Dynamics of picoplankton in the Nansha Islands area of the South China Sea

Dynamics of major picoplankton groups,Synechococcus(Syn),Prochlorococcus(Pro),picoeukaryotes(Euk) and heterotrophic bacteria(Bact) was investigated by flow cytometry for the first time in the Nansha Islands area in the South China Sea.Averaged over the whole investigation area,depth-weighted integrated cell abundance(DWA) of Syn,Pro,Euk and Bact was 1.6(0.4-5.7)×10³,5.4(0.1-7.3)×10⁴,0.7(0.2-2.2)×10³,and 2.3(1.4-3.2)×10⁵ cells/mL respectively.Picoautotrophic cell abundance was low in the northwest part of the Nansha Islands where surface water temperature was low and the upper mixed layer was shallow.Concurrently,a surface maximum vertical distribution pattern was observed in this area.While in the southeast and east zones where temperatures were relatively higher and nitraclines were deeper,picoplankton is abundant and a subsurface maximum around 50-75 m is observed.Coupling of horizontal and vertical distribution patterns of picoplankton abundance and hydrological status was found,suggesting a strong influence of currents and water column structure on picoplankton distribution in the investigation area.Contrary to that in the shelf water in the East China Sea,the relationship between Pro and Bact in the Nansha Islands area in the South China Sea was not significantly negative but weakly positive.Moreover,a similar distnbution pattern of Syn and Pro was observed Possible reasons for these differences in the two marine regimes were discussed.