Temporal changes in the nutrient status of Matsushima Bay after a wastewater plant was destroyed by a tsunami on 11 March 2011

We investigated how the nutrient status of Matsushima Bay was affected when a wastewater plant was destroyed by a tsunami in March 2011. The nutrient concentrations in the seawater and the treated water from the wastewater plant increased just after the tsunami, but decreased again to pre-tsunami levels after 2013. The amount of untreated water that passed through the wastewater plant decreased just after the tsunami. It was estimated that approximately 40.7 × 10 3 m 3 /day of the treated water from the wastewater plant was discharged to Matsushima Bay; therefore, the quantity of effluent from the wastewater plant was less than one percent of the water inflow from Takagi River and Sendai Bay (which are outside Matsushima Bay) to Matsushima Bay. The nutrient concentrations of seawater in Sendai Bay were lower than those in Matsushima Bay. The results suggest that nutrient concentrations in Matsushima Bay after the tsunami did not increase because any untreated or poorly treated effluent was easily diluted by the river flow and the inflow of seawater. Many people were concerned about eutrophication, therefore, because of the decreased functioning of the wastewater plant in Matsushima Bay. Marked eutrophication in the bay was not observed after August 2011.