A LIMNOLOGICAL RECONNAISSANCE 33 



TABLE 18— TEMPERATURE AND SUMMER HEAT-INCOME OF 

 LAKE GENEVA, WIS., AUGUST 30, 1919 



Depth Temp. CaL 



RT 



~"F5 2L3 7802 



5-10 20.1 5957 



10-15 11.9 2568 



15-20 9.4 1512 



20-25 8.7 1058 



25-30 8.0 680 



30-35 7.4 360 



35-40 7.1 121 



40-43 7.0 12 



14.18 20070 



If the heat budgets of lakes are to be fairly compared 

 there must be some common adjustment of the temperature 

 of the epilimnion. This is rapidly affected by the tempera- 

 ture of the air and the relative volume of the epilimnion is 

 so great that its temperature has a large influence in de- 

 termining the heat budget. In Okoboji lake in 1919 more 

 than 75% of the summer heat-income was in the epilimnion 

 and in lake Geneva nearly 70% was in the same stratum. 

 If therefore a lake is visited at the close of a hot period the 

 temperature of its epilimnion will be found to be high, and 

 both the mean temperature and the summer heat-income 

 will be correspondingly large ; while a week later after sev- 

 ei'al days of cool weather the epilimnion may have lost con- 

 siderable heat. This is the case in the two lakes under con- 

 sideration. The surface of lake Okoboji was at 24.7°, while 

 that of Geneva was only 21.4°, and the difference in the 

 m.-5 m. stratum amounted to nearly 1400 cal. in favor of 

 Okoboji, due wholly to a difference in temperature which 

 v\'as itself largely due to the different dates on which the 

 lakes were visited. On the other hand, the temperature of 

 thermocline and hj^Dolimnion in such lakes as these ordi- 

 narily changes very little between the last week of July and 

 the first of September. 



Our observations on Wisconsin lakes show that under 

 similar conditions there is but little difference in the mid- 

 summer temperature of the epilimnia of lakes in the same 

 general region, whatever the size of the lake or the thick- 

 ness of the stratum. Differences are especially small when 



