Tidal effects on diel variations of picoplankton and viruses in the Changjiang estuary

Spatial distributions and seasonal variations of picoplankton （i.e. Synechococcus spp., Prochlorococcus spp., picoeukaryotes and heterotrophic bacteria） and viruses in the Changjiang estuary have been reported in the past. However, short-term variations （e.g. at a tidal timescale） of these organisms and their regulating factors remain unclear. We determined the time-series of fluctuations of picoplankton and viruses with tide simultaneously in flow cytometry in the Changjiang estuary during a cruise in June 2006, in which a tidal model based rectangle equation was applied. The results indicate that high cell abundances of picoplankton and viruses occurred during flood tide and low cell abundances during ebb tide. The period of the surface cell abundance variations was about 13 h, suggesting the surface cell abundances in the Changjiang estuary were largely regulated by tide. However, cell abundances in middle and bottom waters varied in different periods due to influences of tidal induced physical forces such as resuspension and stratification. Therefore, tidal action is an important factor for the diel variations of picoplankton and viruses in the Changjiang estuary.