Green-tide blooms have occurred during each of four years since 2007 in the Yellow Sea of China. Each of the blooms was characterized by a rapid expansion in the area covered by green macroalgae, which caused a series of environmental problems associated with eutrophication. We investigated the biology of the dominant floating macroalgae in these blooms. Our observations suggest that the gametophytes are the primary survival form. Each single blade of the algae thallus is able to produce 2.84×10. Furthermore, of the zoids that are released, about 91.6%–96.4% are able to successfully germinate, forming new plants. Thus, one third per unit weight of the algal thallus is able to produce 0.84×10new individuals during the peak-hour of a bloom. Our results demonstrate that is able to reproduce very rapidly, which is likely a major factor in the spread of green tide blooms in the coastal areas of China.