Influence of the partial removal of reclamation dikes on the bivalve distribution in the Honjo Area of Lake Nakaumi, Southwest Japan

Abstract

The Honjo Area of Lake Nakaumi is characterized as a brackish water area semi-enclosed by reclamation dikes. In accordance with the partial removal of these dikes from 2007 to 2009, we studied subsequent differences in environment, and spatial distribution of bivalves between 2006, 2010, and 2014, to determine the influence of the removal. Arcuatula senhousia was the primary dominant species across all years studied, followed by Ruditapes philippinarum and Macoma incongrua. After the partial removal of dikes, the density of A. senhousia decreased. Three areas were designated according to water depth and bivalve distribution. Area I was shallower than approximately 4 m, and bivalves were recognized at almost all stations in this site during each year studied. Areas II and III were deeper than approximately 4 m, and there were no bivalves in 2006 or 2010. In Area II, bivalves, mainly R. philippinarum, were observed in 2014, while none were identified in Area III. Area II was located near the opening slit of Moriyama dike. The inflow of sea water supplied oxygen to the bottom (greater than 4 m) of a limited region (Area II) and possibly enabled bivalves to inhabit this region in 2014.