Lake Akan in eastern Hokkaido is a dammed lake formed approximately 6,000 years ago by volcanic activity in a caldera depression that dates back to the middle Pleistocene. Its complicated shoreline is characteristic of dammed lakes. The lake is famous as the only native habitat of "Marimo" or fine ball-like masses of the alga, Aegagropila sauteri, 2-15 cm in diameter. It is believed that the globular shape is formed by the aggregation of filamentous alga and the rotary motion due to water turbulence. Similar kinds of spherical algal mass are observed in a few other lakes of Japan, but they are more irregular in shape. L. Akan is also known as the original home of kokanee salmon (Oncorhynchus nerka).

Transparency in the early 1930's was 8-9 m throughout the year, but the lake has recently been eutrophicated to a considerable extent owing to the inflowing waste water from the hot-spring town on the coast in spite of its cool and deep water. The eutrophication exerted a profound influence on the benthic fauna, which had once consisted of Chironomidae, Tubificidae, Chaoborus and Pisidium in its deepest bottom. According to the result of a general limnological survey made in 1973, however, the dissolved oxygen concentration in bottom water was only around 5% of saturation and no organisms were observed beyond a depth of 30 m (1, 2, 3).