CHAPTER V 



ANIMAL COMMUNITIES OF LARGE LAKES (LAKE MICHIGAN) 



1. Conditions 



I. GENERAL (75) 



Lake Michigan lies between 4i°-4o' and 46°-5' N. latitude. Its total 

 length is about 350 miles and its greatest width is approximately 85 miles. 

 Its area is about 25,000 sq. miles. Its greatest depth is nearly 275 meters 

 (900 ft.) and its average depth is approximately 122 meters (400 ft.). 



Within the area covered by our map (frontispiece) there are about 

 3,200 sq. miles. The maximum depth is about 152 meters (500 ft.). 

 It has been estimated that the lake contains 262,500,000,000,000 cubic 

 feet of water. It becomes obvious at once that the lake constitutes one 

 of the most uniform and extensive environments with which we have to 

 deal. 



2. CIRCULATION 



The level of the lake fluctuates from season to season with the 

 amount of rainfall, but we have been unable to find a statement as to the 

 amount of such fluctuation. Changes in atmospheric pressure over part 

 of the lake cause various fluctuations in level, called seiches. In Lake 

 Michigan there is a definite circulation of the surface waters. Here the 

 current moves southward along the west shore (57), around the head of 

 the lake, and northward along the east shore. The rate of flow is 4 to 90 

 miles per day. 



II. Communities of the Lake 1 (80, 81, 82, 8^, 84) 



One of the recognizable animal communities of Lake Michigan is 

 made up of the animals which live freely in the water, either swimming 

 or floating. This community is called the Pelagic or Limnetic com- 

 munity. Other communities are governed directly or indirectly by depth 



x The only published account of the invertebrate fauna of the Great Lakes is 

 that of Lake Superior. From this account and from incidental scattered notes found 

 in various publications cited we have been able to bring together enough data to give 

 an idea of the conditions and life which we may expect future investigations to show. 

 The attempts to study Lake Michigan have been ill-fated. In 187 1, the Chicago 

 Academy of Sciences and the United States Fish Commission co-operated in an 

 attempt to study the fauna of the lake. The work on the vertebrates was published 



73 



