The vertical transport of dissolved and particulate substances in density stratified lakes occurs mainly as a result of turbulent mixing. The intensity of turbulence in the bottom boundary layer is distinctly higher than that in the bulk water, and is probably important for vertical mass transport throughout the entire density-stratified hypolimnion. One possibility of characterizing turbulence is in terms of the energy dissipation rate, i.e. the rate of transformation of turbulent kinetic energy into heat. For turbulent currents, dissipation rates can be obtained from the spectra of time series of measurements of current fluctuations. This method, which allows the intensity of turbulence in lakes, especially in the bottom boundary layer, to be determined with a minimum of experimental effort, was employed in summer 1993 to investigate the properties of the bottom boundary layer of Lake Zug (Switzerland). There are two main results. Firstly, a comparison of friction velocities determined from spectra and from the profile of mean velocities assuming a logarithmic boundary layer coincide within the estimated error. Secondly, values for the bottom drag coefficient derived from friction velocities are about 4 times greater than the usually cited value of 1.6 × 10-3.