Simultaneous determination of picoplankton (i.e., Synechococcus spp., Prochlorococcus spp., picoeukaryotes, and heterotrophic bacteria) and viruses in the Changjiang (Yangtze) River estuary and its adjacent sea area was made using flow cytometry during a cruise in June 2006. The results show that Prochlorococcus in all samples was below detectable level. The abundances of Synechococcus, picoeukaryotes, heterotrophic bacteria, and viruses ranged from 0 . 0 0 to 1 . 2 2 × 1 0 8 cell L？1, 0 . 0 1 × 1 0 6 to 1 . 4 2 × 1 0 7 cells L？1, 8 . 4 0 × 1 0 7 to 4 . 2 9 × 1 0 9 cells L？1, and 1 . 2 0 × 1 0 7 to 1 . 0 6 × 1 0 1 0 particles L？1, respectively. The determined picoplankton groups and viruses distinctly increased with the distance off the estuary and where the maximum abundance that occurred in these groups was different somewhat due to the individual sensitivity to environmental changes. Viral abundance showed a positive correlation with salinity and negative correlations with turbidity and inorganic nutrient concentrations. Positive linear relations were found between Synechococcus, heterotrophic bacteria, and viruses. 1. Introduction Autotrophic Synechococcus spp., Prochlorococcus spp., and picoeukaryotes, together with heterotrophic bacteria, are the principal components of marine picoplankton communities (0.2–2？μm in diameter) [1–4]. These picoplankton groups in the Changjiang estuary have been the subject of research in the past [5–14]. However, few data have been published on picoplankton community structure after the Three Gorges Dam construction. Previous studies were mainly focused on only one picoplankton group rather than simultaneously observing all picoplankton groups, and most of the data in the literature were obtained by epifluorescence microscope, which was not as efficient, sensitive, and precise as flow cytometry (FCM) adopted in this study [12, 13]. Marie et al. [15] reported the method of FCM measurement on marine viruses, providing a glorious prospect on studying natural viral community. Compared with other oceanic regions, the field investigation on viruses in the Changjiang estuary was much less, and, up to now, only a few reports were found [12, 14, 16, 17]. With the construction of the huge Three Gorges Dam (height 180？m, width 2？km) in the middle reaches of Changjiang River, both the amount and the timing of the terrigenous input into the East China Sea will be different [18]. It has been suggested that after the water storage of the Three Gorges reservoir, light availability required for photosynthesis enhanced in the river
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