Estimation of the aquatic plant community area in Lake Shinji reconstructed with the aerial photographs taken by the US forces in 1940's

Abstract

We reconstructed the distribution of aquatic plant community, including submerged plant, in Lake Shinji using aerial photographs taken by the US forces in October 1947. We identified the submerged plant community with the following features: shade and darker than sand and open water; profile, round in shape and existing below the water surface. We found only submerged plant community in the lake. Although Phragmites australis has been widely planted as the restoration project at Lake Shinji, emergent plant community was found only near the river mouths. Floating-leaved plant community or submerge plant community with extended leaves, that protrude from the water surface, were not found in the lake. The area of the submerged plant community was calculated as ca. 3km2. Transparency had been formerly estimated to be 3m, but our results suggest that it was more than 4m in 1947. At present, the lake bottom at a depth of 4m consists mostly of mud. On the other hand, data from 1947 shows that the lake bottom was composed partly of sand, based on its reflection of white light and the existence of a ripple at the bottom. The deepest areas covered with submerged plant community were found at the center of the north shore of Lake Shinji, and its depth was estimated about 4m. Recently, submerged plants with extended leaves, that protrude from the water surface, have invaded Lake Shinji. Our results show that such type of submerged plants are not the same species or the same type of those found in the past which dominated the area.