Echosounding observations of coverage, height, PVI, and biomass of submerged macrophytes in the southern basin of Lake Biwa, Japan

We have developed a procedure to process echosounding data to map the distribution of submerged aquatic macrophytes in the southern basin of Lake Biwa, a water body that has a surface area of 52 km2 and a mean depth of 4 m. Echosounding observations were made along 27 transect lines spaced at 500-m intervals on August 4 and September 2 and 30, 2003. Quantitative vegetation data including percent coverage, mean vegetation height, and percent vegetation infestation were directly determined using image data from the echosounder recorded digitally on videotape. Based on the image data from an echosounder, a regression model was developed for estimating biomass of submerged macrophytes. The regression model using the total echo strength as the explanatory variable could reliably estimate macrophyte biomass up to 300 g m−2. Distribution maps of macrophyte height and biomass suggest that the recent summer decline of submerged macrophytes started earlier in shallow areas (<3 m of depth) than deep areas (>4 m) in the southern basin of Lake Biwa.