Normality of Succession of an Intertidal Community After the Great East Japan Earthquake

Abstract

On March 11, 2011, the Great East Japan Earthquake occurred with an epicenter off the Tohoku area in northern Honshu, Japan. By the Tohoku earthquake tsunami and following ground subsidence, coastal organisms seem to have experienced various physiological and biological effects. We have continuously examined influence of the earthquake to the intertidal organisms at Sakihama in Iwate Prefecture for 4 years since August 2011 by using video camera footage. The surveyed site between E.H.W.S. and E.L.W.S. that has about a 180 cm tidal range experienced a subsidence of 130 cm due to the earthquake. The first colonizers on the newly submerged substrate were barnacles Semibalanus cariosus. The hard uneven substrate formed by the settlement of the barnacle facilitated the recruitment of the green alga Ulva intestinalis first and then the mussel Mytilus galloprovincialis followed. This multilayered community structure increased in species richness for 2 years. However, the species richness subsequently decreased with increasing abundance of the sea urchin Strongylocentrotus nudus which was the causative species in rocky shore denudation in this area before the earthquake. This biological succession was almost the same as that of a previous experiment from 2004 to 2006 which was carried out to examine the ecological roles of S. cariosus using test plates installed at the same location. The main difference between the two biological successions during the two experimental periods was the occurrence of a new alien barnacle Perforatus (Balanus) perforatus after the earthquake. According to the monthly monitoring data of the sedentary organisms on the test plates set on the jetty wall in Sakihama, this barnacle was found for the first time in 2012 and has been settling on them every summer. Just after the earthquake, many barges and tugboats came from various localities in Japan for reconstruction activities along the Tohoku coast. One barge, registered in Niigata, a city on the Japan seacoast where P. perforatus has become common, was anchored near the research site in 2013 and had this barnacle on its hull and fenders. Since the settlement of the alien barnacle occurred after the earthquake, it was likely that these boats played a role in the dispersion of P. perforatus into this area. This possibility implies that the earthquake affected on the intertidal communities not only directly via the tsunamis and subsidence but also indirectly via anthropogenic activities.