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.
Seaweed beds are very important for abalones and sea urchins as a habitat. In Sanriku Coast, these animals are target
species of coastal fisheries. The huge tsunami hit Sanriku Coast facing Pacific Ocean on 11 March 2011. It is needed for
fishermen to know present situation of seaweed beds and understand damages of the huge tsunami on natural
environments to recover coastal fisheries. We selected Shizugawa Bay as a study site because abalone catch of
Shizugawa Bay occupied the first position in Sanriku Coast. To evaluate impact of tsunami on seaweed beds, we
compared high spatial resolution satellite image of Shizugawa Bay before the tsunami with that after the tsunami by
remote sensing with ground surveys to know impact of the tsunami on seaweed beds. We used two multi-band imageries
of commercial high-resolution satellite, Geoeye-1, which were taken on 4 November 2009 before the tsunami and on 22
February 2012 after the tsunami. Although divers observed the tsunami damaged a very small part of Eisenia bicyclis
distributions on rock substrates at the bay head, it was not observed clearly by satellite image analysis. On the other hand,
we found increase in seaweed beds after the tsunami from the image analysis. The tsunami broke concrete breakwaters,
entrained a large amount of rocks and pebble from land to the sea, and disseminated them in the bay. Thus, hard
substrates suitable for attachment of seaweeds were increased. Ground surveys revealed that seaweeds consisting of E.
bicyclis, Sargassum and Laminaria species grew on these hard substrates on the sandy bottom.
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