Geo-historical analysis of tidal flats by the survey using diatom indicators.

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Diatoms are unicellular, eukaryotic, microalgae, and are distributed in most of all the water environments. Their cell walls are highly differentiated and impregnated with silica. The siliceous part of diatom valve is resistant to decay and is often preserved in sediments after the diatom cells died. Thus the diatom can offer us reliable information for sedimentary environments and is a good tool for reconstructing environmental changes. From this point of view, diatom analysis of sediments has been applied to various paleoenvironmental studies.Along most of the Japanese coast, an intertidal diatom Pseudopodosira kosugii occurs abundantly In the successive diatom assemblages of sedimentary sequences formed as a result of the Jomon marine transgression. During the Jomon period, this species was dominant at the transition from marine to freshwater environments and was widely distributed. A present-day habitat of this species, such as exists in Obitsu-gawa river mouth, provides a window onto the former natural environment of the tidal flats. The habitat loss of this species on the northeastern coast of the Seto Inland Sea Is probably due to the deterioration of coastal zones associated with land reclamation. P. Kosugii is sensitive for habitat degradation and thus is a good environmental indicator in the assessment of coastal ecotones.Based on the intertidal diatoms such as P. kosugii in the sediments, mid- to late- Holocene sea level observations have been obtained from two sites in western Kobe, and from a site in Okayama City, Japan. In the tectonically active coast of western Kobe, the local tectonic effect during the Holocene was evaluated by comparison between observations and predictions of sea-level variations (Sato et al., 2001).