42 IOWA STUDIES IN NATURAL HISTORY 



receiving this gas from the air and from the photosynthetic 

 activities of the chlorophyl-bearing organisms. 



The hypolimnion remains practically uniform in tempera- 

 ture after the formation of this stratum so that its capacity 

 for oxygen is scarcely affected during the summer. On the 

 other hand, however, it is cut off from contact with the air 

 by the epilimnion and conditions are not favorable for 

 photosynthesis at that depth ; it is thus deprived of the two 

 main sources from which a supply of oxygen can be obtained 

 and any decrease during the summer remains as a deficiency 

 until the autumnal overturn takes place. Decomposition 

 and the respiration of the organisms which occupy this 

 stratum contribute to the decrease of the oxygen supply and 

 the extent of this decrease depends upon the volume of the 

 hypolimnion and upon the abundance of decomposable ma- 

 terial and living organisms ; if the volume is relatively small 

 and these two processes are fairly vigorous there will be 

 a marked decrease in the supply of oxygen as the summer 

 advances and a portion, or even practically all, of this 

 stratum may be deprived of its dissolved oxygen. 



It will be noted in table 21 that the hypolimnion of Oko- 

 boji lake possessed a relatively small amount of dissolved 

 oxygen at the time of these observations ; the amount varied 

 from 2.23 cc. per liter at 15 m. to 0.86 cc. at a depth of 33 m. 

 At 18 m., however, the amount was a little more than half 

 a cubic centimeter larger than at 15 m. Such differences 

 have been noted in other lakes but the cause of the phe- 

 nomenon has not yet been definitely ascertained. Stated in 

 percentages the quantity of oxygen varied from 36.5 % of 

 saturation at 18 m. to only 10.9% at 33 m. With such a 

 marked decline in the dissolved oxygen at this stage of the 

 summer stratification period it is safe to predict that this 

 gas all disappeared from the lower portion of the hypolim- 

 nion, at least, before the time of the autumnal overturn. 



In the mesolimnion, or thermocline, the quantity of oxy- 

 gen declined from that of the epilimnion above to that of 

 the hypolimnion below; that is, from 5.40 cc. per liter of 

 water at 10 m. to 2.40 cc. at 13 m., while there was a fur- 



