The surrounding area of Mt. Okina-san or Okina Pass in the Oou Mountains is characterized by abundance of smal lakelets formed mainly by landslide. The authors are now conducting the field survey of these lakes from the view points of regional limnology. Lake Nabe-numa is situated at the southern foot of Mt. Okina-san. In this paper, the new findings from the study of the lake are discussed with special reference to the geochemistry of the lake water.
1. The morphometrical data of the lake are summarized as:
surface area: 0.57ha
maximum depth: 7.60m
volume: 1.80×10⁴m³
shore development: 1.17
The bathymetric map reveals that this is formed by landslide.
2. The main solute in the lake water is sodium bicarbonate in the period from summer to winter. Exceptionaly, during the season of rapid thaws, it becomes sodium chloride which derived from the airbone salt in snow deposit transported by monsoon.
3. This lake is poor in dissolved materials, especially in nutrients such as phosphate and nitrogen compounds. While supplied with the drainage basin composed of tuffeons rocks, the amount of silicate in solution is estimated about 20mg/l as SiO₂.
4. In midsummer, the thermal as well as chemical stratification developes distinctly in the lake.
5. Kjeldahl nitrogen and ignition loss of the bottom sediment of the lake are in the lowest level among the lakes in this district.
6. In conclusion, this lake belongs to an oligotrophic lake.