In temperate continental shelf seas, high phytoplankton growth rates are normally restricted to two to three weeks in spring coinciding with the onset of stabilization of the water column and a large supply of nutrients. Thereafter production is slowed down because available nutrients are depleted and increased stability restricts the rate of recycling of nutrients. High levels of production can persist for a longer period wherever nutrients can be made readily available at or near the surface, most notably in upwelling areas and mixed coastal regions, and, to a lesser extent, adjacent to fronts. This paper demonstrates another mechanism whereby a stratified shelf current passes through a small area of intense tidal mixing, entraining nutrient-rich bottom water before becoming restratified. This results in a relatively large area where `spring' conditions persist throughout the summer, injecting many growing phytoplankton into a sea that is otherwise less productive.