Field survey has been carried out to evaluate the water quality stratification related to the water-bloom occurrence in Urushizawa dam without the pollutant load source in the basin during summer season in 2004. From the result obtained, Chl. a concentration was 300μg/l as a highest value at 0.5m water depth in August, and the dominance of Anabena sp. was observed. It was also found that the water bloom may be occurred by the accumulation of the nutrient such as the phosphorus in the only surface layer of the deep dam as well as a shallow lake, when the thermocline was formed. The fluctuation of phosphorus amount in surface layer was verified using the phosphorus balance model.
