The change pattern of the vertical mixing was shown similar to that of the sea surface Chl a, revealing a close relationship between the surface phytoplankton biomass and the vertical mixing in the studied area (Fig. 5). Chl a values started to increase following the enhancement of vertical mixing after September 2009 and peaked in January 2010, when the vertical mixing was the strongest. The abrupt decrease of MLD in February 2010 also coincided with rapidly reduced surface Chl a.