Abstract
We calculated the monthly residence time of seawater in the inner bay of Tokyo Bay during August 2002 to June 2003 and compared it with a monthly residence time of seawater averaged from 1947 to 1974. The residence time of seawater was calculated by dividing the volume of the inner bay by an exchange rate between the inner bay and the outer bay. The exchange rate was calculated by using the monthly averaged salinity in the inner bay and the outer bay, and freshwater discharge in the inner bay. Between the years of 1947 to 1974, estimations from past studies were used to calculate the average monthly salinity. During August 2002 to June 2003, insitu salinity measurements were conducted 22 times at 27 stations within Tokyo Bay. The freshwater discharge into the inner bay was calculated from the catchment area precipitation and water mass imported from the neighboring catchment area for both data sets. From the results, the residence time of seawater during 2002 consisted of an annual mean of 31 days, with seasonal residence times of 19 days during summer and 43 days during winter. On the other hand, during 1947-1974, the mean annual residence time of seawater was 48 days, with summer and winter seasonal durations of 28 days and 79 days respectively. Thus seawater residence time in the inner bay during 2002 was greatly reduced them previous long-term estimations, by approximately 35%.
