Ecological Impacts of the Great East Japan Earthquake and Tsunami and the Following Succession on the Subtidal Macrobenthic Community in Onagawa Bay, Northeastern Japan, with Special Reference to the Dominant Taxon, Polychaetes

Abstract

The Great East Japan Earthquake of March 2011 caused huge tsunami waves. In the present study, field surveys were conducted in the innermost part of Onagawa Bay, from July 2007 to December 2013 to examine whether the benthic community was damaged severely by disturbances caused by the tsunami. Before the earthquake, the benthic community was relatively stable in the species composition and dominated by cirratulid and magelonid polychaetes. The density, biomass, and diversity of polychaetes decreased after the earthquake and tsunami, and the macrobenthic community structure fluctuated for 2 years after the disturbance. In this fluctuation period, spionid and capitellid polychaetes and opportunistic bivalves occurred abundantly, and the initial succession was driven by larval recruitment. In June 2013, the community entered a new state, which is different from the pre-quake community, and is dominated by maldanid polychaetes. Although both density and biomass of polychaetes gradually increased after the period of disturbance, density recovered to the pre-tsunami level, while biomass did not. Ecological impacts due to chemical pollution, derived from tsunami debris, were observed in addition to the physical disturbance of the tsunami. The additive effects by these drivers resulted in the initial succession similar to patterns of biological recolonization following large-scale hypoxia. The polychaete community has shown nonlinear responses during the recovery process, and the macrobenthic community in Onagawa Bay is still in the process of succession though 3 years have passed after the tsunami disturbance.