Thermal Stratification and Tajss of O.xf/^en 33 



Variations in Thermal Stratification 



Bardach - describes die progress of suPiiiner stratification in Lake West 

 Okoboji, Iowa. In diis lake, tliermal stratification normally begins between 

 May 15 and June 1 after die water below 30 meters has already reached 

 10°C (50°F. ) Then, during the summer the hypolimnion warms up 

 further, to 12° or 13°C (53.6° or 55.4°F). 



However, in 1925, 1926, and 1950, when unusually heavy winds were 

 recorded in late spring, West Okoboji did not form a diermocline until 

 very late in the season or not at all, and, if it did form one, it was situated 

 at a greater depth than usual. In some years this abnormal sti*atification 

 consisted of an upper warm layer, below which the temperature gradually 

 dropped as one progressed toward the lake bottom, until at 22 meters 

 (72.2 ft.) the temperature was 6 degrees centigrade lower than in the 

 epilimnion [ep)ilimnion, 20.6°C (69.1°F), and bottom, 14.3°C (57.7°F)]. 

 The vertical change in temperature was less than 1°C per meter so that 

 bv definition no thermocline was present. Nonetheless, there was no 

 evidence that thermal stratification was ever completely broken up during 

 the summer months. As mentioned previously, a similar type of stratifica- 

 tion appears to be characteristic of numerous small artificial impound- 

 ments. 



Fall Overturn 



Summer thermal stratification is broken up in the fall by wind action 

 after the epilimnion cools to a temperature approximately that of the 

 hypolimnion. Gradually, the entire lake begins to circulate, as the winds 

 create water currents across the surface and compensating currents 

 develop across the lake bottom. Water that has remained trapped in the 

 lake depths all summer again comes in contact with the surface layers 

 where free and dissolved carbon dioxide has an opportunity to escape 

 and the dissolved oxygen supply is replenished. 



As fall progresses into winter, the lake water cools to 4°C (39.2°F) 

 and below, and the colder upper layer becomes less dense: In the north 

 a film of ice seals the surface and the lake is again thermally stratified, 

 with the ice and colder water above the mass of water at 4°C. As long as 

 ice covers the lake, very little circulation takes place. Some convection 

 currents may be set up through the mild warming of water in contact 

 with the bottom, but these warming forces are counteracted by colder 

 water immediately under the ice. Once the lake is sealed from the air, 

 the oxygen supply under the ice is dependent upon the photosynthetic 

 activity of algae which in turn is supported by light transmission through 

 the ice. 



