22 NORTHERN FISHES 



deep, and is rich in nutritive elements, but the bottom oxygen may be 

 nearly exhausted during the summer months. 



In some of the very deep lakes that are low in natural fertility, such 

 as Clearwater, Tuscarora, and Saganaga lakes in Cook County, Minne- 

 sota, the volume of water in the lower layer may be enormous. When 

 the lake stratifies in the spring, the lower layer may contain more 

 oxygen than will be used up during the summer by the decomposition of 

 organic wastes. In addition, since these lakes are so infertile that rela- 

 tively few plants and animals are produced in the upper layer and in 

 the shore waters, there is only a small amount of organic material to 

 decay and use up the oxygen. Furthermore the water is very clear, so 

 that sunlight penetrates farther than in the more turbid, fertile lakes, 

 thus enabling oxygen-producing plants to grow at much greater depths. 

 Consequently the deep, cold bottoms in these lakes have sufficient 

 oxygen to support trout and other cold-water fishes. 



