Picoplankton distribution in different water masses of the East China Sea in autumn and winter

Picoplankton distribution was investigated in different water masses of the East China Sea in November, 2006 and February, 2007. The autumn and winter cruises crossed three major water masses: the coastal water mass (CWM), the mixed water mass (MWM), which forms on the continental shelf, and the Kuroshio water mass (KWM). Picoplankton composition was resolved into four main groups by flow cytometry, namely Synechococcus, Prochlorococcus, picoeukaryotes, and heterotrophic bacteria. The average abundances of Synechococcus, picoeukaryotes, and heterotrophic bacteria were (0.63±10.88)×10³, (1.61±1.16)×10³, (3.39±1.27)×10⁵ cells/mL in autumn and (6.45±8.60)×10³, (3.23±2.63)×10³, (3.76±1.37)×10⁵ cells/mL in winter, respectively. Prochlorococcus was not found in the CWM and seldom observed in surface samples in either season. However, Prochlorococcus was observed in the MWM and KWM (approximately 10³ cells/mL) in both autumn and winter. Synechococcus distribution varied considerably among water masses, with the highest levels in KWM and lowest levels in CWM. The depth-averaged integrated abundance of Synechococcus was approximately 5-fold higher in KWM than in CWM, which may be due primarily to water temperature. In the MWM, Synechococcus was resolved as two subgroups; the presence of both subgroups was more common in autumn. Picoeukaryote abundance varied less among water masses than Synechococcus, and heterotrophic bacteria depth-averaged integrated abundance exhibited the smallest seasonal variations with respect to water mass. Correlation analysis showed that relationships between picoplankton abundances and environmental factors (temperature, nutrients, and chlorophyll a) differed among the three water masses, suggesting that the three water masses have different effects on picoplankton distribution (particularly Synechococcus).