Differences in benthic assemblage structures between vegetated and unvegetated habitats in a salt marsh in Lake Shinji, western Japan

To clarify the mechanisms determining benthic assemblage structures in a salt marsh system in the brackish water Lake Shinji, western Japan, differences in the number and density of species, diversity index, and multiple environmental properties (e.g., presence/absence of vegetation, water quality, and sediment condition) were examined between subtidal marshes, including the common reed Phragmites australis, and bare sandy areas. Benthic assemblage structures differed significantly between the two habitats, with the diversity index being greater in the marsh than in the bare sandy area, although effects of habitats on the number and density of species were not significant. Furthermore, similarity percentage analysis indicated that the difference in species composition of the benthic assemblages between the two habitats was primarily due to differential distribution patterns of the dominant species Corbicula japonica, with numbers of larger individuals (> 5.6 mm in shell length) and smaller ones (< 5.6 mm in shell length: representing the size during the first year after settlement) being greater in the marsh than in the bare sandy area. Such differences likely arose from differences in the physical environment, the marsh being characterized mainly by structurally complex marsh roots and stems, and greater sediment organic matter content. These results suggested that sheltering effects against predators and/or organically rich environments are important factors determining benthic assemblage structures in salt marshes.