is a lake in Kushiro

Kushiro, Hokkaido

is a city located in Kushiro Subprefecture, Hokkaidō, Japan. It is the capital city of Kushiro Subprefecture, as well as the most populous city in eastern Hokkaidō....

, Hokkaidō

Hokkaido

, formerly known as Ezo, Yezo, Yeso, or Yesso, is Japan's second largest island; it is also the largest and northernmost of Japan's 47 prefectural-level subdivisions. The Tsugaru Strait separates Hokkaido from Honshu, although the two islands are connected by the underwater railway Seikan Tunnel...

, Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. It is located in Akan National Park

Akan National Park

is a national park located on the island of Hokkaidō, Japan. Along with Daisetsuzan National Park, these are the two oldest national parks in Hokkaidō. The park was established December 4, 1934....

 and is a Ramsar Site

Ramsar sites in Japan

The Ramsar Convention is an international treaty for the conservation and sustainable use of wetlands. Adopted in 1971, it entered into force in 1975 and as of March 2011 has 160 Contracting Parties. Japan was the twenty-fourth party to accede, on 17 October 1980...

.

History

Volcanic activity formed the lake some 6,000 years ago, when a 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

Marimo

, also known as Cladophora ball, Lake ball, or Moss Balls in English, is a species of filamentous green algae found in a number of lakes in the northern hemisphere. A marimo is a rare growth form of the species where the algae grow into large green balls with a velvety appearance...

 (Aegagropila sauteri), aggregations of algae that form into spherical shapes 2–15 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
    Chironomus plumosus
    Chironomus plumosus, also known as the buzzer midge, is a species of non-biting midge that occurs throughout areas in the Northern Hemisphere.-Adult:...
    
    
  • Tubificidae
    Tubificidae
    The Naididae are a family of clitellate oligochaete worms like the sludge worm, Tubifex tubifex. They are key components of the benthic communities of many freshwater and marine ecosystems....
    
    
• Fish:
  • Cyprinus carpio
  • Carassius
    Carassius
    Carassius is a genus in the ray-finned fish family Cyprinidae. The species in this genus are commonly known as Crucian carps, though this term often specifically refers to C. carassius. The most well known is the goldfish , which was bred from the Prussian carp...
    
    
  • Hypomesus olidus
  • Leuciscus hakonensis

See also

• List of Special Places of Scenic Beauty, Special Historic Sites and Special Natural Monuments
• Ramsar sites in Japan
  Ramsar sites in Japan
  The Ramsar Convention is an international treaty for the conservation and sustainable use of wetlands. Adopted in 1971, it entered into force in 1975 and as of March 2011 has 160 Contracting Parties. Japan was the twenty-fourth party to accede, on 17 October 1980...