A coupled model, employing turbulence closure physics and cell-quota threshold limitation biology, has been used to simulate the evolution of the vertical distribution of phytoplankton in a seasonally stratified shelf sea. Predictions are compared with observations from the North Sea. Variations in surface wind stress, which episodically weaken the thermocline barrier to nutrient input to the photic zone, are shown to make an important contribution to the formation of a midwater chlorophyll maximum in Summer.