Survey of lotus root habitat under the lake bottom using a single-beam acoustic technique: In Lake Izunuma

The distributions and population dynamics of plant species are modified by environmental factors. The grazing intensity of herbivores may have major effects on aquatic plants. Lake Izunuma is located in the northeast of Japan and is one of the most famous wintering grounds for swans. The foraging activities of whooper swans (Cygnus cygnus) were observed frequently during the winter of 2011-2012. During the spring, the distribution of lotus (Nelumbo nucifera) differed in various locations and it seemed to be affected by the predation pressure of swans. Understanding the interactions between lotuses and whooper swans is essential for the appropriate control of lake ecosystems. Therefore, a more efficient monitoring system is needed to assess the status of the lake. In this study, we originally integrated a single-beam acoustic measurement system and conducted the in situ investigation of lotus root habitat under the lake bottom. The layer of maddy sand and lotus root were clearly detected by the measurement method. This novel survey method will contribute to sustainable management in the rapidly changing underwater environments found in lakes.