A field survey has been performed both in the estuary of the Banjo river and in the Saeki bay in order to investigate the effect of water quality on the clam habitat in tidal flat. The result of water quality measurements shows that dissolved oxygen depletes due to fine sediment in the bottom of the Banjo river. Dissolved oxygen profiles near the sediment/water interface on the tidal flat are simulated based on the experimental results of the oxygen consumption rate by the sediment. The dissolved oxygen concentration profile varies considerably near the sediment/water interface and rapidly becomes zero just below the interface regardless of the shear velocity, because all DO that reach the sediment surface are utilized instantly. There is not enough oxygen both at the sediment surface and inside the sediment. A tidal flat that has biologically active sediment accumulating is therefore an inappropriate place for the clam habitat.

No abstract

In Banjo estuary, clam resources declined once, but recovery of the population is observed . To reveal potential of the fishery ground and major factors on clam life history, patterns of waves and currents around the river mouth sand bar and water temperature changes were analyzed. Patterns of wave directions and nearshore currents show the function of the sand bar as the bather of waves to protect habitats of estuary organisms. Long-term measurements of the water temperature indicated its severe changes might effect the survival of clam pelagic larvae and larval clams . Conservation and management of the river mouth sand bars dedicate estuary ecosystems and fishery.

In the Banjo River (Saiki, Oita), the population of clam drastically decreased after 1994 and has not been recovered yet. In order to make the reason clear some field surveys on water quality (:DO , salinity, water temperature, etc.), sediment quality and flow patterns were carried out from 2003 to 2004 in the Banjo estuary and Saiki Bay. Measurement results by ADCP showed the dominant flood current through the shorter navigation channel and the dominant ebb current through the longer one. In addition, water body tended to be stagnant in a neap tide and oxygen deficiency was measured in the longer channel . It was confirmed that temporal variation of DO concentration in the Banjo estuary depended on both flow patterns of tidal current and a redox conditions in the sediment.