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Akan-Mashū NP, Hokkaido
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Few phytoplankton live in Mashu’s nutrient-poor water, so they absorb less light and help the lake look intensely blue. Their summer increase can lower clarity and shift the color toward green.
Fish stocking changed Mashu’s food web: rainbow trout were introduced in 1926, then kokanee salmon in 1968–74. Researchers linked the later shift toward tiny plankton to declining clarity—and a less vivid blue.
Scientists measure clarity by lowering a 30-cm white disk until it vanishes. In 1931, it remained visible to 41.6 m—Mashu’s recorded high, a transparency mark that remains unbeaten.
At Lake Mashu’s Observatory 1 rest house, the lake’s signature color becomes a snack: visitors can order “Mashu Blue” soft-serve, a local edible tribute to the vivid view outside.
Three typhoons struck Hokkaido in August 2016, raising Mashu’s water level nearly 1 m and burying a research boat in mud. By September, visibility fell to 6.3 m, the lowest in the monitoring record then.
Mashu’s catchment has no human pollution sources, so contaminants detected in the lake can flag substances carried from far away in rain. Five neighboring towns and agencies help monitor its water annually.
To study the blue, scientists carry instruments down Mashu’s steep caldera wall, then shoulder collected water back uphill. The lake is a protected special zone; visitors cannot simply descend to its shore.
