Abstract: A total of 102 species of algae were found in the synthetic biological pond system(SBPS)during the three-year operation. The annual mean values of cell densitv andstanding biomass were 1.84?08 ind,/l and 32.7 mg/l, respectively. Maximal densityand biomass in the systemwere observed in April-June, The algae of Chlorophyta,es-pecially Chlorella,were dominant in population density, but Cryptomonas contributedwith the highest biomass during most ofthe year. An attempt to reduce the phytoplankton number in effiuents using ecological modi-fication with aquatic vascular plants was made successfully.As a water hyacinth area wasset up at the front of the outlet, the algal eell density in effiuents was reduced by 20-65%. It shows that the macrophyte inhibited the growth ofalgae.The phenomenon of stratification of flagellates was notable when Euglena was dominant.