RESPIRATION AND OXYGEN. 47 



from 3.36 to 6.84 c.c. In the soil-water of rapidly growing pine 

 forest the range was from 0.88 to 3.30 c.c, the average being approxi- 

 mately 2 c.c. in contrast to 8.2 c.c. at saturation. In pine forest 

 with poor growth, the range was from to 0.88 and the average 0.13 

 c.c, half of the situations showing no oxygen whatever. 



Birge and Juday (1911) have thoroughly investigated the gases 

 in Wisconsin lakes. They have found that the amount of oxygen 

 decreases during the winter in Lake Mendota, the decrease being 

 sHght from 1 to 15 meters, but often falhng to zero in the bot- 

 tom water before the ice breaks up in the spring. The layer 

 beneath the ice may reach 130 per cent of saturation on clear days 

 when the algae receive enough sunhght for photosynthesis. The per 

 cent of saturation due to the evolution of oxygen by algae was great- 

 est at 4 to 5 meters, where a maximum of 364 per cent was reached. 

 The spring overturn occurs about the time of the disappearance of 

 the ice and results in the equal distribution of the gases throughout 

 the depth of the lake, the oxygen being 8 cc per liter. The process 

 of decay reduces the amount of oxygen in the lower water, so that it 

 has entirely disappeared at 18 to 22 meters before the middle of 

 July. During August it may be entirely absent below 10 meters, 

 and is present in small quantities until the fall overturn in October. 

 During the four months, June to September, the amount in the 

 surface 5 feet varied chiefly between 5 and 8 c.c. per Hter, with fre- 

 quent periods of supersaturation for the surface. The amount be- 

 fore the autumnal overturn in October was about 5 c.c. to a depth of 

 15 meters, while after the overturn about the same amount occurred 

 to 22 meters. From this time the oxygen in the upper water gradu- 

 ally decreased, due to the falHng-off of photosynthesis and the rapid- 

 ity of decay. The amount tends to rise again in November and to 

 approach saturation in December. 



The demands of the algae of Lake Mendota for carbon dioxid are 

 greater than the supply of this gas in the free state. The excess 

 demand is met by the half-bound carbon dioxid, and this results in 

 changing the water from acid to alkahne. The free carbon dioxid is 

 uniform from top to bottom as a result of the autumnal circulation. 

 The amount of carbon dioxid, and hence the acidity, increases dur- 

 ing the winter, reaching a maximum of 5 to 9 c.c. near the bottom 

 in March. The spring overturn brings the entire body of water near 

 to the neutral point, after which it becomes increasingly alkaline 

 until the latter part of May, when the water below 18 meters devel- 

 oped free carbon dioxid and became acid. From June to October, 

 the water was alkahne above 8 feet and acid below 15 feet, the 10 

 and 12 foot depths being acid in midsummer. The overturn in 

 October again made the water uniformly alkaline in reaction. 



Chambers (1912) found the oxygen-content of a lagoon to vary 

 from 3.2 c.c. per hter, June 23, to the saturation-point (7.8 cc.) on 



