Harmful algal blooms (HABs) have become an environmental issue of global concern due to their wide distribution and sudden outbreak. The existing research mainly focuses on the attribution of the long-term trend of algal blooms, in which eutrophication has been considered as the main driving factor. However, this trend-based analysis framework does not reliably predict the occurrence of specific water blooms, largely due to insufficient explanation of the interaction mechanism between persistent drivers and event triggers. In this study, Hulun Lake was taken as an example to distinguish the red tide driving factors at annual, monthly and daily scales to clarify the interaction between persistence maintenance factors and short-term meteorological trigger factors. On an annual scale, HABs have shifted from localised, low-frequency events to widespread, high-frequency outbreaks, peaking in 2022. Water blooms mainly occurred in July–August, concentrated in the semi-enclosed southwest and northern bays. Total nitrogen and precipitation are key long-term predictors, reflecting the role of eutrophication and hydrological variability. On the monthly scale, HABs were divided into four types according to frequency and severity. This classification shows that meteorological activation and nutrient structure together determine the bloom pattern. HABs are more likely to occur under ' warm, humid, calm and nutrient-rich ' conditions. The key thresholds are: air temperature >19.7 °C (Interquartile coefficient of variation, IQRCV = 9 %), relative humidity >61 %(IQRCV = 21 %), wind speed <3.2 m/s(IQRCV = 37 %). On the daily scale, the formation of HABs is driven by cumulative meteorological effects. When 7-day air temperature >103 °C (IQRCV = 77 %), relative humidity >380 % (IQRCV = 77 %), wind speed <3.0 m/s (IQRCV = 77 %), the risk is further increased. These findings support a multi-scale strategy that combines watershed nutrient control with real-time meteorological monitoring.
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