LAKE DYNAMICS 21 



lishment of stratification, resulting in a thermoclinc and in making 

 only certain parts of lake bottoms available to animals. As the water 

 warms during the summer, the warm water, being lighter than the 

 cold, rises to the surface. By the time summer is well established, a 

 layer of warm water, the epilimnion, usually from 30 to 45 feet thick, 

 lies over the deeper and heavier cold water, the hypolimnion. The 

 intervening area is called the thermochne. In very deep lakes the cold 

 water may have a summer temperature as low as 39° to 40° F. (4° C.) . 



The cold water is so heavy that it remains at the bottom, and all 

 the heat from the sun is absorbed by the warm upper layer before 

 any of it can reach the cold layers beneath. This stratification becomes 

 so pronounced that after awhile these two layers do not mix. Even at 

 times when the wind, blowing heavily in one direction, piles the warm 

 layer on one side and tilts the bottom layer, the currents in the upper 

 layer may slide over the lower layer without mixing. During the spring 

 and fall, when the lake is either warming or cooling, there comes a 

 time when the temperature of the upper layer is the same as that of 

 the lower layer, and then the thermocline disappears. At this period 

 there is complete circulation or mixing from top to bottom in the 

 deepest parts. 



The temperature stratification just described is found only in lakes 

 of 25 or more feet in depth and not more than several miles in diameter. 

 In larger lakes, like Mille Lacs and Red Lake, the action of the wind 

 and waves creates powerful undertows and keeps the water well mixed 

 at all times, preventing stratification. The size of the body of water 

 with the consequent action of wind and waves probably also accounts 

 for the fact that no definite stratification occurs in Lake Superior 

 (Eddy, 1944) . 



The effect of stratification in fertile lakes is stagnation in the lower 

 layers. The oxygen is soon depleted by any life present and by the 

 decomposition of the bottom muds and organic materials continually 

 setthng down from above. Experiments have been made by lowering 

 different species of fishes in wire baskets to different depths in various 

 lakes. In summer the fishes died in a few minutes when lowered to 

 depths below 30 feet, while those closer to the surface always lived. 

 In the fall and spring, when there was no stratification, the fishes lived 

 in baskets at all depths, thus indicating that it was the lack of oxygen 

 and not the pressure that killed these fishes. 



The deep bottoms may be rich in fertility, but because of the lack 

 of oxygen, only a few worms and other low forms can live there. How 

 these forms live without oxygen is one of the interesting biological 

 problems. Because of the lack of sunhght at this depth, plants cannot 

 grow and form oxygen. Lower LaSalle Lake in Hubbard County is an 

 example. It is one of the deepest lakes in Minnesota, being 218 feet 



