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

本项目旨在揭示后藤内海沙脊地区因采沙而恶化的环境恢复过程以及沙脊在海洋生态系统中的作用。主要结果如下：通过对采空区底部地形的高精度长期监测，发现有相当一部分采空区沙波运动，表层沉积物由砾石向细粒沉积转变，而在其他潮汐分流较弱的地区，地形无明显变化。沙脊恢复过程的数值实验表明，如果周围地区有足够的沙子，沙粒堆积海床的上升速度为每年一到几毫米，因此开采到40m深的海床需要十几到几万年的时间才能恢复到10m深。沙脊对海洋生态系统的作用的野外调查表明，大潮期间，沙丘表面的底栖微藻被强流抬高，导致有机质供应给附近地区，营养物质也被供应到表层，促进初级生产力。这意味着沙脊不仅为一些底栖生物提供了合适的栖息地，而且对提高周围地区的生产力也起到了非常重要的作用。实验室实验表明，生活在沙脊中的普通刺鱼通过扰乱沙面和向沙子中引入溶解氧，也起到促进有机物分解、提高生产力的重要作用。

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.