A LIMNOLOGICAL RECONNAISSANCE 39 



Fixed carbon dioxide. Table 21 shows that the quantity 

 of fixed carbon dioxide in West Okoboji lake amounted to 

 38.58 cc. per liter of water from the surface to a depth of 

 14 m. Below this depth the amount w^as somewhat larger, 

 ranging from 39.59 cc. at 15 m. to 41.20 cc. at 33 m, the 

 bottom. 



At the close of the vernal period of circulation the quan- 

 tity of fixed carbon dioxide is substantially the same at ail 

 depths ; but soon after the water becomes stratified in early 

 summer there is an appreciable difference between the epi- 

 limnion and the hypolimnion. There is a decrease of the 

 fixed carbon dioxide in the upper water, due in part to the 

 rise in the temperature of the water which tends to make 

 the carbonates less soluble, and in part to the fact that the 

 chlorophyllaceous organisms of this stratum draw upon the 

 half-bound carbon dioxide. This process gives the water 

 an alkaline reaction and thus tends to precipitate the car- 

 bonates, more especially the calcium carbonate. The sub- 

 merged aquatic plants growing in the shallow water, such 

 as Chara, also remove a considerable amount of calcareous 

 material from the upper water. 



The water of the hypolimnion soon comes to have a dis- 

 tinctly acid reaction, which is due to the free carbon di- 

 oxide that is liberated in the decomposition of organic mat- 

 ter in this region and in the respiration of the various 

 organisms that inhabit this stratum. Thus any carbonates 

 that are precipitated in the upper water and sink into this 

 region will be redissolved; at the bottom where the water 

 contains the largest amount of free carbon dioxide the fixed 

 carbon dioxide is largest in amount because this acid water 

 dissolves some of the calcareous material in the bottom mud. 

 As the summer period of stratification advances the differ- 

 ence in the fixed carbon dioxide content of the upper and 

 lower water becomes more marked, the maximum differ- 

 ence being found a short time before the autumnal overturn 

 takes place. 



Half-hound carbon dioxide. When the water of a lake 

 is neutral or acid in reaction to phenolphthalein the half- 

 bound carbon dioxide is regarded as equal in amount to the 



