This paper describes water quality processes in Ohfunato Bay. In Ohfunato bay, the bottom layer becomes oxygen deficient in summer, due to the formation of a thermocline. This oxygen-deficient water mass promotes the dissolution of nutrients from sediments, especially phosphorus.
　In this investigation, it was found that the bottom water inside the bay was carried outside on the ebb tide, depending on the strength of the thermocline. Numerical simulations using a two-layer model confirmed that internal waves carry bottom waters from the bay.
　On the ebb tide, water inside the bay flows out in the surface and bottom layer. On the flood tide, water outside the bay flows in through the middle layer. The oxygen-deficient water mass carried out during one tide was estimated as 0.4% to 1.2% of the total oxygen-deficient water mass inside the bay. The amount of oxygen supply from outside the bay was estimated about 1/3 of oxygen consumption in the bottom layer.
　Finally, mention is made about water quality management strategies using this mechanism briefly. This mechanism has an effect on the amount of dissolved oxygen at bottom water inside the bay.