Estimated distribution and biomass of a submerged macrophyte community in Lake Biwa's South Basin using an echo-sounder

Abstract

We estimated the distribution and biomass of a submerged macrophyte community in the Lake Biwa's South Basin using an echo-sounder. Echograms corresponding to GPS locations were obtained during a cruise along 14 latitudinal transects at 1-km intervals. Based on the echograms, we drew a distribution map of the submerged macrophyte community. We also made a model to estimate its biomass from the echograms. The submerged macrophyte community was estimated to have a biomass of 7.1×104t in dry weight; the estimated macrophyte area was 32 km2 and accounted for 58% of the whole basin. These quantities were approximately equal to or grater than those before the 1960s, the time when the community decline became apparent. We compared the distribution map to an inferred submerged macrophyte area from transparency. In 1994, the macrophyte community was absent from many part of the euphotic zone where it would thrive. In contrast, the macrophyte community was present in most part of the euphotic zone in 2001. The submerged macrophyte community is considered to have gradually expanded to the empty bottom where sufficient light was available after 1994.