Field observations were carried out in Banzu intertidal flat in Tokyo Bay throughout two days in the summer of 2000,in order to clarify the spatio-temporal fluctuations of water quality during a tidal cycle. Concentrations of DO, ON (TN-DIN) or OP (TP-PO4-P) at the offshore station were higher than those at the onshore station, whereas DIN and PO4-P had an inverse trend, during submergence. During flood tides, an abrupt increase in the concentrations of DIN and PO4-P was observed around a shoreline.
　We constructed a box model, incorporating both the release flux of inorganic nutrients from sediment and the nutrient uptake rate by overlaying water, that were experimentally obtained in situ at each tide. The observed fluctuation of nutrients during ebb tide was well reproduced by the model. However, agreement was poor during flood tide. These results strongly indicate that the increase in nutrient concentrations during flood tide was attributed to the temporally increased effluxes of nutrients from the sediment and/or the physical resuspension of sediments at the shoreline.
　The mass balance of TP during a tidal cycle showed that the tidal flat acted as a sink of TP (1μ mol m-2 h-1) during daytime, in turn, source of TP (30μ mol m-2 h-1) during nighttime. The influx of organic N and organic P to the tidal flat and the efflux of inorganic nutrients from the tidal flat were commonly dominant processes when evaluated through a tidal cycle. Possibility that the light intensity during exposure affect the sediment-water flux of nutrients was indicated.