Response of phytoplankton functional group to spring drought in a large subtropical reservoir

Global warming has caused an increase in the frequency and duration of droughts worldwide. Droughts could trigger large changes in physico-chemical conditions and phytoplankton community in waterbodies, resulting in a shift in the phytoplankton community. Spring diatom blooms in reservoirs have been increasingly observed in the past decade in the Taihu Lake basin. The aim of the present study is to elucidate the impacts of droughts on aquatic environment and to determine the driving factors for the succession of the phytoplankton functional groups based on the analysis of data collected during spring from 2009 to 2020 in the Daxi Reservoir. The unimodal relationship between 1-month aggregated precipitation index and phytoplankton species richness indicated the competitive exclusion occurred in extremely drought period. The structural equation modeling indicated that drought-related low water level conditions intensified sediment resuspension, and increased the phosphorus-enriched nonalgal turbidity in the Daxi Reservoir. Concurrently, a steady shift in the Reynolds phytoplankton functional groups from L0, TD, J, X2, and A (phytoplankton taxa preferring low turbidity and nutrient conditions) to TB (pennate diatoms being adapt to turbid and nutrient-rich conditions) was observed. The increased TP and non-algal turbidity in addition to the lowered disturbance contribute to the prevalence of Group TB. Considering the difficulties in nutrient control, timely water replenishment is often a feasible method of controlling the dominance of harmful algae for reservoir management. Finally, alternative water sources are in high demand for ensuring ecological safety and water availability when dealing with drought.