Factors of Water Quality and Feeding Environment for a Freshwater mussel’s (Anodonta lauta) Survival in a Restored Wetland

Wetland plays important roles in river ecosystems. However, the area of wetland decreased considerably due to excessive construction and land use with economic development. For recovering river ecosystem, the wetland named Azame-no-se was restored in the Matsuura River, which is located in Saga Prefecture in Kyushu, Japan. Anodonta lauta, a freshwater mussel that lives in the lentic water of wetlands, played an important role in a river. However, A. lauta lost its habitat and became endangered in Japan. A. lauta was found surviving in the Azame-no-se Wetland; however, only recently researchers have begun to understand the mechanisms that allow this mussel to now prosper there. This study investigated mussel survival mechanisms in the restored Azame-no-se Wetland. Mussel motility, coverage of Trapa japonica (an aquatic plant) and water quality were investigated in three pools (Shitaike, Ueike, and Tomboike) of the Azame-no-se Wetland and in still water in the Matsuura River (May 2011–2012). A feeding behavior experiment was also conducted in Ueike Pool two times in 2012. Results showed that this pool had abundant nutrients (thanks to its high flood frequency) that supported the growth of certain phytoplankton species possibly important for A. lauta. Dissolved oxygen was another important factor. Although Shitaike Pool shared similar conditions with Ueike Pool, the mussel became extinct when the dissolved oxygen decreased because of the lush growth of T. japonica in Shitaike Pool. This study indicated that the factors essential for A. lauta’ survival in the restored wetland were dissolved oxygen (DO), nutrients, and phytoplankton species. These results could be helpful for understanding functions of restored wetland and further offer suggestions for managing river–floodplain system.