A shallow brackish lagoon, Gamo Lagoon (Sendai, Miyagi Pref.), was attacked by the tsunami of 7.2 m height on 11 March 2011. In June to August 2011, we conducted a series of survey to assess the Tsunami-induced changes in environments and macrozoobenthic community structure in this lagoon. Lagoon topography and vegetation were traced using a GPS, and macrozoobenthos and environment were assessed at more than 60 stations in the area.

After the tsunami, marsh vegetation mostly disappeared and sand bar was destroyed at several points. Sea water directly entered the lagoon for several months until the reestablishment of sand bar. Highly reduced mud in inner lagoon was flushed out, and the sediment became more sandy. Bivalves were distinctively declined, and the gastropod Cerithidea rhizophorarum inhabiting the marsh was nearly extinct. In contrast, densities of dominant polychaetes (e.g.,Pseudopolydora cf. kempi, Hediste spp., and Capitella sp.) and amphipods (e.g. Corophium sp.) were as high as those before the disaster. Although the tsunami largely modified the lagoon environment, dominant polychaetes and amphipods, which were characterized by opportunistic life cycle, had rapidly recovered their population within 5 months after the disturbance.