THE CURRENT STATE OF MACROBENTHIC COMMUNITY AND ENVIRONMENT IN LAKE SHIBUNOTSUNAI WHERE A TRAINING WALL FOR PREVENTION OF RIVER MOUTH CLOSURE WAS CONSTRUCTED

Lake Shibunotsunai, located at the end of the Shibunotsunai River system, is a small brackish water lake that yields brackish water clam and smelt. Here, a training wall was constructed to prevent river mouth closure due to coastal sand drift. In this study, we investigated the water quality and sediment of the lake and the benthic community and examined the current state of the lake environment. During the survey period, the salinity of the lake was 5 psu or less and remained under 1 psu for one year or more, and the occurrence of blue-green algae was confirmed. The bottom of the lake had a mud content of 50% or more, an ignition loss of 10% or more, and a mud content of 90% or more near the inflowing river in most areas except the eastern lakeshore. The dominant species of macrobenthos are chironomid larvae and Capitellid polychaete, and the density of constituent species common to the good fishing grounds of the brackish water clam in Japan was very low. From the above results, it was suggested that the open state of the river mouth training wall strongly affects the water quality, sediment, and organisms in the lake.