50 Interrelationships of Fishes and Lake Habitats 



the source of additional under-ice oxygen is gone, and in a relatively short 

 time the current supply of oxygen may be completely used up by the respi- 

 ration of living plants and animals and the demands of organic decay.^' ^^ 



In 1945, Greenback ^^ published results of a study of the physical, 

 chemical, and biological conditions in ice-covered lakes (Michigan). He 

 measured dissolved oxygen, pH, carbon dioxide, alkalinity, biochemical 

 oxygen demand, and light penetration in these ice-covered lakes, to de- 

 termine what factors or combinations of factors were responsible for the 

 death of fishes, and to develop more efiFective methods of preventing 

 winter fish kills. The amount of dissolved oxygen appeared to be the most 

 important single factor in relation to death or survival. This oxygen con- 

 centration might change gradually or rather suddenly, depending upon 

 other conditions associated with the body of water in question. For ex- 

 ample, at Green Lake ( Michigan ) Station 5, oxygen at the surface ( under 

 ice) changed from 1.8 ppm on February 5, 1943, to 9.8 ppm on February 

 8, an increase of 8.0 ppm in 3 days, or at the rate of 2.7 ppm per day. 

 The most abrupt decline was noted in Pasinski's Pond (Michigan) Station 

 27, where the oxygen fell from 12.3 ppm on February 12, 1940, to 2.4 ppm 

 on February 14, at a rate of 5 ppm per day. A delicate balance often exists 

 between the processes which produce oxygen (photosynthesis of plankton 

 algae) and those that use it up.^^ As light is essential to photosynthesis, 

 its transmission through the ice and snow covering a lake or pond is 

 extremely important (Figure 3.1). Measurements of light penetration 

 show that about 85 per cent will pass through 7.5 inches of clear ice, and 

 as much as 11.5 per cent, through 15 inches of ice that is cloudy on top. 

 However, 1 inch of crusted snow limited the light penetration through the 

 snow only to between 10 and 17 per cent of the light that fell on the 

 snow's surface, and 5 inches of dry snow allowed the transmission of 

 only 2.5 per cent of the available light.^^ Clean fresh snow allowed the 

 greatest light penetration, clean wet snow the next greatest, and granular 

 snow the least. 



While the rate of photosynthesis is dependent on many factors, it is 

 conceivable that there is a range of light intensity sufficient to stimulate 

 a level of photosynthetic activity during which the oxygen output will 

 exactly equal the oxygen demands of the aquatic environment. This is a 

 dangerous condition because it may depreciate rapidly into a situation 

 of oxygen shortage. There is reason to assume that the amount of light that 

 penetrates 1.5 to 2 feet of moderately clear ice (without snow) is enough 

 to satisfy the requirements for photosynthesis.^^ Further, the evidence is 

 conclusive that a heavy snow cover on ice so greatly reduces the amount 

 of light entering the water, regardless of the clarity of the ice, that 

 photosynthesis of phytoplankton is completely stopped. 



