In order to estimate nutrients budget of a tidal flat, a field observation was conducted in autumn in Yatsu tidal flat which is connected to Tokyo Bay only through two rivers. Measured water level showed asymmetric variation with relatively longer duration for low tide, due to nonlinear effect of the semi-enclosed water body connected to the open sea by narrow and shallow rivers. It was also found that the Yatsu tidal flat was the source of nutrients for Tokyo Bay in autumn , owing to the

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... owth of ulva in spring and summer. Thus, Yatsu tidal flat is considered to be a temporal buffer for storing nutrients. The outflow of SS exceeds the inflow, resulting in the change of surface sediments from mud to sand.