is a lake in Kushiro, Hokkaidō, Japan. It is located in Akan National Park and is a Ramsar Site. History Volcanic activity formed the lake some 6,000 years ago, when a lava dam was formed. The lake used to have a clarity of 8–9 meters in the 1930s. Pollution from local hotspring resorts has decreased the transparency to 3–4 meters. Flora and fauna The lake is famous for the marimo (Aegagropila linnaei), aggregations of algae that form into spherical shapes 2–30 cm in diameter. Other flora of the lake include the following: Phragmites communis Nuphar sp. Potamogeton crispus Hydrilla verticillata Myriophyllum verticillatum Vallisneria gigantea Melosira italica Asterionella formosa Synedra Kokanee salmon (Oncorhynchus nerka) are native to Lake Akan. Other fauna of the lake include the following: Zooplankton: Daphnia longiremis Bosmina coregoni Eurytemora affinis Bottom: Chironomus plumosus Tubificidae Fish: Cyprinus carpio Carassius Hypomesus olidus Leuciscus hakonensis Climate See also List of Special Places of Scenic Beauty, Special Historic Sites and Special Natural Monuments Ramsar sites in Japan References Lakes of Hokkaido Ramsar sites in Japan Volcanic crater lakes Volcanoes of Hokkaido Calderas of Hokkaido VEI-7 volcanoes Lava dammed lakes