Quantitative Evaluation of the Impact of the Great East Japan Earthquake and Tsunami on the Rocky Intertidal Community

Abstract

It has been widely assumed that tsunamis associated with mega-earthquakes severely damage coastal ecosystems. In no previous studies, however, have the community-level impacts of tsunamis been credibly quantified. We evaluated the impacts of the tsunami following the Great East Japan Earthquake (the 2011 Tohoku earthquake tsunami) on species abundances in a rocky intertidal metacommunity encompassing 30 km of shoreline located 150-160 km north-northwest of the epicenter of the earthquake. For this purpose, we generated an indicator of the tsunami impact-the change in mean abundances between before and after the tsunami, standardized to population variability before the earthquake-for each of five sessile and six mobile species. We then used the averaged values of this indicator as a quantitative measure of tsunami impacts for the entire community and for the sessile and mobile subgroups. The tsunami had an overall significant negative impact on the rocky intertidal community at the regional scale, although the negative impact seemed smaller than those of similar large-scale disturbances on rocky intertidal communities. There was a significant negative impact of the tsunami on mobile species, but not on sessiles. The results suggest that mobiles on rocky shores are more vulnerable than sessiles to tsunamis. At a regional scale, the rocky intertidal community suffered minor damage from the 2011 Tohoku earthquake tsunami.