Glossary
Transparency
Transparency is a measure of how clear a lake's water is. A white disk is lowered into the water, and the depth at which it disappears is recorded. The larger the value, the clearer the water.
How it is measured
The national lake survey defines transparency as follows.
A white disk 25–30 cm in diameter is lowered into the water; transparency is the average of the depth at which it disappears and the depth at which it reappears as it is raised again.
The report treats transparency as an easy way to compare how polluted lakes are. Transparency falls as a lake becomes more eutrophic, and also changes with the seasons as the amount of plankton changes. According to the report, clean lakes in Japan measure 5–10 m or more, and polluted lakes about 1–4 m.
Even in the same lake, the value changes with the weather, season and water conditions on the day it is measured. The transparency figures in this catalog are from the 1991 survey.
Japan's clearest lakes
In the 1991 survey, 13 lakes nationwide had a transparency of 10 m or more, while most were 5 m or less. The top three were Hokkaido's Lake Mashu (28.0 m) and Lake Kuttara (22.0 m), and Akanuma in Aomori (18.2 m). By origin, caldera lakes were the clearest, averaging 10 m.
Hokkaido's clearest lakes
6 lakes in this catalog have a transparency of 10 m or more.
- Lake Mashu 28 m (caldera lake, oligotrophic)
- Lake Kuttara 22 m (caldera lake, oligotrophic)
- Lake Shikotsu 17.5 m (caldera lake, oligotrophic)
- Lake Panketo 14 m (dammed lake, oligotrophic)
- Lake Penketo 11.3 m (dammed lake, oligotrophic)