The global surface temperature has increased by about 0.74 degrees C over the past 100 years, and the frequency of extreme weather has increased as well. We used the state-of-the-art complex, dynamic, mechanistic model GOTM-FABM-PCLake to quantify the impacts of extreme summer warming on a summer-stratified temperate Danish lake. Simulated values of all calibrated parameters (water temperature, DO, NO3, NH4, TN, PO4, TP and Chl.-a) agreed well with observed values over the whole calibration and validation period and generally exhibited the same seasonal dynamics and inter-annual variations as the monitoring data. A series of climate scenarios with different summer heat wave frequencies and duration were set up to quantify the effects on the ecosystem state of the lake. Our simulations showed that summer surface mean TN will decrease with rising summer heat wave frequencies, while summer surface mean TP and Chl.-a and the biomass and proportion of cyanobacteria will increase. Following a summer heat wave, the lake approached baseline conditions in the autumn, but with increasing frequency of heatwaves the recovery period increased. Our results suggest that compliance with existing legislation, such as EU's Water Framework Directive, will become increasingly challenging in a future scenario with increased temperatures and more frequent heatwaves.