[Analysis of Mechanism and Start-up Thresholds of Seasonal Algal Blooms in a Northern Eutrophic Stratified Reservoir].

Seasonal algal blooms produce a high risk for water supply safety. To explore the mechanism of seasonal algal blooms in northern eutrophic stratified reservoirs, the combination of taxonomic and functional classifications, local weighted regression (LOWESS), and Boundary line analysis (BLA) were employed to obtain the succession features and environmental thresholds of seasonal (e.g., spring and summer) algal blooms, based on the long-term and high-frequency monitoring from 2017 to 2020 in Lijiahe Reservoir. The results showed that:① the succession and response mechanisms of algal blooms were different in spring and summer. In detail, Chlorophyta, Bacillariophyta, and Dinoflagellates (e.g., low-temperature, small, high surface-to-volume genera) dominated in spring, whereas Chlorophyta, Bacillariophyta, and Cyanobacteria (e.g., high-temperature, large or colonial, low surface-to-volume genera) dominated in summer. The differences in physiological and morphological characteristics of algae were the internal cause triggering seasonal algal blooms. ② The main drivers of algal blooms were different in spring and summer. Spring blooms were controlled by water temperature (WT), mixing depth (i.e., ), and light availability (i.e., /), whereas summer blooms were jointly influenced by WT, , /, and total phosphorus (TP). The differences in the changes of the major drivers were external causes triggering seasonal algal blooms. ③ The water environment thresholds starting seasonal algal blooms were different in spring and summer. The thresholds of WT, , and / in spring were>9.4℃, <10.9 m, and>0.24, respectively, whereas the thresholds of WT, , /, and TP in summer were>16.0℃, <11.6 m, >0.16, and>0.011 mg·L, respectively. Based on the research on the mechanism of seasonal algal blooms and related thresholds, this work will provide a reference for the control of subsequent algal blooms.