Waku-ike is a striking salt-water lake which was formed during the landslides caused by the great Zenkôji Earthquake on May 8, 1847, in the northern part of Nagano Prefecture. The writer, having visited the lake three times in 1954 and 1956 respectively and once in 1960, investigated chemical and physical characters and plankton of that lake. The results obtained are as follows :
1) The maximum temperature and the pH values of surface water increased in summer and decreased in autumn, of both 1954 and 1956.
2) The metalimnion developed in the layers 12 meters deep in August 1960; the water temperature varied from 22.8°C in the upper layer to 10.3°C in the lower layer; the pH values ranged from 8.2 to 7.2; the dissolved oxygen of the surface layer was 154%, but the layer below the depth of metalimnion was anaerobic.
The surface water contained only a trace of hydrogen sulphide, but the water from the depth of 2m to the bottom contained as much as 13.614.8mg/l H2S, where a sulphur bacteria, Chromatium sp. abounded in large amount.
3) The plankton consisted of 24 species, of which 17 were the phytoplankters and 7 were the zooplankters. The dominant species of plankton were as follows :
Anabaenopsis Raciborskii
Nitzschia sp.
Brachionus calyciflorus var. amphiceros
4) In this lake we have seen a phenomenon called “Mizukawari”, which means the suden change of water colour from dark green to yellowish brown, with the death of a large number of freshwater fish inhabiting there. It is considered that this phenomenon of “Mizukawari”is caused by some sudden changer of circumstances in autumn circulation period.
