Based on the two layer box model for Bays of Tokyo and Mikawa the transport of gravitational circulation was estimated and compared with the river discharge in winter and summer. The transport of the circulation exceeds the river discharge remarkably, but the ratio of the former to the latter is smaller in summer than in winter. It suggests that the gravitational circulation is likely to be suppressed in summer, because of its strong density stratification. The amount itself of the transport, however, is larger in summer than in winter in Tokyo Bay, and comparable in the both seasons in Mikawa Bay. It is attributed to the abundance of the river discharge in summer. Moreover, the residence time of river water is shorter in summer than in winter in the respective bays. Therefore, we can conclude that the sea water in the lower layer of bay is not so stagnant as believed intuitively but circulates actively even in the warm season.

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Areal surveys were conducted in August 1995, February 1996 and July 1996 in the vicinity of the planned tidal flat at the Port of Sakai-Semboku, Osaka Bay to clarify characteristics of the seawater and bottom sediment qualities and occurrence of benthic animals. Laboratory experiments were also carried out to examine the response of a benthic fish to hypoxia, using young marbled sole Pleuronectes yokohamae which has lower tolerance to hypoxia among benthic animals in the study area. Hypoxia and/or anoxia developed in summer at the lower seawater layer in the port, and this led to an increase in sulfide content and a decrease in the oxidation-reduction potential of the bottom sediment. As a result of such environmental deterioration, the number of benthic animal species was reduced (macrobenthos to less than 11 species ; megabenthos to less than 8 species) and no benthic animals were found at the innermost part of the port. In contrast, improvements of dissolved oxygen saturation and bottom sediment quality, together with the restoration of benthic fauna were observed in winter. Concentrations of total sulfide and pheophytin of the bottom sediment tended to increase with increasing depth of water. In addition, the total organic carbon and total nitrogen concentrations of the sediment showed the same trend as total sulfide in the areas shallower than ca. 12m. Both numbers of species and individuals of macrobenthic animals tended to increase with decreasing depth of water, except for shallow bottoms in the mouth of the Yamato River and Dejima Fishing Port. Laboratory experiments revealed that the mortality rate of young marbled sole was 0%, 50% and 100% when exposed at 22.4-24.3°C for 24 hrs to oxygen saturations of 30%, 25% and 19%, respectively. Breathing frequency of the young marbled sole increased at 40-50% saturation, compared to 92-100% oxygen sufficient seawater. Based on the results of field surveys and laboratory experiments, we concluded that dissolved oxygen should not drop below 30% for longer than 1 day and be kept higher than 50% on average in summer to maintain sound benthic populations in the study area.

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The REE contents increase in the following sequence : hydrothermal crusts ≤ suboxic-diagenetic nodules < oxic-diagenetic nodules < hydrogenetic crusts. With the above order, a positive Ce anomaly and a Sm maximum when normalized to deep-sea water become remarkable. These phenomena are controlled by kinetics rather than by REE concentrations in the environments where nodules and crusts are precipitated. The Sm maximum is explained by the competition between REE adsorption on oxides and REE stabilization by their complexation with ligands in seawater. The contents of readily oxidized elements such as Co, Pb, Ce and Pt in nodules and crusts decrease with increasing water depth. Their contents are also controlled by kinetics rather than by their concentrations and redox potentials in the depositing environments. The contents of oxyanionic elements are not so varied among nodules and crusts of different origins in comparison with those of cationic elements. This phenomenon is attributed to the adsorption of oxyanions on oxides without their substitution for the elements forming the oxides, because molecules of oxyanions are large. The adsorption is a rapid process relative to the substitution.

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