Seasonal Variation in Residence Times of Two Neighboring Bays with Contrasting Topography

Residence time is an important parameter for evaluating the physical and biochemical characteristics of estuaries and bays. The residence times of two neighboring South Korean bays with contrasting topographies, Gwangyang Bay and Jinju Bay, were calculated for winter and summer using a three-dimensional model. Gwangyang Bay, with considerable industrial development nearby, has a relatively large volume and depth, whereas Jinju Bay is shallow and is primarily used for aquaculture. River discharge in the two bays is high in summer and low in winter, in part due to the influence of the East Asian Monsoon. Temperature and salinity in both bays are vertically homogeneous during winter but stratified in summer. The mean residence time in Gwangyang Bay is more than 20 days in winter due to its large volume, but less than 5 days in summer due to increased gravitational circulation driven by large volumes of river discharge and a horizontal density gradient. However, the mean residence time of Jinju Bay is less than 5 days in both seasons due to its small volume and shallow depth. These results reveal that Gwangyang Bay is vulnerable to industrial pollution during winter, but that it may remove pollutants efficiently in summer. The shallower Jinju Bay is likely to remain free from anthropogenic eutrophication caused by nutrient inputs from aquaculture due to its short residence time throughout the year.