20 ILLINOIS BIOLOGICAL MONOGRAPHS [20 



and chemical, and are the direct results of atmospheric, or at least non- 

 aquatic conditions. The prime cause of these changes is the warming of the 

 water due to the activity of the sun's rays during the spring and summer, 

 and the subsequent cooling again as the sun drops to the north. During 

 the winter, when the lake is covered with ice, the aquatic environment is 

 in a state approaching stable equilibrium and a constant temperature, 

 equal oxygen and carbon dioxide distribution is found from top to bottom. 

 Under these conditions there is a complete circulation of the water and, 

 because of the uniform distribution of the gases, the entire lake is acces- 

 sible — at least insofar as the gas content of the water is concerned — to 

 fish life. This is particularly important for the deep water species as 

 Leuciscus artedi, for example, as will be discussed in the section dealing with 

 the cisco. Thus the lake starts out in the early spring with a uniform 

 temperature normally below 4°C, and a uniform gas distribution. With 

 the warming of the surface water, the complete circulation of the water is 

 checked and the lake becomes divided into two regions, separated from 

 each other by a sharp break in temperature. This break is the thermocline, 

 the warmer region above it is known as the epilimnion, the cooler region 

 below it as the hypolimnion. The position of the thermocline is at first 

 in deep water, but with the continued warming of the water the thermocline 

 crawls slowly upward until in mid-summer the hypolimnion greatly 

 exceeds the epilimnion in depth. These temperature changes which check 

 the free circulation of the water result in marked changes in the gas 

 content of the water, as the oxygen content cannot be restored, nor the 

 carbon dioxide distributed. The result is that the hypolimnion becomes 

 an area deplete in oxygen and replete in carbon dioxide, forming a region 

 into which fish may venture only for a short period of time. Thus, through 

 the formation of the thermocline, a large portion of the lake becomes 

 unsuited for fish life, and a consequent movement of the deep water fish 

 to shallower waters results. This seasonal cycle has been well worked out 

 for many Wisconsin lakes by Birge and Juday, including many of the 

 Waukesha county lakes. 



The importance of these seasonal changes in oxygen, carbon dioxide 

 content and temperature variations of the water will be fully discussed 

 in their relation to the distribution of fish life in the section dealing with 

 the life history of the cisco (Leuciscus artedi). Suffice it here to say that the 

 behavior of the thermocline offers barrier conditions which force the deep 

 water species of fish to leave the deep places and seek more favorable 

 conditions of gas content of the water. The serious results which accrue 

 when the thermocline forces these deep, cold water species up into the zone 

 of warm surface water to which they are not tolerant, will also be discussed 

 in that section. 



