In order to understand the water quality condition of Dacha Lake in Wuhan,composition of phytoplankton in the lake was investigated from June 2010 to May 2011.A total of seven phyla,59 genera and 89 species of phytoplankton were identified from qualitative and quantitative samples.Based on the species and quantity of phytoplankton,biological assessment of water pollution and eutrophication condition was conducted by using the methods of indicator organism and biotic diversity index (Margalef and Shannon-Weiner index).In the phytoplankton community,Chlorophyta and Cyanophyta were dominant in both alga species and quantities,followed by Bacillariophyta.Of the pollution-indicating phytoplankton,the algae indicating α-ms and β-α-ms types of eutrophication were dominant,accounting for 48.72% and 23.08% of the total,respectively.The Margalef index and Shannon-Weiner index for phytoplankton ranged from 1.45 to 2.19 and from 1.64 to 2.95,respectively.The results indicated that the water quality of Dacha Lake was of moderate pollution (α- or β- moderate pollution),and the trophic status was in the state of eutrophication (α-ms or β-α-ms),mainly in the type of α-ms as revealed by the pollution-indicating algae.