Estimation of vertical diffusion coefficient and oxygen consumption rate in the interior western parts of the Ariake Sea [Japan]

In this study, in order to clarify the generation mechanism of hypoxic water that occurs frequently in the interior parts of the Ariake Sea, various field observations were conducted in summer of 2005 and vertical diffusion coefficient and oxygen consumption rate in the study area were analyzed by the two-layer box model using Saga Prefecture research data in 1972-2000. Temporal variations of DO near sea bottom during 23 July-17 August in 2005 were strongly influenced by typhoons and strong wind-induced mixing of sea water and by the advection of hypoxic water from offshore area, besides tide and current velocity. From the data observed on 16 August in 2005, mixing condition of water column in the west coastal area was the weakly mixed-type and density stratification and pycnocline were formed at 3-5m depth from surface sea water at the time of occurrence of hypoxic water. There was a relationship between vertical diffusion coefficient obtained by the thermal stratification model and stratification parameter. That is, vertical diffusion coefficient decreased exponentially with increasing of stratification parameter. Thus, the development of density stratification in the water column was considered to restrict the ability supply of O2 from surface to lower layers. From Saga Prefecture research data in 1972-2000, seasonal variations of density difference between surface and lower layers and density stratification parameter in the interior western parts of the Ariake Sea were clarified. That is, density difference raised and density stratification parameter became high values in summer due to increasing of freshwater input and surface warming. Conversely, density difference dropped and density stratification parameter became low in winter due to decreasing of freshwater input and surface cooling. Next, vertical diffusion coefficient and oxygen consumption rate in the study area were analyzed by the two-layer box model. As a result, seasonal variations of vertical diffusion coefficient were closely related to that of density stratification parameter. That is, vertical diffusion coefficient tended to increase in summer-autumn and to decrease in winter-spring. On the other hand, oxygen consumption rate tended to be positive (O2 consumption) in spring-summer and to be negative (O2 production) in autumn-winter.