HYPOLIMNETIC HEATING IN CASTLE LAKE, CALIFORNIA’

A S-year series of summer temperature profiles was used to calculate the coefficient of eddy conductivity in a small (20.1 ha) mountain lake. The relatively high value obtained (6.22 ~10~ ’ g cm-l set-I) prompted us to evaluate the role of direct solar heating in the hypolimnion using data on total daily solar radiation, the spectral curve of sunlight, and the light transmission properties of the lake water. It was found that an estimated 65-85s of the heating in the upper hypolimnion could be accounted for by direct solar heating. It is postulated that in Castle Lake direct solar heating rather than turbulent mixing is responsible for the shape of the hypolimnetic thermal profile. LNTRODUCTION Limnologists have often noted that the vertical temperature profiles of lakes show a regular series of changes during the summer heating period. As time progresses, the epilimnion becomes thicker, the gradient in the metalimnion becomes steeper, and tem-peratures increase at all depths. This is evi-dent in profiles from Castle Lake, California, taken on three dates in the early summer of 1963 ( Fig. 1). Th e original impetus for this study was a desire to use these temperature changes to estimate the vertical coefficient of eddy conductivity following the proce-dures successfully applied to a number of lakes by Hutchinson ( 1941, 1957). In the process, some additional information was obtained on the manner in which the thermal profile may be formed in Castle Lake and in other similar bodies of water. Castle Lake is located in northern Cali-fornia (Siskiyou County, T.39N, R.5W, S.13). Its basin is formed in a glacial cirque in the Klamath Mountains at an altitude of 1,706 m. It has a surface area of 20.1 ha and a maximum depth of 35 m. The lake surface is protected from strong wind action by the cirque walls and trees that surround th