Did huge tsunami on 11 March 2011 impact seagrass bed distributions in Shizugawa Bay, Sanriku Coast, Japan?

Seagrass beds play important roles for coastal ecosystems as an ecosystem engineer and also as a habitat for fish and mollusks as spawning, nursery and feeding grounds, and provide us important ecological services. On 11 March 2011, huge tsunami hit Sanriku Coast, Japan, after the big earthquakes occurred in Northwestern Pacific Ocean. Seagrass beds were distributed on sandy or muddy bottom in Shizugawa Bay, Sanriku Coast. Thus, remote sensing research was conducted to evaluate impact of the tsunami on seagrass bed in Shizugawa Bay, Sanriku Coast. GeoEye-1 multi-band imageries taken on 4 November 2009 and 22 February 2012 were analyzed to map seagrass beds before and after the tsunami, respectively. Analysis of the former imagery showed seagrass beds were distributed in sheltered bottom against waves along the coast corresponding to seagrass distributions obtained through inquiry to fishermen and references on seagrass bed distributions before the tsunami. Analysis of the latter imagery indicated that seagrass bed distributions on 22 February 2012 were less than on 4 November 2009. Seagrass beds in the bay head disappeared while some seagrass beds remained behind the points along the north coast. This was verified by the field survey conducted in October 2011 and May and October 2012. Since the tsunami waves propagated into the bay along the longitudinal axis of the bay without crossing both sides of the bay, they produced only big sea-level changes during the propagation along the both sides from the center to the bay mouth. Their energy is concentrated the bay head and removes seagrass with sand and mud substrates. On the other hand, the tsunami higher than 12 m could not completely destroy seagrass beds due to topographic effect protecting seagrass from strong force by the tsunami. Thus, all seagrass weren’t destroyed completely in Shizugawa Bay even by the hit of the huge tsunami.