Light is one of the primary resources in regulating phytoplankton growth and productivity.Recent satellite observations have started to report optimal light intensities for phytoplankton blooming in various physiological and ecological environments.To investigate the optimal light intensities required for phytoplankton spring bloom development in the East Sea (ES), we constructed a 10-year climatology of the mean photosynthetically available radiation (PAR) in the mixed layer at the onset times of spring blooms using satellite data and in situ temperaturedepth profiles.The PAR estimates in the southern ES (<40N) are nearly uniform, with an optimal value of 4.5 2.1 E m -2 day -1 ; this value is comparable to the PAR in the western North Pacific and the North Atlantic, where bloom initiation has been explained by a shallowing of the mixed layer depth and an increase in the light availability.The PAR estimates for the northern ES are also nearly uniform, with an optimal value of 16.0 7.3 E m -2 day -1 .However, the PAR is about three times larger in the northern ES than in the southern ES.The value of the northern ES is comparable to the PAR estimate in the Southern Ocean, which is known as a high-nutrient and low-chlorophyll region, owing to iron limitation and zooplankton grazing pressures.These distinct regional differences in the PAR estimates indicate that a variety of physiological and ecological conditions can control the onset of spring blooms.