The Role of Circulation in the Development of Hypoxia in Ise Bay, Japan

Seasonally occurring hypoxia in Ise Bay, Japan was studied by comparing flow structures and ages of water parcels. Age is defined as the elapsed time after isolation of the water parcel from the oxygen source. In this bay, bay water is strongly stratified and advective transport by the estuarine gravitational circulation dominates from April to October. In contrast, the strong tidal current keeps the water column well mixed and oxygen-rich from top to bottom in the bay-mouth strait, which connects the bay with the Pacific Ocean. The estuarine circulation in Ise Bay is influenced by the effect of the Earthʹs rotation; the well-mixed water intrudes into the eastern half of the bay under the pycnocline with the land on its right (looking down current). Surrounded by this strait water, relatively cooler water (cold pool) remains on the bottom in the western half of the bay, isolated from the seasonal heating. Comparing the temperature of water under the pycnocline in the bay with the temperature of the strait water, we can estimate an age (τ) of water parcels. This age coincides with the mean residence time, which has traditionally been used. This method gives an age to the water parcel at a point under the pycnocline in the bay.%Oxygen concentration under the pycnocline indicates a high correlation coefficient (R2>0•8) with age. The decrease of oxygen concentration during the period τ gives the oxygen consumption rate (OCR). The OCRs under the pycnocline were calculated from May to August. The OCR is approximately homogeneous independent of location in the bay. Moreover, monthly values of OCRs are also approximately constant from May to August, despite the mean temperature increase from 14•4 to 20•4 °C during this period. Known physical processes explain 80% of the spatial and temporal variation in oxygen concentration. In contrast, biochemical processes provide homogeneous OCR and play only a minor role in the variation in oxygen concentration.