The past and the present of vegetation and environment in a disappeared habitat of the freshwater alga Marimo (Aegagropila linnaei) in Lake Akan, Hokkaido, Japan

Although Lake Akan is famous as the habitat of green alga Marimo, a special natural monument of Japan, two of four Marimo populations in this lake were lost as a result of developmental activities, such as deforestation, by the middle of the 20th century. In this study, the past distribution of Marimo in Shurikomabetsu bay, one of the disappeared habitats, was presumed through detection of Marimo remains in sediment, and present status of the environment including springwater from lake bottom, which is considered to influence growth of Marimo, has been surveyed with the aim of restoration of the habitat. Most of Marimo remains were contained in the sediment collected at depth of 1.2 to 2.0m in two of six transects investigated. In these areas, substratum was mainly composed of mud, and submerged plants consisting of Ceratophyllum demersum and Potamogeton maackianus covered the lake bottom thickly. On the other hand in the transects of the neighborhood, change in the electric conductivity of interstitial water indicating the existence of springwater was detected, and the scoria with diameter of less than 1cm was observed as sedimentary layers in the depth of about 0.4m under the bottom. Existence of springwater and scoria is already known at Churl bay, a current Marimo habitat located in the northern part of Lake Akan. The result obtained here shows a possibility that deposition of mud and changes of vegetation disturb the functions of such environmental factors for generating the Marimo population.