長野県北安曇郡鎌池の現況

Abstract

Kama-ike is a small but deep mountain lake in sickle shape that lies at an elevation of 1, 190 m in the northernmost part of Nagano Prefecture in Central Japan. It lies at 36°51′N. Lat. and 137°58′E. No detailed survey has been made since Dr. A.TANAKA sounded this lake in 1919 for the first time. The writer's investigations were carried on in this lake in August 1972, in regard to re-soundings as well as to some limnological features. The following morphometrical features were obtained by the 139 soundings in total : the superficies of the lake 27, 100m2, excluding 175m2 of the island areas ; the greatest depth 16.2m and the mean depth 6.87m, and the shore development 1.71. Its volume is calculated as 1, 950, 000m3. The results indicate that this lake has become shallowed by 1.8m during the fifty-one years since TANAKA's first soundings. This means that the decrease of depth has progressed at the rate of 1.5cm a year. The thermocline was seen at the layer of 2-6m deep, at which zone the dissolved oxygen was supersaturated in summer. In the hypolimnion the amount of dissolved oxygen decreased greatly as indicating the progress of eutrophication to a certain extent. The pH values were 7.4 at the lake surface, then gradually lowering as 6.8-6.9 and became reversed to 7.0 in the bottom water.