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Picoplankton are the component of plankton in the size range 0.2-2.0 micrometers, because of their sensitivity to the water environmental change, they can be effectively used as early indicators of eutrophication. This paper is focusing on the relative and absolute abundance of picoplankton in different eutrophic status freshwater bodies. The distribution of picoplankton (Prochlorococcus, Picoeukaryotes and heterotrophic bacteria) in Chenhang Reservior, Dianshan Lake, the middle section of Huangpu River and Bailianjing River which represent different eutrophic status was studied during May to June, 2009. According to the traditional definition for the eutrophication of lakes (reservoirs), The Chenhang Reservoir and Dianshan Lake represented a mesotrophic condition and a light eutrophic condition, respectively. While the middle reach of Huangpu River and Bailianjing River represented a mesoeutrophic and hypereutrophic state, respectively. In Chenhang Reservoir and Dianshan Lake, Prochloroccus and Picoeukaryotes were both found, the orders of magnitude of Prochloroccus and Picoeukaryotes cell abundances in both waters were 105 ind/mL and 103 ind/mL, respectively. In the middle reach of Huangpu River and Bailianjing River, only Picoeukaryotes were detected, and the orders of rmagnitude of Picoeukaryotes cell abundances were 103 ind/mL. The orders of magnitude of heterotrophic bacteria cell abundances in four fresh water systems were among 105-105 ind/mL. So it could be conclude that Prochloroccus was dominant in mesotrophic and light eutrophic water, while in mesoeutrophic or hypereutrophic water, Picoeukaryotes was the dominant species. The cell abundances of autotrophic picoplankton in the Middle reach of Huangpu River and Bailianjing River were lower than that of in Chenhang Reservoir and Dianshan Lake. However, the biomass percentage of picoplankton to total phytoplankton increased by the increasing eutrophication status. [ABSTRACT FROM AUTHOR]