荒瀬ダム撤去前後における球磨川水系潮間帯の底生生物の出現パターンの変化

Few studies have investigated the response of estuarine ecosystems to high dam removal with embankment heights exceeding 15 m. Focusing on the Arase dam removal project in the Kuma River system, we surveyed the intertidal zones in the Kuma and Mae River estuaries between 2011(before the removal)and 2018(after the removal)to assess the changes in sediment conditions and benthic fauna. A total of 14 surveys were conducted in the fall and spring, with a total of 178 sites, of which 138 sites were used for the analysis. Coarse-graining was observed in the spring of 2012 and 2014, which may have been associated with the permanent gate opening of the Arase Dam in 2010 and the flush in 2011. The results of the benthos community analysis indicated that the changes in the fauna at each site were not based on the specific survey period or season. Therefore, it is possible that the dam removal had no significant impact on the estuarine benthic community. However, the number of infaunal species in both estuaries increased from the fall of 2012 to approximately 2014. During the same period, the occurrence of the mud and ghost shrimps increased, as were the number of symbiotic species that inhabit these shrimp burrows. The coarse-graining of the sediment in the intertidal zones may have expanded the habitat of these species, which are commonly found at sand and sandy mud flats. Since the relationship between the observed changes in the benthos community and the Arase Dam removal has not been fully verified, the dynamics of sediment transportation during the removal should be further evaluated.