Abstract:In this study, we evaluated the bacterial populations in Miyun Reservoir in different seasons from Nov. 2006 to Nov. 2007 to determine the overall water quality of the reservoir. The bacterial counts in water samples collected from the reservoir were determined by flow cytometry and by culture in LB medium. In addition, the bacterial diversity and dominant groups were also evaluated by PCR-DGGE. The results revealed that the bacterial counts were different according to the season, but that the average counts, which were determined to be 104 per ml by flow cytometry and 102-103 CFU per ml by culture in LB medium, were below the national standards for resource water. In addition, determination of the bacterial Shannon-Weaver diversity index revealed that the diversity also varied by season. Specifically, diversity was very low in winter (approximately 3.0 in November 2006 and 2007), but increased to as high as 3.3068 in the spring and summer (April and July, 2007), after which it began to decrease, with a value of 3.1246 being observed in autumn (September 2007). Furthermore, the dominant groups of bacteria observed in Miyun Reservoir included β-Proteobacteria, Actinobacteria, Cytophaga/Flexibacter/Bacteroides (CFB) phylum and Cyanobacteria. Taken together, the results of this study will help to understand future changes in the water quality of Miyun Reservior.