Distribution of living and dead diatoms in tidal wetlands of northern Japan: relations to taphonomy

This paper presents the results of a survey of the contemporary diatom distributions in four tidal marshes of eastern Hokkaido, northern Japan. Distribution of living diatom cells, as defined by cluster analysis, have relations to vegetation and substratum, and clearly to tide level. Comparison between living and dead diatoms in each cluster provides synecological information relating to diatom taphonomy in the tidal wetland environments. Four assemblages are recognized and they are summarized as follows. (1) An upland assemblage consisting of autochthonous freshwater diatoms adapted to relatively dry conditions. (2) A high marsh assemblage formed at or slightly above mean high tide level (MHTL) with only few allochthonous components. (3) A low marsh assemblage formed below MHTL. Dead valves within this assemblage are repeated widely redistributed by tidal processes. This assemblage is therefore a ‘mixed assemblage’ comprising a combination of allochthonous and autochthonous diatoms. (4) A tidal flat assemblage. On death diatom valves in tidal flat are immediately redistributed by tidal currents, and the dead diatom assemblage here has the characteristics of an ‘autochthonous’ or ‘residual’ assemblage. Autecological perspectives based on the distribution patterns can advance these interpretations. Dead valves of the epiphytic diatom Cocconeis scutellum, which possesses a raphid valve (R-valve) and a rapheless valve (P-valve), are common on two transects but only P-valves are widely distributed beyond the seaweed zone which represents the actual habitat of this species. Such selective processes are related mainly to the life form characteristics of monoraphid species: only R-valves remain attached to their substratum on death. The tychoplankton Paralia sp. is also a widely distributed species. In the case of Paralia, their long chains float more readily than other specimens are transported easily by tidal currents, and then trapped in entire tidal environments. These observations suggest it is likely that concentrations of only P-valves of C. scutellum or Paralia in the tidal marsh deposits should be treated as allochthonous components in the diatom assemblage.