Aquatic ecosystem was studied in Lake Paro to monitor the effects of the Keumkangsan Dam construction in North Korea. Water quality, pirmary productivity, and microbes were surveyed bimonthly in 1987. The texture and composition of sediments, and meterological aspects were also investigated. The most remarkable change of water quality was the increase of turbidity. due to the dam construction. Overall chemical and biological aspects were greatly changed because of the high turbidity. The concentration of phosphate increased and subsequently phytoplankton standing crop increased. The effect of phosphate increase was compensated by the decrease of light penetration, and, therefore, areal primary productivity was not so high as expected from the chlorophyll concentration. From the standing crop of microbes, the trophic state of Lake Paro was evaluated as mesotrophy. As for the horizontal variation of microbial activity, the microbial heterotrophy was higher in the upstream regions than in the downstream regions. Sediment types are almost clayey silt and the mean sizes of sediments were homogeneous through the whole lake area. Organic carbon contents of the sediment seemed to be influenced by seasonal variation of primary productivity. Air temperature on Lake Paro was about 2 ℃ lower and the relative humidity was about 10% higher than that of Chuncheon. The evaporation from lake surface estimated from the comparison of air temperature and surface water temperature was highest in autumn.