Offshore wind farms (OWFs) have been increasingly installed in stratified regions of the coast in the past years. In tidally influenced shelf seas, the addition of wind turbines to the water column implicates power removal from the tidal flow by the structures, which is subsequently fed into turbulence and mixing of the stratification. We use high-resolution large-eddy simulations to quantify the additional turbulence and mixing generated by single monopile foundations and study its effect on different scenarios of temperature stratification comparable to the conditions found in the German Bight of the North Sea -- an area of concentrated OWF development and planning. The results suggest that enhanced turbulent dissipation and vertical scalar transport are strongly concentrated in a narrow region of the wake of single foundation structures. A bulk mixing efficiency of approximately 0.1 is obtained, whereby the additional mixing promoted by single foundations is only a fraction of that generated by the bottom mixed layer. The impact of this enhanced mixing by OWFs on the stratification in the North Sea is assessed using a simple parameterisation motivated by the simulations, as well as estimates of the stratification budget of the North Sea using regional model simulations and field measurements. The results suggest that the mean rate of stratification build-up in the study region is of the same order of magnitude as the combined mixing promoted by multiple turbine foundations as found in OWFs. Moreover, if built over extensive regions of tidal shelf seas, OWFs could potentially contribute to significant changes in the natural stratification regimes and dynamics across the water column over a seasonal scale.