38 IOWA STUDIES IN NATURAL HISTORY 



chemical diflferences which become more marked as the 

 period of stratification advances. The differences which 

 v/ere found about the middle of the summer period of strati- 

 fication in West Okoboji lake are shown in tables 7 and 21 

 (pp. 10 and 44). 



Samples of water from the different depths of the lake 

 were obtained by means of a water bottle and the carbon 

 dioxide and dissolved oxygen were determined for the dif- 

 ferent strata. The former was determined by the Seyler 

 method, using phenolphthalein and methyl orange as indi- 

 cators for the titrations with standard acid and alkali. The 

 Winkler method was used for the determination of the dis- 

 solved oxygen. (For descriptions of these methods see 

 Birge and Juday, '11, pp. 13-23). 



CARBON DIOXIDE 



Carbon dioxide is usually present in lake waters in three 

 different states; that is, as fixed, half-bound, and free car- 

 bon dioxide. The quantity of fixed and half-bound is de- 

 pendent upon the amount of the substances present with 

 which this gas is usually combined, chiefly calcium and mag- 

 nesium. The fixed carbon dioxide is that portion which is 

 held in a close chemical union, most generally with calcium 

 and magnesium, and forms the carbonates of these sub- 

 stances. The half-bound is held in a rather loose combina- 

 tion by the carbonates, converting them into bicarbonates. 



In addition to these a lake water may hold in solution a 

 certain amount of carbon dioxide in an uncombined state 

 and this constitutes the free carbon dioxide. From the bio- 

 logical standpoint the half-bound and free carbon dioxide 

 are the most important since they constitute the sources 

 from which the aquatic chlorophyllaceous organisms obtain 

 their carbon dioxide for photosynthesis. No portion of the 

 fixed carbon dioxide is directly available for photosynthesis, 

 but indirectly the quantity of it is a very important factor 

 because the amount of half-bound carbon dioxide is depend- 

 ent upon the quantity of carbonates present in the water. 

 In a water that gives a neutral or acid reaction with phe- 

 nolphthalein it is assumed that the quantity of half-bound 

 carbon dioxide is equal to that of the fixed. 



