Nitrogen cycling in Suo-Nada is analyzed using a numerical ecosystem model that consists of upper and lower layers separated by a pycnocline. Primary production is higher in autumn and spring compared to those in other seasons, and lowest in winter. In spring, primary production in the lower layer is comparable to that in the upper layer. The main path of nitrogen cycling is DIN (Dissolved Inorganic Nitrogen) → phytoplankton → detritus → DIN. The decomposition rate from DON (Dissolved Organic Nitrogen) to DIN increases in the season with smaller primary production. Because the DIN supplies from the land and the lower layer are small, nutrient is depleted in the upper layer during spring to summer. Therefore the primary production in the upper layer is mainly limited by DIN. Furthermore, since the outflow flux of nitrogen is small, DIN is mainly supplied by the decomposition, and mostly consumed by primary production. In the lower layer, DIN is also mainly supplied by decomposition, and light intensity is more critical for primary production than DIN concentration. The inner part of Osaka Bay is a highly eutrophicated area, characterized by large outflow flux compared to sedimentation flux. In contrast, outflow flux in Suo-Nada is relatively small to sedimentation flux, since Suo-Nada has relatively weak estuarine circulation. These results suggest that if the nutrient load from land to Suo-Nada would increase, the environment of Suo-Nada would become worse, for instance, red tide area would expands, and it would take a longer time to recover than Osaka Bay.

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Seasonal variations in nitrogen and phosphorus budgets in Tokyo Bay were estimated by use of a box model. The consumptions of DIN and PO₄-P in the upper layer were large in the stratified season and small in the vertically mixed season. The production of DIN in the lower layer was small in the stratified season and a little large in the vertically mixed season. But the production of PO₄-P in the lower layer was large in the stratified season and small in the vertically mixed season. The sedimentations of TN and TP to the bottom showed a tendency to be large in the stratified season and small in the vertically mixed season. But the sedimentation of TP was not as large as shown in the sedimentation of TN. The productions of DIN and PO₄-P through the year in the lower layer compared to terrestrial loads were 48 and 146%, respectively. These differences between N and P behaviors were caused by desorption of P from the sediment in the stratified season.

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[in Japanese]

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[in Japanese]

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[in Japanese]

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