Spatial changes in a macrozoobenthic community depend on restoration methods in historically squeezed coasts in a brackish lagoon

Abstract

The coastal shallows in the Kugushi-ko lagoon (western Japan), were restored to mitigate the historical coastal squeeze using two different methods: sand nourishment (SN), which is commonly used worldwide, and natural deposition (ND) method, which facilitates the deposition of river-derived sediments using local knowledge (LK) on habitat characteristics of the historically commercial Shijimi clam (Corbicula japonica). In this study, macrozoobenthic assemblage structures were examined to clarify how the different restoration methods affected them. A total of 24 taxa occurred with a predominance of molluscan species in 18 samples (two SN sites and one ND site, six sampling plots each). In the ND site, the macrozoobenthic community was dominated by reed-associated snails and Shijimi clams, linking to a higher whole-benthos density than SN sites. Cluster analysis identified two groups of sampling plots that had different macrozoobenthic communities, and four shallow-water inhabiting species were detected as indicator species for one cluster involving all sampling plots at the ND site. Distance-based redundancy analysis (dbRDA) demonstrated that water depth and reed vegetation explained the variations in assemblages. Furthermore, within the cluster, a subcluster consisting of all plots in the ND site was identified. dbRDA showed that the dominant species in the ND subcluster were associated with reed vegetation and finer grain size.The present results suggest that the ND method likely restored the reedvegetated shallow habitats characterizing the macrozoobenthic community structure, and that LK-based habitat restoration practices may offer new options for restoration programs in estuarine and coastal seas.