“Black Water Aggregation”(BWA)is one of the most serious environmental hazard so ocurred in Lake Taihu.Based on the investigation in relevant areas during Apr.-Oct.,2009,the BWA status was analyzed.11BWA were observed in western Lake Taihu,Meiliang Bay and Gonghu Bay.Comparison of blue-green algal blooms,water level,water-quality in recent years gave in sight to the further understanding of BWA. The algal bloom was less serious in intensity,frequency and scale in 2009th an in 2007 and 2008,which reduced the possibility of BWA outbreak.In addition,the ecological dredging project undertaken in northern Lake Taihu since 2007 also functioned for the less serious BWA in 2009.Investigations of water quality revealed that the lake was still under meso-eutrophic as a whole whereas the western Lake Taihu, Meiliang Bay and Lake Zhushan was hyper-eutrophic. In order to mitigate and control the BWA,measures such as algae salvage,ecological dredging,source control and interception and water diversion from the Yangtze River to Lake Taihu.More efficient investigation in relevant BWA-sensitive areas and set up of monitoring-warning system for BWA are favored for early assessment and instant handling of BWA,which offers technical guarantee for water supply in Lake Taihu.