CHAPTER VII 
ANIMAL COMMUNITIES OF SMALL LAKES 
I. INtTRODUCTION 
. 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 Jake 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 our 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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