Factors to determine the functions and structures in natural and constructed tidal flats

The purpose of this study was to evaluate the effects of design criteria and environmental conditions of the site of created tidal flats, on the structure and function of the man-made ecosystem. Seven constructed tidal flats and three natural tidal flats were studied taking into consideration the difference in the location (wave height and tidal current), inflow of river water, slope of tidal flat and age after the construction was completed. Parameters studied were physico–chemical and biological characteristics of soils and rate of respiration. The natural tidal flats had higher contents of silt, nitrogen and organic matter compared to the constructed ones. The natural ones had a reductive zone below 2 cm, whereas the constructed ones had an aerobic zone from the surface to below 20 cm. The bacterial population in the soil of the constructed tidal flats were one to two orders of magnitude lower than that in the natural ones. Biomass of macrobenthos and microbial respiration rate, however, were not significantly different between the natural and the constructed tidal flats. There was an exceptional constructed tidal flat with similar physico–chemical and biological characteristics as the natural ones. It is most probable that sufficient conditions to have similar tidal flats to natural ones are not only the slope, but also the location in enclosed bay or calm coastal area. Thus, to make man-made tidal flats with the same characteristics as those in natural one, man-made tidal flats should be designed and/or located to enhance the accumulation of silt on tidal flat. It is important to select a place having low water motion and to construct a flat with small slope.