We monitored an Eisenia bicyclis kelp bed during a survey of the rocky coast subtidal zone of Shizugawa Bay, the Sanriku Coast, northeastern Honshu, Japan, from 23 July 2008, to detail the biodiversity, which was subsequently directly impacted by the 2011 Great East Japan Earthquake (GEJE). To assess temporal changes in abundance of the dominant canopy forming kelp E. bicyclis and in the distribution patterns of macroalgae along a water depth gradient, percent coverage of macroalgae has been observed in permanent quadrats set near the lower limit of the Eisenia bed and in quadrats set along a water depth gradient. The GEJE, which induced huge tsunami waves and coseismic seafloor subsidence, occurred during the monitoring survey period and also affected the coastal communities in Shizugawa Bay. After the GEJE, the cover of E. bicyclis within the permanent quadrats near the lower limit of E. bicyclis gradually declined, and reached zero by July 2014. Also in the line transect survey, the offshore (deep) edge of the Eisenia bed showed a tendency to shift shoreward (upward) after the GEJE; the Eisenia bed near the pre-earthquake offshore (deep) edge declined and finally disappeared after the GEJE. Combined with results of the permanent quadrat and line transect surveys, the post-earthquake gradual decline and subsequent complete disappearance of the Eisenia bed within the permanent quadrats probably indicates an upward shift of the deep edge of the subsided kelp bed. Gradual change in the E. bicyclis bed over 2 years after the GEJE is a unique opportunity to document the response of a kelp bed to coseismic subsidence, demonstrating the slow and prolonged recovery process of E. bicyclis to subsidence caused by the mega-earthquake to the pre-earthquake depth zone.
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