Preliminary Report of Impacts of the 2011 Earthquake and Tsunami and Subsequent Events on Macrobenthic Community in a Shallow Brackish Lagoon in Sendai Bay, Japan

Abstract

We have been monitoring the spatio-temporal change of macrobenthic community inhabiting soft sediment in Gamo Lagoon which is located on the north side of the Nanakita River Estuary in Sendai Bay, Japan since 2011 earthquake and tsunami disturbance. Unstable condition of estuary topography has been observed to continue until 2017. Just after the disturbance, topography was catastrophically damaged, and macrobenthic community was almost all disappeared by flowing outward and/or being buried in soft sediments caused by tsunami. Soon after that, some opportunistic polychaetes and amphipods were observed to appear, and they sharply increased their population. The secondary series of disturbance, namely river mouth closing and typhoon-derived floods occurred, the area lasted completely in freshwater condition, and that following drastic decline in density again. Adding to these initial sudden changes within a year, successive events and serious diurnal hypoxia in summer were observed every year from 2012 on. Dominant species consisted of firstly small polychaetes and amphipods and later adding bivalves repeatedly appear and disappear during the period. Furthermore, the extraordinary intense deluge occurred in 2015 and the second tsunami arrived in the estuary following Fukushima Earthquake in 2016. The collapse of the balance of the sediment dynamics is suggested to be the worst and major cause which drove the estuary to an unstable environment and macrobenthos community continued to be vulnerable until 2017. Not only natural disturbances but continuous anthropogenic disturbances such as reconstruction of seawalls and dikes have been carried out around the estuary since 2011.