A LIMNOLOGICAL RECONNAISSANCE 41 



at the time of stratification, and the free carbon dioxide 

 shows an appreciable increase in quantity as the season 

 advances; the largest amount is found at the bottom just 

 before the overturn takes place. 



DISSOLVED OXYGEN 



During the vernal and autumnal periods of circulation 

 this gas is also substantially uniform in its distribution 

 from surface to bottom ; the largest amount is found at the 

 close of the latter period because oxygen is more soluble in 

 cold than in warm water. With the rise in the temperature 

 of the water in the spring there is a corresponding decrease 

 in the amount of oxygen but the water remains nearly or 

 quite saturated with this gas as long as the vernal cirula- 

 tion continues. When stratification becomes established in 

 early summer, however, it is soon followed by appreciable 

 differences in the quantity of oxj^gen in the different strata. 



The temperature of the epilimnion continues to rise until 

 the summer maximum is reached which means a decreased 

 capacity for dissolved oxygen. Decomposition and respira- 

 tion also tend to decrease the supply of this gas but the 

 water of this stratum is kept in circulation by the wind and 

 thus freely exposed to the air which tends to keep the supply 

 near the saturation point. Another source of supply is the 

 oxygen liberated in this stratum during the process of pho- 

 tosynthesis. In general, then, the quantity of oxygen in 

 the epilimnion does not fall very far below the saturation 

 point during the summer period of stratification and, under 

 favorable conditions, the stratum may even become super- 

 saturated. 



On July 31, 1919, the dissolved oxygen in the epilimnion 

 of West Okoboji lake amounted to 5.80 cc. per liter at the 

 surface and 5.40 cc. at a depth of 10 m. While these quan- 

 tities represent an abundant supply of this gas, neverthe- 

 less they are both well below the point of saturation, the 

 former being 95.5% and the latter only 86.6% of the amount 

 required for saturation. This indicates that the water of 

 the epilimnion was being depleted of its supply of oxygen 

 through decomposition and respiration faster than it was 



