Dynamic drivers and thresholds of algal blooms under different thermal structure in a deep reservoir: Biomass and vertical distribution perspectives

High temperatures and stable stratification are recognized as the primary conditions for algal bloom formation. However, the frequent occurrence of algal blooms in low-temperature mixed water bodies suggests that changes in thermal structure will reshape the relationship between environmental factors and algal blooms. Investigating the key environmental factors associated with different thermal structures is essential for understanding the complex mechanisms that underlie bloom formation. Utilizing high-frequency in-situ monitoring data, this study used a machine learning algorithm (boosted regression trees, BRT) to quantify the contribution rates and thresholds of environmental factors affecting biomass (chlorophyll a , Chl- a ) and vertical distribution (Morisita's Index, MI) during the stratification and mixing periods. The findings indicate that environmental factors during the stratification period exhibit significant vertical heterogeneity, whereas those during the mixing period are more uniform. In terms of biomass, during the stratification period, nutrient-related factors such as ammonium nitrogen (NH₄ + -N) and nitrate nitrogen (NO₃ − -N) contributed most significantly to Chl- a , with contribution rates of 16.5 % and 15.7 %, respectively. During the mixing period, the oxidation-reduction potential (ORP) and water temperature (WT) exerted the most significant impact on Chl- a , with a total contribution rate exceeding 85 %. Regarding vertical distribution, Chl- a was a highly contributing factor to MI in both periods (contribution rate > 30 %), promoting vertical aggregation after exceeding the thresholds of 40 μg/L and 65 μg/L in the stratification and mixing periods, respectively. In addition, MI during the stratification period was also affected by thermal stratification and hydrodynamic factors, with S N below 310 J/m 2 or flow rates exceeding 250 m 3 /s being unfavorable for algal aggregation. This study reveals the dynamic differences and thresholds of the drivers of algal blooms under various thermal structures, providing a scientific basis for the precise prevention and management of algal blooms in deep reservoirs. • Changes in environmental factors lead to blooms under different thermal structures. • The drivers and thresholds of bloom of various thermal structures were determined. • Nutrient is the main driver of algal bloom in the stratification period. • The phased differential management in algal bloom management was emphasized.