The study investigates the impact of summer heatwaves on the growth and spread of harmful cyanobacteria, specifically *Microcystis*, in Lake Nieuwe Meer, a hypertrophic lake in the Netherlands. During the 2003 summer heatwave, which was one of the hottest recorded in Europe, intermittent artificial mixing failed to control *Microcystis* blooms. The researchers developed a coupled biological-physical model to understand the mechanisms behind this failure. The model combines a phytoplankton competition model with a one-dimensional hydrodynamic model driven by meteorological data. Key findings include: 1. **High Temperatures Directly Favor *Microcystis***: High temperatures increase the growth rates of *Microcystis*. 2. **Stability of the Water Column Reduces Vertical Mixing**: High temperatures also increase the stability of the water column, reducing vertical turbulent mixing. 3. **Combined Effects of Meteorological Conditions**: The combination of reduced cloudiness, low wind speeds, and high air temperatures led to a higher summer abundance of *Microcystis* compared to any single factor in isolation. 4. **Model Predictions**: The model accurately predicted the observed temperature and turbulence structures, as well as the population dynamics of *Microcystis*, diatoms, and green algae during the 2003 heatwave. 5. **Ecological Success of *Microcystis***: The positive buoyancy and high-temperature optimum of *Microcystis* contributed to its ecological success during the heatwave. The study warns that climate change, characterized by warmer summers with reduced wind speeds and cloud cover, may increase the threat of harmful cyanobacterial blooms in eutrophic freshwater ecosystems.The study investigates the impact of summer heatwaves on the growth and spread of harmful cyanobacteria, specifically *Microcystis*, in Lake Nieuwe Meer, a hypertrophic lake in the Netherlands. During the 2003 summer heatwave, which was one of the hottest recorded in Europe, intermittent artificial mixing failed to control *Microcystis* blooms. The researchers developed a coupled biological-physical model to understand the mechanisms behind this failure. The model combines a phytoplankton competition model with a one-dimensional hydrodynamic model driven by meteorological data. Key findings include: 1. **High Temperatures Directly Favor *Microcystis***: High temperatures increase the growth rates of *Microcystis*. 2. **Stability of the Water Column Reduces Vertical Mixing**: High temperatures also increase the stability of the water column, reducing vertical turbulent mixing. 3. **Combined Effects of Meteorological Conditions**: The combination of reduced cloudiness, low wind speeds, and high air temperatures led to a higher summer abundance of *Microcystis* compared to any single factor in isolation. 4. **Model Predictions**: The model accurately predicted the observed temperature and turbulence structures, as well as the population dynamics of *Microcystis*, diatoms, and green algae during the 2003 heatwave. 5. **Ecological Success of *Microcystis***: The positive buoyancy and high-temperature optimum of *Microcystis* contributed to its ecological success during the heatwave. The study warns that climate change, characterized by warmer summers with reduced wind speeds and cloud cover, may increase the threat of harmful cyanobacterial blooms in eutrophic freshwater ecosystems.