Some characteristics of the factors sensitive to water-quality are numerically investigated to a mathematical model prepared for the prediction of the effect by bottom sediment improvement works as one of the countermeasures against marine eutrophication.
　The nutrient cycle between seawater and sediment is formulated in the two-layer model. From the application to the bay of Osaka, the following facts are confirmed; i) the horizontal distribution of I-P (Inorganic Phosphate), O-P (Organic Phosphate), COD (Chemical Oxygen Demand) and DO (Dissolved Oxygen) are closely related with tidal flow, tidal residual flow, open boundary condition, horizontal dispersion coefficients, etc., ii) the increase of vertical flow or vertical mixing causes to act as to homonize between layers and to increase COD and O-P in each layer by the promotion of photosynthesis in upper layer, iii) settling rate is also affective to water-quality, and iv) unaerobic condition in the bottom region of inner bay is successfully reproduced by the three-layer model.