Impact of the Great East Japan Earthquake on the seaweed (Eisenia bicyclis) habitat at Oshika Peninsula, Miyagi, Japan

Abstract

The objective of this study was to examine the effects of the earthquake and tsunami event on the seaweed (Eisenia bicyclis) habitat and environmental conditions at Tomarihama on the Oshika Peninsula by comparing them before and after the event. After the event, due to the land subsidence caused by the effects of the earthquake, water depths increased by approximately 1.0 m, and the underwater-light intensities between November 2012 and May 2013 were significantly lower than those before the event. In July and August 2011, loss of E. bicyclis due to the tsunami was limited. On the other hand, sea urchins, the primary herbivore, largely decreased after the earthquake. The decrease in the sea urchin density led to the reduction of their grazing and resulted in the increase of juvenile E. bicyclis in the seaward areas of the transects. Some of these juveniles survived and grew into adult E. bicyclis, leading to an expansion of E. bicyclis habitat. The appearance of adult E. bicyclis in the seaward areas suggested that light conditions were sufficient for the survival of E. bicyclis, even though the underwater-light intensity was reduced by the earthquake. In 2015, the density of sea urchins appeared to increase gradually, and then the forests of adult E. bicyclis in the seaward areas were largely decreased by the grazing of sea urchins. After the event, sediment deposits on the rocky shore were consistently confirmed, while showing a fluctuation at each monitoring place, and juvenile E. bicyclis individuals decreased in relation to the increase in sediment deposits on the sea floor. This result suggests that the increased sediment deposits on the rocky shore negatively affected the recruitments of E. bicyclis.