Phytoplankton, the dominant marine primary producers, are considered highly sensitive indicators of ecosystem conditions and changes. East China Sea (ECS) includes a variety of oceanic and coastal domains that collectively challenge the understanding of phytoplankton dynamics and controls. This study evaluated the seasonal and interannual variability of phytoplankton in the ECS and the underlying environmental determinants based on 22-year satellite chlorophyll (Chl-a) data and concurrent environmental variables. The seasonal spring bloom was found in the ECS classically driven by increased stratification, which are associated with increases in sea surface temperature (SST), photosynthetically available radiation (PAR), net heat flux (NHF), and reduced wind mixing. The most significant Chl-ainterannual variability is present in a triangle area surrounded by three SST fronts in the southern ECS during springtime. Anomalously high Chl-a(~30% increase) occurred with lower SST, NHF, and enhanced wind mixing during negative Pacific Decadal Oscillation (PDO) and ENSO modes. This seems to be contrary to the stratification control model that fit to the seasonal spring bloom observed in this region. More front activities during negative PDO and ENSOcould be associated with Chl-aincrease in this triangle area. Contrary to this mixing control scenario, significantly Chl-aincrease (~36% increase) also developed during the positive PDO and ENSO modes after 2014 in the condition of higher SST, NHF, and weaker wind mixing following the stratification control scenario. This study used a biologically-relevant objective regionalization of a heterogeneous area to elucidate phytoplankton bloom dynamics and controls. Our analyses highlight the triangle area in the ECS for its region-specific linkages between Chl-aand multiple climate-sensitive environmental drivers, and the potential structure and functioning variability in this region.