Abstract
The water quality of Isahaya Reservoir was calculated with a numerical model, which mainly consists of organisms such as phytoplankton, zooplankton, benthos and fish, and inorganic nutrients such as nitro-gen and phosphorus. As a result, it was considered that the removal of planktons from the reservoir contributes effectively to the improvement of water quality because nutrients transition from water to the organisms was estimated at high level, while it remains problems on initial/ running costs and throughput for the purification equipments. On the other hand, the nutrient transition to benthos and fish was estimated at low level. Therefore, the removal of benthos and fish should not be adopted. Since the most of nutrient loads into the reservoirwere derived from thebasin, and were taken into planktons and were used forthe primary production causing organic pollution of reservoir, the bypass of loads from basin was expected to avoid extra organic pollutant. The effectiveness in 50 % nitrogen load bypass was estimated at 37 % reduction of nitrogen budget in the reservoir. And as the release and resuspension of nutrients from sediment were not negligible, the 50 % reduction of suspension by sand capping on the sediments might deserve 17 % reduction of nitrogen budget.
