Atmospheric heatwaves expand hypoxic zones in a deeply stratified mega-reservoir

Atmospheric heatwaves are emerging as a dominant stressor on freshwater ecosystems under global climate change. However, the impacts of atmospheric heatwaves on hypoxia formation and dissipation in deep lakes remain insufficiently understood. Here, we used the high-frequency buoy data and meteorological data (2017–2024) to examine the effects of extreme high air temperature events on thermal stratification and dissolved oxygen (DO) in Lake Qiandao, a deep subtropical mega-reservoir in China. On average, Lake Qiandao experienced four atmospheric heatwave events per year, with a total duration averaging 27 days per year and the 2022 event setting a record duration (55 days). Prolonged atmospheric heatwaves and consequent enhanced thermal stratification reduced subsurface DO by more than 32 % and expanded deep hypoxic zones vertically, covering up to 40 % of the water column during extreme events. Our findings reveal a 14-day lag between heatwave-induced stratification intensification and hypoxia development, highlighting a delayed but predictable ecosystem response. Future projections suggest that the duration of atmospheric heatwaves could increase fivefold by 2100 under high greenhouse gas emissions scenarios, substantially exacerbating hypoxia risks. These findings underscore atmospheric heatwaves as major drivers of hypoxia in lakes and call for urgent nutrient control and climate mitigation efforts to safeguard water quality and ecosystem resilience. • Atmospheric heatwaves amplify lake warming and intensify thermal stratification. • Intensified stratification suppresses oxygen mixing and expands hypoxic zones. • Hypoxia development exhibits a lagged response to thermal stratification. • Future warming projects a substantial expansion of summer hypoxic zones.