34 



IOWA STUDIES IN NATURAL HISTORY 



lakes of the same order of size or depth are compared. In 

 order therefore to eliminate so far as possible differences 

 in temperature due to weather condition of short period and 

 to employ temperatures which show the effects of the sea- 

 son as a whole, we have been accustomed to assume a mid- 

 summer temperature of the epilimnion at the average max- 

 imum ordinarily reached. This for our lakes is 23°, so that 

 T-4=19°. This temperature may ordinarily be assumed 

 without any necessity of modifying the lower part of the 

 temperature curve to fit the change in the upper part. 



The effect of this assumption in Okoboji and Geneva 

 lakes may be seen from the following table, in which tem- 

 peratures and calories below 10 m. remain as observed, but 

 the epilimnion is given a temperature of 23°. This reduces 

 the budget of Okoboji and raises that of Geneva. 



TABLE 19— SUMMER HEAT-INCOME OF OKOBOJI AND GENE- 

 VA LAKES COMPARED WITH EPILIMNION AT 23° 



Under these conditions the heat budget of Geneva is about 

 1500 cal. larger than that of Okoboji, and most of the excess 

 is in the lower water. The budget of Geneva below 15 m. is 

 about 3740 cal. and that of Okoboji is 2000 cal. The larger 

 volume of the lower water more than makes up for the 

 lower temperatures of lake Geneva. In the 10 m.-15 m. 

 stratum the reverse is true; here Okoboji has nearly 550 

 cal. more than Geneva. T-4 is nearly twice as great, and 

 this more than compensates for the smaller value of RT. 



The table therefore shows very clearly the effect of the 

 volume of the lower water on heat gains. The temperature 



