Novel hydrodynamic observations 40 m from an offshore wind turbine monopile show that the turbulent tidal lee wake doubles the drag acting on the seabed, potentially enhancing sediment transport and impacting the seabed habitat and the organisms that utilise it. It also enhances the vertical mixing of the water column, which drives the transport of heat, nutrients and oxygen. As offshore wind farms rapidly expand into deeper waters, array-scale wakes may have significant ecological impacts.