Abundance, carbon biomass and composition of picoplankton along a transect (fromthe Pearl River estuary to Dongsha Island) in the northern South China Sea were measured by flow cytom-etry (FCM); and the vertical structure, composition variation, relationship between heterotrophic and au-totrophic picoplankton as well as the controlling mechanism of diel variation were studied. Results showedthat along the horizontal direction, both Synechococcus (SYN) and picoeukaryote (PEUK) were highest inabundance and dominated autotrophic picoplankton in the Pearl River estuary, but decreased
