Temporal and spatial variation in the inflow of oceanic water into Tokyo Bay and accompanying change in circulation in the bay were studied with emphasis on the movement of the hypoxic water mass. Longitudinal distributions of hydrographic conditions, oxygen concentration and nutrient concentrations were repeatedly measured in Tokyo Bay from the bay head to off the bay mouth in summer, 1998. Changes in the density in the upper several tens of meters in the ocean induce inflow and outflow in the adjacent shallow bay. When water density increased in the upper ocean, heavy saline water intruded into the lower layer in the bay. This intrusion forced the "bottom hypoxic water mass" to the bay head and finally lifted it up to the middle layer, forming a "subsurface hypoxic water mass". The subsurface hypoxic water mass spread within a thin layer just under the pycnocline and part of it spread out of the bay. When the intruding heavier water retreated, the hypoxic water mass followed this movement and was displaced to the bay mouth from the central bay. The bottom hypoxic water mass dominates north of Yokohama, while the subsurface oxygen minimum appears frequently south of Yokohama. The movement of the hypoxic water mass may influence nitrogen and phosphorus transport in the bay.

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Ecological studies on the cyst-forming armored dinoflagellates were made in Onagawa Bay, northeast Japan during the period from 1990 to 1994. This article reviews mainly the population dynamics of Scrippsiella spp.(mostly S.trochoidea) which is the dominant group among the dinoflagellate community in the bay. As the first step, factors controlling in situ cyst germination of Scrippsiella spp.were considered on the basis of germination experiments in the laboratory, abundance of the vegetative cells in the water column and in situ environmental parameters. Then, a new sampling device, "germinating cell trap/sampler", was developed to measure in situ germination rates of dinoflagellates cysts on the surface sediment. Using the trap/sampler and sediment traps, in situ germination rate and cyst deposition rate, respectively, were monitored for Scrippsiella spp., and seasonal change in abundance of the vegetative cells in the water column was also investigated. The results obtained, provided a comprehensive understanding of the population dynamics of Scrippsiella spp. Consequently, its life history strategy as well as the roles of the cysts could be defined. Further relationships in seasonality between in situ germination phenomena and the occurrence of vegetative population of some armored dinoflagellates other than Scrippsiella spp.are then presented.

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