Influence of the physical forcing of different water masses on the spatial and temporal distributions of picophytoplankton in the northern East China Sea

• Seasonal survey of picophytoplankton was conducted in the northern East China Sea.
• Five picophytoplankton groups responded differently to the water mass fluctuation.
• Two geographical groups showed different features seasonally.
• Pro was useful as an indicator species to estimate changes the Kuroshio intrusion.
• Verifying a strong influence of the Changjiang discharge and the Kuroshio intrusion.

To investigate the temporal–spatial distribution of picophytoplankton in relation to different water masses in the northern East China Sea (ECS), picophytoplankton abundance and biomass were investigated using flow cytometry with environmental factors in May, August, and November 2010 and February 2012. Five picophytoplankton populations, including Prochlorococcus (Pro), low and high orange fluorescence Synechococcus (Syn1 and Syn2), and small and large sized picoeukaryotes (SPEuks and LPEuks) were discriminated, and each group responded differently to the water mass distribution. Syn1 was the predominant group in the picophytoplankton abundance in all seasons, indicating ecological importance as a primary producer in the northern ECS. LPEuks was a minor group in terms of the abundance but they contributed significantly to the total picophytoplankton carbon biomass in spring and winter due to their larger size among picophytoplankton groups. Two geographical groups were distinguished in the northern ECS. One group (including Syn1 and LPEuks) was distributed mainly in the western area affected by the Changjiang discharge and Chinese coastal waters, and the other group (including Pro, Syn2, and SPEuks) dominated picophytoplankton abundance in the eastern area characterized by the Kuroshio intrusion. Pro was detected in the eastern side of the study area during each survey, except in May, indicating the possible effect of the low Kuroshio intrusion on the northern ECS in late spring. This study shows that the distributions of these picophytoplanktons are strongly influenced by the seasonal fluctuations of the Changjiang discharge and Kuroshio intrusion in the northern ECS.