Spatial and temporal variation of the Synechococcus population in the East China Sea and its contribution to phytoplankton biomass

Variations in the spatial and temporal distribution of the Synechococcus population in the East China Sea were investigated in five cruises of the R/V Ocean Research I between May 1996 and October 1998. On the continental shelf, a two-phased (warm >20°C; and cold <20°C) seasonal cycle with 10-fold variation was found in the Synechococcus population. In the cold season (winter to late spring), when temperature is probably a limiting factor, a low density Synechococcus population was homogeneously distributed over the entire continental shelf. A dome-type spatial distribution pattern appeared in summer, that is, population densities were high toward the middle of the sea. This area of higher density moved toward the outer shelf in fall. No seasonal variation was found in the Coastal Water, probably because in summer the low transmission means that light is limited below the surface. The average carbon biomass of Synechococcus in the East China Sea ranged from 0.09 (early spring) to 0.90 gC m−2 (fall). Based on carbon biomass, Synechococcus accounted for less than 25% of the total photosynthetic plankton in cold season and about 50% in the warm season. Our results suggest that the cyanobacterium Synechococcus is a major contributor to the seasonal variation of phytoplankton in the East China Sea.