A spatial and temporal variation in physiochemical parameters in the southeastern Yellow Sea (YS) is in-vestigated in the spring and summer of 2009 to 2011. Nutrient show a strong negative relationship with chlorophyll a (Chl a ) concentration in spring, and the subsurface chlorophyll a maxima (SCM) layer was associated with the nitracline in summer. In summer, the SCM was usually found within or above the pyc-nocline and at the depths of shoals from the open sea to the coastal sea due to tidal and/or topographical fronts in the southernmost study area. High Chl a concentrations were found in the central southern YS, where the YS cold water layer expanded under the pycnocline and encountered water masses during spring and summer. After a typhoon in the summer of 2011, Chl a concentration increased, especially in the central southern YS, where cold waters occurred below the pycnocline. The results suggest that the development of thermohaline fronts may play an important role in the growth and accumulation of phytoplankton biomass in the upper layer of the southeastern YS during spring and summer.