32 Artificial Aquatic Habitats 



weight) both above and below this temperature. Let us consider the fact 

 that soon after the ice melts in the spring, the temperature of the water 

 in a lake rises to 4°C, bringing the entire lake to a uniform density. Then, 

 winds blowing across the lake surface pile the water up on the windward 

 side, and in order to compensate for this, water passes downward across 

 the bottom of the lake to the upwind side. The entire lake begins to 

 circulate from top to bottom. As spring advances, however, there are 

 days when the wind blows lightly or not at all, and the sun beating down 

 on the lake begins warming the water on the surface, causing it to become 

 less dense ( lighter ) than the colder water below. After the surface water 

 has warmed a few degrees above the water in the lake depths, thermal 

 stratification has begun and no ordinary wind will cause the two to mix. 



The surface water will mix with itself down to a depth of several feet 

 or yards (meters), the depth depending upon the wind velocity and the 

 area of lake surface acted upon. Thus the warm surface layer (epi- 

 limnion) tends to be thicker on large lakes than on small ones. The 

 temperature of the epilimnion is about the same from top to bottom, but 

 this may vary a few degrees during days when the surface is warming 

 rapidly and winds are light. 



Below the epilimnion is a layer of water (the thermocline or meta- 

 limnion) where the temperature of the stratum decreases rapidly as one 

 progresses downward, that is, one degree centigrade per meter (about 

 1.7°F per yard). The thermocline may vary in thickness in different 

 lakes and at different times during the period of stratification. Although 

 in large lakes the thermocline usually is a thinner layer than the 

 epilimnion, in small ponds it may continue from the lower edge of the 

 epilimnion to the pond bottom. 



In large deep lakes the volume of water below the thermocline ( hypo- 

 limnion) tends to show a fairly uniform temperature. Where there is a 

 significant temperature decrease as one moves downward, that change is 

 less than 1°C per meter. As mentioned previously, lack of dissolved 

 oxygen makes the hypolimnions of some lakes uninhabitable for most 

 aquatic organisms. The rapidity and extent of eutrophication is dependent 

 upon the volume of oxygenated water and the amount of organic decay 

 and respiration placing demands upon the oxygen. For example, after 

 the spring period of complete circulation, very deep and relatively in- 

 fertile lakes have a very great volume of oxygenated water in the 

 hypolimnion, and the oxygen-consuming organisms and processes are 

 proportionately small. In these lakes, the oxygen is not used up in the 

 hypolimnion during summer stratification, and they are inhabited by all 

 kinds of oxygen-requiring organisms, including such fishes as lake trout, 

 white fish, and walleye. This lake type is called oligotrophic. 



