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A 10,000‐km² hypoxic ‘dead zone’ forms, during most years, in the central basin of Lake Erie. To investigate the processes driving the hypoxia, we conducted a 2‐yr field campaign where the mixing in the lake interior during the stratification period was examined using current meters and temperature‐loggers data, as well as > 600 temperature microstructure profiles, from which turbulent mixing was computed. Near‐inertial Poincaré waves drive shear instability, generating ∼ 1‐m amplitude and 10‐m wavelength high‐frequency internal waves with ∼ 1‐mdensity overturns that lead to an increase in turbulent dissipation by one order of magnitude. The instabilities are associated with enhanced vertical shear at the crests and troughs of the Poincaré waves and may be correlated with the local gradient Richardson number. Poincaré wave–induced mixing should be an important factor when the Burger number < 0.25 …