Species composition and seasonal dynamics of phytoplankton in the shallow, eutrophic Lake Donghu were studied during a two years period (Sep. 1999 - Aug. 2001), and the historical changes of phytoplankton over the past 20 years were also reviewed with emphasis on the mechanisms underlying these changes. The impacts of both filter-feeding fishes and nutrient levels on phytoplankton were studied through enclosure experiments. The phytoplankton biomass in the lake during the period of Sep. 1999- Aug. 2001 was much lower when compared with many other eutrophic lakes with similar or even lower nutrient concentrations. It was probably due to both high grazing pressures by filter-feeding fishes and light limitation. Phytoplankton in Lake Donghu is mainly composed of flagellated Cryptomonas and Gymnodinium eucyaneum, centric Cylotella spp and Melosira ambigua, small sized cyanobacteria and Chlorococcales, most of which were typical species in condition of high grazing and high nutrient level. In the 1999 enclosure experiment when phytoplankton were dominated by nanophytoplankton, an outburst of Daphnia occurred in fishless enclosures where phytoplankton biomass was the lowest and water transparency significantly increased. While in the 1992 enclosure experiment when the phytoplankton were dominated by Microcystis bloom, the addition of filter-feeding fishes resulted in great declines in phytoplankton biomass, in contrast to the grass carp addition and the control. It suggested that the impact of filter-feeding fishes on phytoplankton might also be dependent on the composition of phytoplankton. Nutrient addition experiment was carried out to test the role of N/P ratios on phytoplankton in Lake Donghu. It was found that neither nitrogen nor phosphorus addition had significant effect on the composition and biomass of phytoplankton and dense Microcystis bloom occurred in all the enclosures. It was concluded that suitable biological (absence of filter feeding silver carp and bighead carp) and physical environment (high transparency and high pH) combined with saturated nutrient concentration contribute to the nuisance Microcystis bloom in the enclosures. The effect of sediments and water with different nutrient contents on phytoplankton was tested. Both nutrient and phytoplankton biomass were significantly higher in enclosures with nutrient-saturated sediment than that with nutrient-poor sediments. Consistently dense Microcystis blooms only occurred in enclosures with nutrient-saturated sediment. However, water with different nutrient contents had no significant impact on phytoplankton biomass (n=64, p=0.11) especially when the sediment was rich in nutrient. In Lake Donghu, the biomass of phytoplankton remained at relatively low levels with little change in species composition since the disappearance of cyanobacterial blooms in mid-1980s. Before the mid-1980s, Lake Donghu was classified as hypereutrophic, dominated by heavy cyanobacterial bloom. Afterwards, the lake shifted to a so-called holotrophic state, characterized by abundant flagellates and C-strategy species by the definition of Reynolds. The shift between the two statuses was most likely induced by the change in stocking rate of filter-feeding fishes.