Temporal Variations in Water Quality and Its Controlling Factors in Sendai Bay, Northern Japan

We examined changes in water quality parameters (water temperature, salinity, DO, and nutrients) in Sendai Bay before and after the Great East Japan Earthquake (GEJE). Principal component analysis (PCA) showed that, after the GEJE, DO and nutrients (DIN and DIP) decreased, and water temperature and salin-ity increased compared to before the GEJE. The highest decrease in nutrients was observed in spring at the bottom layer, followed by summer and autumn at the bottom layer. Intrusion of water mass with high tem-perature, salinity, and low nutrient concentrations increased in spring after the GEJE. In addition, regres-sion analyses suggested that spring intrusion of the Oyashio current affected water quality and increased nutrient concentrations in Sendai Bay, and the mean southernmost latitude of the Oyashio in spring has tended to shift to the north in recent years. Therefore, the nutrient decrease in spring after the GEJE was likely due to effects of the increased intrusion of water mass with low nutrient concentrations from the Kuroshio Current, and the decreased intrusion of water mass from the Oyashio current. The nutrient de-crease at the bottom layer observed in summer and autumn was likely due to decreases in nutrient supply from the bottom sediment caused by the change in bottom sediments after the GEJE.