The advance and retreat of the thermocline on the continental shelf

An important factor in the seasonal cycle of phytoplankton growth is the establishment and breakdown of the seasonal thermocline (Sverdrup, Johnson & Fleming, 1946). When surface heating is sufficient, more buoyancy is produced than can be dissipated by wind and tide. The stabilization of the water column allows the phytoplankton sufficient time in the surface layer to utilize the light energy for increased net production and the spring bloom is triggered. At the end of the summer period the erosion of the thermocline may result in an increase of nutrients in the upper layers. If production has become limited by surface depletion of nutrients during the summer, then the thermocline breakdown may result in an autumnal bloom particularly if a period of calm sunny weather follows. The importance of the thermocline development and season length is clearly illustrated in plankton studies of the North Atlantic, for example Robinson (1970).