Aim of this paper is to reveal major causes of seasonal variability of phytoplankton biomass in the seas adjacent to Japan by relating changes of transparency to those of environmental factors. Generally, phytoplankton biomass begins to increase when the Sverdrup's critical depth exceeds the upper mixed layer depth in spring, and decreases in summer with development of the seasonal pycnocline. After the summer minimum, it increases temporally in early autumn when an upper mixed layer becomes deeper and yet shallower than the critical depth. Critical depth model is applicable to waters in the transitional/subarctic Pacific and the Japan Sea. On the other hand, in the Pacific off the southern midcoast of Japan and the East China Sea, seasonal variability of phytoplankton biomass is not fully explained by the critical depth model. These waters are characterized by low concentration and no seasonality of nutrients. Significant negative correlations were observed between surface nutrient concentrations and transparency. These facts indicate that the physical conditions such as critical depths, mixed layer depths and solar irradiance as well and the chemical condition like nutrients availability are responsible for the seasonal variability and annual level of phytoplankton biomass in the seas adjacent to Japan.

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Conductivity-temperature-turbidity surveys were conducted along ten transects from 31.5° to around 27°N across the shelf edge zone in May 1995 (Leg 1; 10-16 May and Leg 2; 19-26 May). During Leg 1 survey, a well-developed turbid layer extended from the shelf to the shelf edge at the central transects located from 28° to 29°N. On the contrary, the development of the turbid layer was significantly weaker at the same locations during Leg 2 survey. At the northern and southern transects, the turbidity was quite low throughout the present survey. During Leg 1 the continental coastal water which had relatively low salinity and high turbidity was observed on the outer shelf, suggesting the coastal water supplies particulate matter to the shelf edge zone. It was also observed that high turbidity in the bottom layer was related to the well-developed mixing layer. On the other hand, during Leg 2, the high turbidity water diminished on the outer shelf while no intrusion of the continental coastal water and well-develped mixing layer was observed in this area. These observation showed the temporal and spatial variability of the turbidity was quite large on the shelf edge zone, suggesting the seaward transport of particles from the shelf is a sporadic event.

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This article reviews the author's studies concerning climatology and variability of North Pacific Subtropical Mode Water (NPSTMW) based on hydrographic data analysis, for which the Okada Prize for 1997 was awarded. Cited topics are (i) systematic differences in NPSTMW properties between summer and winter observed in the 137°E repeat section and their implication on the NPSTMW formation and circulation processes, (ii) the NPSTMW formation area revealed by close analysis of the wintertime mixed layer climatology, (iii) the NPSTMW circulation depicted as formation, migration and decay of the pycnostad in the hydrographic climatology, and (iv) interannual variation of the NPSTMW properties and its relation to the atmospheric forcing.

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The nitrogen isotopic ratio (δ^&lt15&gtN) of sedimentary organic matter is a newly developed chemical tracer which can be used to reconstruct paleo-biogeochemical condition of ocean surface water. Firstly in this article, factors controlling the δ^&lt15&gtN values of sinking particles are discussed on the basis of spatio-temporal variations of sinking particle δ^&lt15&gtN in the western Pacific Ocean. And it is concluded that nitrate mass balance between surface and sub-surface water layers determines the δ^&lt15&gtN of sinking particles through large isotope fractionation process during uptake of nitrate by phytoplankton, although some constant alteration processes during sinking and sedimenting of particles must also affect the absolute values of δ^&lt15&gtN in sediment cores. Secondly in order to demonstrate the potentials of δ^&lt15&gtN in paleoceanography, glacial/interglacial changes in the water current and upwelling system in the western equatorial Pacific Ocean are reconstructed using sedimentary δ^&lt15&gtN values, and the changes of intensity and location of upwelling system are clearly illustrated.

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