A semi-closed estuary, Ohmura Bay, consists of two parts characterized by geographic and oceanographic features. One is a deep and vertically well-mixed area near the mouth of the bay; the other is a shallower, rather fiat and stratified area. A second thermocline and oxygen-deficient water are usually formed in the bottom layer of the latter stratified area in summer. We observed temperature and salinity profiles at 18 points over Ohmura Bay from June to September once or twice a month in 1994. To examine the physical processes in the evolution of stratified structure, numerical experiments were carried out, and the structures of the tidal residual current and density current based on the data observed in July, August and September were obtained. The results showed that the tidal residual current would not have much influence on the circulation in the stratified area of the bay. On the other hand, the density current generated by the horizontal difference of stratification intrudes into the stratified area of the bay. The level of intrusion depends on the structure of stratification. In July and August, the mixed water in the bay-mouth area intrudes into the intermediate layer of the stratified area. Therefore, the bottom water in the stratified area is isolated from the intruded high-oxygen and high temperature water, resulting in formation of a distinct second thermocline, In September, the stratification is weakened, and the water in the mouth area flows into the bottom layer of the stratified area, the low-oxygen and low temperature water is pushed out from the central part of the stratified area; then, the second thermocline disappears.

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Phosphorus cycling in the inner part of Osaka Bay is analyzed using a numerical ecosystem model which consists of euphotic and aphotic layers. Since the accuracy of this model is ± 62% except for POP in the aphotic layer, it is useful to discuss the characteristics of phosphorus cycling and lower trophic level ecosystem in the inner part of Osaka Bay. The main path of phosphorus cycling in the euphotic layer is DIP → phytoplankton → detritus→ DOP → DIE, or detritus → DIP. DIP in the euphotic layer is mainly supplied by decomposition of DOP and detritus (49% of total supply on annual average). Decomposition is controlled by water temperature. DIP transport from the aphotic layer to the euphotic layer (24% of total supply on annual average) is almost equal to DIP supply from rivers and rain (27% of total supply on annual average). The most effective limiting factors on primary production are DIP concentration in summer and water temperature in winter.

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In this review article, the author introduces his research career with special reference to his motivation and thoughts on the subjects. His first research field was estuarine circulation, diffusion and dynamics. Then he extended his research field to coastal oceanography in which he developed the tide-killer filter and proposed the importance of the coastal boundary current from the view point of material transportation ability. Next, in the 1980s, he was involved in the Ocean Mixed Layer Experiment (OMLET) project which was Japanese contributions to the World Climate Research Program (WCRP).During the project, he began to study variabilities of surface water masses such as the North Pacific Subtropical Mode Water by analyzing historically archived oceanographic data. He also solved the so-called XBT fall-rate problem in collaboration with an international task force. In addition, he extensively conducted the ocean monitoring using volunteer observing ships (VOS). His group first succeeded n the monitoring of the Kuroshio current system using a ferry boat shuttling between Tokyo and the Ogasawara Islands. Further, his group entered the research field of large scale air-sea interaction including decadal/interdecadal time scale climate change found in the North pacific.

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