Temporal and Spatial Changes in a Coastal Ecotone in Shizugawa Bay, Sanriku Coast Due to the Impacts of the Tsunami on 11 March 2011 and the Following Artificial Impacts

Abstract

The Great East Japan Earthquake produced the huge tsunami on 11 March 2011, which hit and changed a coastal ecotone consisting of ecosystems that forms a transition zone between the land and the sea and provides ecosystem services for human societies and habitats for marine living resources along the Sanriku Coast. Therefore, it is important to identify temporal and spatial changes in the ecotone and its succession under natural-system processes and the following human impacts on the terrain. This is particularly relevant in Sanriku Coast, Japan, because tsunami events commonly repeat at intervals of several decades to 100 years. Since October 2011, we have observed a succession of the coastal ecotone in Shizugawa Bay, Sanriku Coast, which was seriously impacted by the tsunami. The tsunami did not seriously damage seaweed beds on rocky substrates, but it did impact seagrass beds on sandy substrates in the bay head due to large displacement of sand being moved by the tsunami. However, the seagrass beds may have fully recovered by 2014 in most areas except those near the river mouth where restoration and construction activities to raise the land for houses to a safe height have caused turbid waters. The tsunami restored tidal flats and saltmarshes along the coast, which had been destroyed by land reclamation before the tsunami event. However, they are being destroyed again by public works, such as the construction of huge seawalls, roadways, and the elevation of the ground level by piling up sand. The area over which the seaweed beds are distributed has increased because the hard substrates needed for seaweed attachment (such as fragments and debris of buildings, broken seawalls or wave-dissipating concrete blocks) were shifted seaward from the land and shore by the tsunami. The seaweed beds not seriously affected by the tsunami have gradually decreased in size because of the removal of hard substrates from the bay. They are now suffering grazing pressure due to an increase in the number of sea urchins since 2014 (3 years after the tsunami) due to the resulting of lack of fishing pressure on sea urchins. The Government of Japan and local governments decided to construct high seawalls to protect against large-scale tsunamis along the Sanriku Coast based on computer simulations. The Government of Japan decided that Environmental Impact Assessment Law is not apply to the construction of huge seawalls with a height of ~8 m above the sea level with deep and wide bases because the law doesn’t include construction of seawalls and embankments along the shore and the river bank. These seawalls prevent materials and organisms from flowing or migrating between the land and the sea and often destroy tidal flats and saltmarshes forming the coastal ecotone. It is necessary to keep constant material flows between the land and the sea to maintain healthy coastal waters and prosperous, sustainable coastal fisheries.