6 4 



AQUATIC CONDITIONS 

 TABLE V (76) 



It will be noted that there is a little less than one atmosphere 

 increase in pressure for each 10 meters (33 feet) in depth because water 

 is very slightly compressible. According to this, animals in the deepest 

 parts of Lake Michigan are living under a pressure of about 375 pounds 

 to the square inch. 



5. BOTTOM 



The character of materials and topography of the bottom are very 

 important to animals living on the bottom, but it has its effect also on 

 free swimming animals as a determining factor in the amount of sedi- 

 ment. 



The kind of bottom is important because many animals are 

 dependent upon solid objects for attachment and are absent from 

 bottoms made up of fine materials. Others must burrow into mud 

 or creep on sand and gravel. This will be discussed later in special 

 cases, particularly in streams. 



Topography of the bottom in shallow water is important in lakes 

 locally in affecting wave-action and currents, and through these, bottom 

 vegetation and temperature. Ward (75) noted such effects but did 

 not carry the work far enough to solve any of the problems involved, 

 which are usually local. In lakes, bottom materials are most important 

 in shallow water, because of their effect in connection with wave-action, 

 the amount of sediment in suspension, and the stability of the bottom. 

 The bottom materials of lakes vary greatly locally. Taking Lake 

 Michigan as an example, if we were to see the region about Chicago 

 denuded of all vegetation, we would be able to appreciate the fact that 

 there are bowlder deposits, gravel deposits, sand, clay, and bare rock. 

 Evidently the ice sheet left the same kind of bottom materials strewn 

 with the same irregularity in the bottom of the lake as on the land. 

 Apparently wave-action has not affected them below 25 meters (85 feet). 

 The waves of Lake Michigan are believed not to move sand below 

 9 meters (30 feet). It is thought that, during the Champlain stage, the 

 lake stood at a level 60 feet below its present level. Along the north 

 shore there is a cliff at this level with sand deposits lying on the side 

 toward the deeper water. Inside of this is an area of clay and then, next 



