Combined in situ measurements of bottom boundary-layer turbulence and the diffusive oxygen fluxes at the sediment water interface were conducted in a medium-sized lake. The internal seiche driven periodic forcing produced an oscillatory boundary layer featuring distinct deviations from steady-state law-of-the-wall theory such as a pronounced phase lag between the current velocity and the turbulent dissipation of 1.5 hours. The oxygen flux into the sediment was controlled by a diffusive boundary layer, whose thickness was strongly affected by the periodic bottom boundary layer turbulence.