The Yellow Sea is a eutrophic sea and the South China Sea is an oligotrophic one.Several cruises were carried out respectively in March and May to investigate bacterial abundance (BA),bacterial productivity (BP),chl a,and nutrients in the two seas.In March the sea temperature was low and there was no stratification in the Yellow Sea.The water mixed vertically to compensate the nutrients consumed by phytoplankton so that the biomass was quite high.While in May the higher sea surface temperature formed steady stratification which stopped nutrients transferring upward in both the Yellow Sea and the South China Sea.So the biomass was lower in the Yellow Sea and lowest in the South China Sea in May compared to that in the Yellow Sea in March.And also the phytoplankton size structure was quite different: larger algae (>5μm) dominated the Yellow Sea while smaller ones were in the majority in the South China Sea.