Influence of freshwater input and bay reclamation on long-term changes in seawater residence times in Tokyo bay, Japan

In this study, we demonstrated that the residence time of seawater in Tokyo bay, Japan, has changed over time because of fresh water imported from neighbouring catchments and reclamation of portions of the bay. The residence time of seawater was estimated from data collected in 2002, and compared with re-estimates of the residence time based on data from 1947 to 1974. We also used a numerical model to examine the effects of changes in the freshwater input and in the surface area of the bay on the seawater residence time. Our analysis showed that the seawater residence time decreased by 35% from the period prior to 2002. The effects of the increase in freshwater input and the decrease in the sea surface area over this period on the decrease in residence time were almost the same. Eighty percent of the increase in freshwater input resulted from water imported from neighbouring catchment areas. In terms of the relationship between seawater residence time and water quality, the effects of the imported fresh water were almost the same as those of bay reclamation. Copyright (C) 2011 John Wiley & Sons, Ltd.