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Ofunato bay is highly enclosed because of a tsunami breakwater. Therefore, water quality problems, such as eutrophication and ocean anoxic, have been caused in the inside of the bay. Field observation was carried out to clarify the characteristics of water quality in Ofunato bay in 2003 and 2004. The purpose of this paper is to study the seasonal variation characteristics and the load of nutrients. The amount of inflow of nutrients was calculated using the material balance model using field observation data. It shows that a lot of nutrient is released from sediment when anoxic water has occurred. Unusual increase of phytoplankton was observed when river inflow was increased. They have great effect on water quality.