The coastal front modulates the timing and magnitude of spring phytoplankton bloom in the Yellow Sea

Major seasonal quasi-stationary fronts on shelves play an important role in regulating the spatiotemporal variations in the phytoplankton community. However, knowledge of their effects on the timing and magnitude of spring phytoplankton bloom (SPB) remains limited. Here, based on decadal satellite data (2003-2020), we examine the climatological relationship between the Shandong coastal front (SCF) and SPB in the Yellow Sea. The results show that the onset of SPB occurs either in March (∼56% of the seasons examined) or in April (44%). The peak of SPB most often occurs in April (∼56% of the seasons examined) or is advanced to March (16%) or delayed to May (28%), and that the peak ranges from 1.04 to 2.54 mg Chl-a m^-3. The onset of SPB matches with lower turbulence, particularly when the rate of generation of turbulent kinetic energy (TKE_RT) reaches zero. A higher magnitude of bloom is associated with a greater change in front and a lower TKE_RT. The in situ observations along the SCF transects in the Yellow Sea indicate that weakened SCF in spring associated with a shallower mixing layer enhances the transport of nutrients from the coastal to the shelf waters. Weakened frontal structure and atmospheric forcing in spring can further increase the water stability and decrease turbulence in the upper waters. The variation in hydrodynamic conditions allows shelf phytoplankton to stay longer in the upper waters with sufficient light and nutrients and consequently generate a Chl-a peak. The results suggest that the seasonal changes in front intensity and structure and turbulence are important prerequisites for initiating SPB on the shelf, and that further determines the magnitude of SPB.