Changes in topography, vegetation, sediment characteristics, and benthic invertebrates of coastal ecotones in Gamo Lagoon (Miyagi Prefecture, Japan) after the disturbance of 2011 Great East Japan Earthquake

Gamo Lagoon, a shallow brackish lagoon in Sendai, Miyagi Prefecture, northeastern Japan, was struck by a 7.2-m-high tsunami on 11 March 2011. The tsunami washed away the sediment and vegetation in the lagoon, leading to changes in lagoon topography, sediment characteristics, and areas of tidalflats and vegetation. Reed marsh and sand dune vegetation (SDV) mostly disappeared after the tsunami, while the endangered opportunistic halophyte Suaeda maritima became proliferated in the bare upper tidal zone formed by the tsunami. It is expected that reed marsh and SDV are fully recovered respective in 2030 and 2026, while S. maritima will disappear due to the competition with reed. In 2011, lagoon sediment become courser, organic poor, and more oxidized due to the removal of reduced mud and deposition of sea sand, and opportunistic macrozoobenthos including polychaetes and amphipods rapidly recovered their population size within three months after the tsunamis. Sand deposition caused by a typhoon and Fukushima earthquake tsunamis in 2016 also modified lagoon topography and sediment characteristics, resulting in the increase in macrozoobenthic density and diversity in 2017. Contrastingly, water stagnation caused by the topographical changes and restoration works after 2017 negatively affected the density and diversity of macrozoobenthos in Gamo Lagoon. Present results demonstrated the impacts of pulsed disturbances including tsunami, typhoon, and restoration works on the lagoonal ecosystem, and the process of recovery and succession of coastal ecotones for 13 years following the 2011 tsunamis.