LIMNOLOGICAL STUDIES OF KARLUK LAKE 
413 
of August 15, 1926, the highest. The curve for 1926 shows that the epilimnion and 
the thermocline were more sharply defined in that year than in the other years. At a 
depth of 30 meters the highest temperature (7.1° C.) was noted on July 12, 1930, and 
the lowest (5.9° C.) on July 9, 1929. The two curves for 1929 show the temperature 
changes which took place in the upper 30 meters of water between July 9 and Sep- 
tember 8 of that year. Between these two dates there was a decrease in the temper- 
ature of the upper 5 meters; but there was a marked rise in temperature between 5 
and 25 meters, with a slight increase between the latter depth and 30 meters. The 
bottom temperatures in the deepest water of Karluk Lake ranged from a minimum 
of 4.1° C. (39.4° F.) to a maximum of 4.9° C. (40.8° F.). (Table 4.) 
Table 4. — Summer temperatures of Karluk Lake at station 1 
[The readings are indicated in degrees centigrade at the different depths] 
No winter temperatures have been taken in Karluk Lake, so that the winter 
mini mum is not known definitely. Some observations made in the spring of 1931, 
however, serve as a basis for estimating the minimum. The covering of ice disap- 
peared from Karluk Lake on April 20, 1931, while the ice disappeared from the lakes 
of northeastern Wisconsin on April 13 to 15, 1931, or only about a week earlier 
than on Karluk. On May 2, 1931, the temperature of the surface water of Karluk 
Lake was 2.4° C. (36.3° F.) and that at 126 meters was 2.6° C. These results are 
similar to those obtained on Wisconsin lakes soon after the disappearance of the 
ice, and it seems probable that the temperature cycle in Karluk Lake is substantially 
the same in winter as that of the Wisconsin lakes where the mean temperature at 
the time of freezing is about 1° C. Thus a minimum temperature of 1° C. may be 
used for the computation of the heat budget of Karluk Lake. The summer heat 
income represents the gain above a temperature of 4° C. 
Table 5 shows the annual heat budget, the summer heat income, and the gain 
in heat which took place between the two sets of summer temperature readings in 
three of the years in which observations were made. The heat budgets represented 
in the table ranged from 28,200 gram-calories per square centimeter of surface on 
July 19, 1927, to 35,800 gram-calories on August 15, 1926. At the time of the July 
observations in 1927 to 1929, inclusive, the heat budgets amounted to 28,000 to 
29,000 gram-calories per square centimeter and a further gain of 2,400 to 5,800 calories 
was made between the July readings and those of August or September. The heat 
budgets-indicated for September 3, 1928, and September 8, 1929, probably represent 
the maximum budgets for those years as it does not seem probable that the water 
gained any heat after these dates. The budgets for August 15, 1926, and August 
13, 1927, may not represent the maximum for those years, but the gain in heat after 
the middle of August is probably small in that latitude, except perhaps in years when 
it is unusually warm in the latter half of August and in early September. 
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