40 IOWA STUDIES IN NATURAL HISTORY 



fixed carbon dioxide ; but in waters which give an alkaline 

 reaction the quantity of the latter is greater than that of 

 the former. The excess of the latter is equivalent to the 

 amount of free carbon dioxide that would be required to 

 give the water a neutral reaction ; in table 21 the deficiency 

 is indicated by a minus sign in the column showing the free 

 carbon dioxide. 



Free carbon dioxide. The epilimnion is preeminently the 

 zone of photosynthesis and where the chlophyl-bearing or- 

 ganisms are abundant a rather large supply of carbon di- 

 oxide is required for this process. The demand thus be- 

 comes larger than the supply of free carbon dioxide and the 

 organisms draw upon the supply of half-bound carbon di- 

 oxide. Since this leaves an excess of normal carbonates the 

 water has an alkaline reaction and the degree of alkalinity 

 depends upon the amount of half-bound carbon dioxide that 

 has been used. It has been found that more than 80% of 

 the half-bound carbon dioxide may be thus utilized. 



As a result of the activities of the chlorophyl-bearing or- 

 ganisms the epilimnion of Okoboji lake was distinctly alka- 

 line at the time of these observations. (See table 21, p. 

 44) . This alkalinity was equivalent to 4.28 cc. of free car- 

 bon dioxide ; that is, it would have required 4.28 cc. of this 

 gas per liter to make this water neutral. The water at a 

 depth of 5 m. showed the same degree of alkalinity as that 

 at the surface; but at 10 m. it was distinctly less alkaline 

 and became neutral at 13 m. 



The water of the hypolimnion gave a distinctly acid re- 

 action; a maximum of 1.72 cc. of free carbon dioxide per 

 liter was noted at 33 m. This is the usual condition in lakes 

 of this type. Much decomposition takes place in this stra- 

 tum which furnishes a constant supply of free carbon di- 

 oxide and an additional amount is derived from the respira- 

 tion of the organisms that inhabit this region. Since the 

 water of the hypolimnion is cut off from the air during the 

 stratification period and since this gas diffuses very slowly 

 through water, there is very little loss of free carbon dioxide 

 during such periods. As a result the lower water soon 

 gives an acid reaction, even when it is distinctly alkaline 



