Quantification of Bottom Water Age by Using Temperature Based Age Index Model and Its Relationship with Bottom Water Dissolved Oxygen Concentration in Ise Bay, Japan

Abstract

 The age of bottom water which indicates the period in which oceanic water stays at the bottom of the inner bay basin after intrusion from the bay mouth was calculated to verify whether the age is a good index for the development of bottom water hypoxia. The spatially distributed water quality data was utilized in this study for three consecutive year in 2012, 2013 and 2014. The data was observed monthly on sixteen monitoring stations by Suzuka fisheries laboratory of Mie prefectural government. To calculate the age of bottom water, a temperature-based age index model was used, the model based on the assumption that bottom water in the Ise Bay originates from the bay mouth (Lower strait water) in the predominate stratified period, i.e., summer season and age of the bottom water is the elapsed time from the beginning of intrusion from the bay mouth. From the analysis of age index model, it was found that the age of bottom water was higher in the center of the bay and water can be of 60 days old whereas near the bay mouth it was much younger and its age varied from 0 to 10 days. In September 2014, the entire basin accounted for younger water mass with a maximum age of 10 days at the center as compared to other years. Overall bottom water age in Ise Bay has also been calculated by taking an average of all stations, and it was found that the maximum average age occurred in June 2013 while the minimum one occurred in September 2014. The bottom water oxygen concentration indicated a high average correlation coefficient (R² >0·70) with bottom water age in June, July, and August, whereas low average correlation coefficient (R² <0·30) in September.