36 IOWA STUDIES IN NATURAL HISTORY 



larger range of variations than has already been shown. 

 We may therefore infer that the summer heat-income may 

 range from about 20,000 cal. to 29,000 cal., or that with the 

 epilimnion at the standard temperature of 23° the range 

 will be from 21,000 cal. to 28,000 cal. Another 1000 cal. 

 may be added to the maximum if we impose the warmest 

 observed epilimnion on the warmest observed temperature 

 for the lower water. 



In the case of Okoboji the limits are not thus known. 

 Temperatures of 1919 are probably close to the minimum 

 for the lower water, and therefore no heat-income is likely 

 to be lower than 19,000 cal. to 20,000 cal, with the epilim- 

 nion at 23°. But the volume of the lower water is so small 

 that its temperature has very little influence on the heat- 

 income ; and the same fact also brings it about that but little 

 work is needed to raise the temperature of the lower water 

 much above the minimum. In 1916 weather conditions 

 were such as to favor a very high temperature. Yet the 

 total heat-income (Table 20) was only 2000 cal. above that 

 of 1919 at observed temperatures. A temperature much 

 above that of 26.5° for the upper five meters is hardly to 

 be expected ; a mean gain of one degree at all depths of the 

 water below 10 m. means only 460 cal. total gains, and one 

 degree in the hypolimnion below 15 m. means only about 

 240 cal. Under such conditions we can hardly expect the 

 maximum heat-income to rise much above 24,000 cal.-25,000 

 cal. The latter figures would involve a mean temperature 

 of the hypolimnion close to 20°, or more than 4° above that 

 of 1916. 



Even such extreme temperatures do not seem excluded 

 from possibility — at least not by the amount of work in- 

 volved in the distribution of heat at such high tempera- 

 tures. The distribution of heat in Okoboji in 1916 when 

 the bottom water reached 15° required 1981 g. cm. ; the dis- 

 tribution of heat in Geneva in 1899 demanded 2752 g. cm. 

 or nearly 800 g. cm. more than that of Okoboji. It would 

 thus seem possible for the wind to do more work on Oko- 

 boji and under favorable circumstances raise the tempera- 

 ture of the lower water above 15°. If we assume that the 



