Studies on the function of sandridge-centric ecosystems in Seto Inland Sea and the restoration processes of the ecosystems from the deterioration due to sand mining

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The present project aimed at revealing restoration processes of deteriorated environments of sandridge areas in Seto Inland Sea due to sand mining and roles of the sandridges in marine ecosystem. The major results are as follow.The long-term high accuracy monitoring of bottom topography in the mined area found out movement of sand wave and change in surface sediment from gravel to fine sediment in quite a small part of the mined area, while no topography change was found in other areas where tidal cm-rents are weak. The numerical experiments on the restoring processes revealed that, if there is enough son of sand in the surrounding area, the speed of sea bed rise by accumulation of sand is one to several millimeters per year and hence it will take ten to several tens of thousands of years for the sea bed mined to 40m deep to recover to 10m deep.The field surveys on the roles of sandridges on marine ecosystem revealed that, in spring tidal period, benthic microalgae on the sand surface is raised up by strong current resulting in supply of organic matter to nearby areas, and nutrients are also supplied to the surface layer enhancing primary production. This means that the sandridges not only supply suitable habitat for some sorts of benthic organisms but also play very important role of enhancing the productivity of the surrounding areas. The laboratory experiments revealed that the common lancelet Branchiostoma japonicum living in sandridges also play an important role of enhancing decomposition of organic matter resulting in higher productivity by disturbing sand surface and introducing dissolved oxygen into the sand.