Physico-chemical, organic, and microbiological factors were measured from April to November, 1991 in Daechung Reservoir. Concentration of inorganic nutrients and eutrophic state varied according to the geological characteristics and water depth of each research site. In spring, allochthonous organic material from upstream was the main source of reservoir eutrophication. Autochthonous organic compound from Microcystis in summer act also as another source, especially at the post-summer season. Availability of the autochthonous organic compound by bacterial community was increased after some time interval. Organic matters distributed from spring to summer play a role of potential source of eutrophication phenomena in each year and bacterial activity contributed to the increase of organic concentration at the water column. In this study, primary productivity at June, 1991 measured by 1.5mgCm^-2day^-1 when blooming of Microcystis started. 84.11% and 35.29% of it was excreted and uptaken by bacteria, respectively at the same time.