The transport and material balance of nutrients in the Yamaguchi Estuary were studied using environmental impact assessment (EIA), in order to investigate the effects of the reclamation of tidal marshes. The main methods utilized were: 1. a topographical survey; 2. pollutant load analysis; 3. water sampling during one tidal cycle; 4. sediment quality analysis; 5. box model analysis of water quality; 6. modelling of the ecological system including fish.

The main results obtained were as follows: 1. after the reclamation of 348 ha, the volume of the estuary, tidal prism, and tidal marsh area decreased by 6.5%, 12%, and 11%, respectively; 2. pollutant loads have increased from 1.38 to 1.69 t/d for N and from 0.09 to 0.18 t/d for P; 3. upward transport of suspended solids (SS) was observed under fine weather conditions, especially in the summer; 4. sediments in marsh areas exhibited higher values of oxygen consumption rate and nutrient recycling rate; 5. the net settling velocity of SS in the river mouth area was 1 to 3 m/d; 6. most of the SS transported up the estuary seemed to be flocculent organic matter; 7. the results of the box ecosystem model simulation indicated that there will be a net input of fish from the open sea area.

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