Effects of the 2011 Great East Japan Earthquake and tsunami on two kelp bed communities on the Sanriku coast

To determine the effects of the tsunami on kelp bed communities, we examined two kelp beds that had been surveyed before 2011 on the Pacific coast of Honshu Island, Japan. One survey was conducted in Tomarihama on the Oshika Peninsula, which is dominated by the large perennial brown alga Eisenia bicyclis, and the other survey was conducted in Nagane in Otsuchi Bay, where the large annual brown alga Saccharina japonica var. religiosa dominates. In Tomarihama, the tsunami was associated with a limited loss of E.bicyclis individuals. In contrast, the population of sea urchins, the primary herbivore occurring at high densities, drastically decreased after the earthquake, and a large occurrence of juvenile E.bicyclis was then observed over a broader depth range owing to the decrease in grazing pressure. Some of these juveniles survived to adulthood, and with survival over this extended depth range, the E.bicyclis bed expanded to deeper areas. In Nagane, the biomass of S.japonica var. religiosa displayed no significant change before and after the earthquake. A definite reduction in sea urchins was not observed after the earthquake. Thus, the impact of the earthquake and tsunami widely differed between Tomarihama and Nagane.