Factors shaping the macrobenthic animal assemblages in a tidal flat newly-formed by the Great East Japan Earthquake

In 2011, the Great East Japan Earthquake and tsunami struck the Pacific coast of northeastern Japan and created new intertidal habitats, which are ideal places to examine the roles of spatial and environmental variables shaping community structure since historical effects caused by resident species are precluded. In this study, therefore, we examined the relative importance of environmental and spatial variables in determining the community structure of the benthic macrofauna in the Higashi-Yachi tidal flat. During the study period, from July 2015 to June 2016, a total of 36 macrobenthic invertebrate taxa were found in the tidal flat. Among these, 14 taxa were abundant and in general significantly different in the abundance among the stations, but not among seasons. Redundancy analysis showed that most polychaetae such as Heteromastus spp. and Notomastus spp. were abundant in stations near the destroyed points of embankments, while the sand bubbler crab Scopimera globosa was abundant at stations with higher elevations. In addition, the analysis showed that 39% of the spatial and temporal variations in the community structures was explained by the examined environmental and spatial variables. The effects of spatial variables such as the elevation of ground level and distance from the destroyed parts of the embankment explained 18.7% of the variations, while the effects of temporally changing variables such as temperature and nutrient contents in the sediment and overlapping effects of these variables explained 7.4 and 12.7% of the variation, respectively. These results suggest that the macrobenthic animal community in this tidal flat was assembled by not only the environmental conditions but also the spatial configuration of the seawater flow and/or bottom elevation in the tidal flats.