Survey of submerged aquatic vegetation with Multi-beam Echo Sounder in Towada Lake of Japan

Submerged aquatic vegetation (SAV) is a potent water-purifying agent enhancing integrity of lake ecosystems. Therefore, mapping the distribution of SAV in lakes is very important for managing the water quality and serves to conservation purposes. Current method for mapping SAV, i.e. collecting plant samples from shore, is not applicable to mapping wide areas. Here we report the novel method for mapping and quantifying the volume of SAV based on the use of Multi-beam Echo Sounder. The new method was used in Towada Lake, northeast Japan during the summer of 2010. This method includes a Multi-beam Echo Sounder, the Antennas of RTK-GPS and Azimuth Measurement. SAV was scanned with Multi-beam Echo Sounder after taking pictures with an underwater camera and sampling with the plant body extraction device. Using vegetation echo data and lake bottom echo data, we calculated height, coverage, and volume of SAV and produced a distribution map of SAV. In Towada Lake SAV distributed at bottom depths from 2 to 20 m with an area of 1948m² and volume of 1324m³. The volume changed with the depths, being 393m³, 64m³, 424m³, 308m³ and 134m³ at 4-6m, 6-8m, 8-10m, 10-12m and 12-18m depths, respectively. Our study demonstrated that Multi-beam Echo Sounder is an effective tool for mapping and quantifying SAV distribution including its visualization in three dimensions.