CHAPTER VII 



ANIMAL COMMUNITIES OF SMALL LAKES 



I. Introduction 



Lakes are difficult to classify on the basis of animal relations. This 

 is because size, shape, exposure to wind, depth, and age are all important 

 in determining conditions that affect animals. A classification into 

 coastal lakes and morainic lakes will serve our purposes best, because, 

 other things being equal, it represents age and depth (near Chicago). 



Morainic lakes are depressions in the moraine due to irregularities of 

 deposition, which stand below ground-water level. They are of various 

 sizes. We shall apply the term lake only to those bodies of water that 

 are large enough to produce an area of at least a few square rods of 

 sandy shore, which supports gilled snails, mussels, etc. The principal 

 lakes included in our area are shown on the map facing p. 52. The 

 largest of these are the Fox, Pistakee, Maria, and Grass lakes in northern 

 Illinois; Hudson, Cedar, Stone, and Flint lakes in Indiana; and Paw 

 Paw and Pipestone lakes in Michigan. The only coastal lakes of any 

 size are Wolf Lake and Calumet Lake. These are located in the old Lake 

 Chicago plain. 



I. CONDITIONS IN LAKES 



Depth is important in determining the conditions at the bottom, but 

 is of little importance to the other parts of the lake. Little is known of 

 the depths of our lakes. Exposure to wind is of importance in affecting 

 the waves and circulation of the water (see p. 61), both of which are 

 important to animals. A lake well protected by high hills will be likely 

 to be less affected by wind than others. Shape is also a factor. Long 

 lakes whose long axes are parallel with the direction of the prevailing 

 winds are more strikingly affected by the wind than those with the long 

 axis at right angles to the wind. 



Waves are never large on small lakes, but are usually effective in 

 determining the kind of bottom by controlling erosion and deposition. 

 The general circulation of all pur lakes has not been studied. On 

 account of their small size it is probable that the deeper ones at least 

 have an incomplete circulation like that indicated in Fig. 11, p. 61. 

 Those that get warmed throughout in summer probably have a complete 

 circulation. The dissolved content of the waters of lakes is usually 



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