.