Distributional changes of the kelp community at a subtidal reef after the subsidence caused by the 2011 Tohoku Earthquake

Abstract

The subsidence caused by an earthquake can be regarded as a large-scale depth manipulation experiment that can provide valuable information about the formation of marine subtidal communities. To assess the causes of subtidal zonation patterns in macroalgae, we established a permanent survey area (4 m wide × 30 m long offshore) and conducted a long-term monitoring survey of the kelp Eisenia bicyclis and its associated assemblages on a rocky shore of the Oshika Peninsula in northeastern Japan where a 0.9 m subsidence occurred due to the 2011 Tohoku Earthquake. After the subsidence, the distribution of E. bicyclis expanded toward the shallower area, but the density of adults remained unchanged while recruitment declined at the offshore margin. In contrast, the perennial species Sargassum confusum and Sargassum siliquastrum tended to occupy the intermediate depth area and their vertical ranges of distribution did not change over time. No clear change was observed in the distribution of grazers. The most dominant herbivorous snail Omphalius rusticus was evenly distributed over the survey area while the distribution of the sea urchin Hemicentrotus pulcherrimus was limited to the shallower area. We hypothesized that the distributional expansion of the perennial Sargassum species toward the shallower area was prevented by active snail grazing during the microscopic stages of the plants as well as spatial competition with the kelp E. bicyclis. Survival of the subsided E. bicyclis at the lower limit was longer than predicted, indicating that a subsidence event could produce time-lagged effects.