Glossary
Lake types (nutrient level)
Lake type classifies lakes by how many nutrients their water contains and how much plankton and other life grows in them. According to the report, a lake's transparency falls as it becomes more eutrophic.
Two groups
Lake types are first divided into two groups.
- Harmonic: lakes where the makeup and growth of living things are not skewed toward particular species and are governed mainly by the amount of nutrients. Eutrophic, mesotrophic and oligotrophic lakes belong here.
- Disharmonic: lakes where an unusually large amount of a substance in the water (acid, iron or humic substances) governs biological production. Acidotrophic, siderotrophic and dystrophic lakes belong here.
Six categories
| Category | Definition (national lake survey) |
|---|---|
| Eutrophic | A lake rich in nutrients, with active biological production |
| Mesotrophic | A lake between eutrophic and oligotrophic |
| Oligotrophic | A lake poor in nutrients, with low production |
| Acidotrophic | A lake whose water has a high hydrogen-ion concentration and is acidic |
| Siderotrophic | A lake whose water contains a large amount of iron |
| Dystrophic | A lake whose water contains humic substances and is yellow-brown and slightly acidic |
Definitions are translated from definition 3 in the report.
Which lakes become which type
The report summarizes the relationship between origin and lake type as follows.
- Caldera and volcanic lakes: about half are oligotrophic, and a certain number are acidotrophic. This is thought to be because they lie at high elevations where few nutrients flow in, and because of the geology and soils created by volcanoes.
- Lagoon-origin lakes: half are eutrophic, and about 20% are dystrophic. They lie in lowlands that are easily affected by human activity, and those formed at river mouths collect nutrients from their whole catchment.
- Dystrophic lakes: found in lowland wetlands by the sea and in mountain valleys, where humic substances flow in from peat bogs in which plants have accumulated without fully decomposing at low temperatures.
Hokkaido has many dystrophic lakes
In the 1991 survey, 37 of Japan's 63 dystrophic lakes were in Hokkaido, reflecting the island's many wetlands. Oligotrophic lakes are also the most numerous type in Hokkaido, including caldera lakes such as Lake Shikotsu and Lake Mashu. The only siderotrophic lake in Japan is in Fukushima Prefecture; there are none in Hokkaido.
| Lake type | Japan (478 lakes) | Hokkaido (1991 survey) | This catalog |
|---|---|---|---|
| Eutrophic | 158 (33.1%) | 35 | 35 |
| Oligotrophic | 124 (25.9%) | 43 | 43 |
| Mesotrophic | 105 (22.0%) | 4 | 4 |
| Dystrophic | 63 (13.2%) | 37 | 35 |
| Acidotrophic | 20 (4.2%) | 3 | 4 |
| Siderotrophic | 1 (0.2%) | 0 | 0 |
| Unknown | 7 (1.5%) | 5 | 6 |
National and Hokkaido figures are from tables III-1-3 and III-1-4 of the report. The counts for this catalog differ slightly because apparent errors in the report have been corrected.
Find Hokkaido lakes
Sources
- Environment Agency of Japan, 4th National Survey on the Natural Environment: Lake Survey Report (national edition), March 1993 (Japanese) pp. 16 (definition 3, table III-1-3), 18 (table III-1-4), 19, 34 (definition 4)