70 Types of Aquatic Environment 



Such lakes, when their basins He above the level of 

 the permanent water table, may sometimes be drained 

 by sinking wells through the soil of their beds. This 

 allows the escape of their waters into the underlying 

 limestone. Sometimes they drain themselves through 

 the widening of their underground water channels. 

 Always they are subject to great changes of level conse- 

 quent upon variation in rainfall. 



Enough examples have now been cited to show how 

 great diversity there is among the fresh-water lakes of 

 North America. Among those we have mentioned are 

 the lakes that have received the most attention from 

 limnologists hitherto ; but hardly more than a beginning 

 has been made in the study of any of them. Icthyolo- 

 gists have collected fishes from most of the lakes of the 

 entire continent, and plancton collections have been 

 made from a number of the more typical : from Yellow- 

 stone Lake by Professor Forbes in 1 890 and from many 

 other lakes, rivers and cave streams since that date. 



Lakeside laboratories — On the lakes above mentioned 

 are located a number of biological field stations. That 

 at Cornell University is at the head of Cayuga Lake. 

 That of Ohio State University is at Put-in- Bay on Lake 

 Erie. The biological laboratories of the University of 

 Wisconsin are located directly on the shore of Mendota 

 Lake, and from that point as a center a comprehensive 

 study of the very numerous lakes of the State of Wiscon- 

 sin is in progress. Other lakeside stations are as follows : 



That of the University of Michigan is on Douglas 

 Lake in the northern end of the southern peninsula of 

 Michigan. This is an attractive sheet of water at an 

 altitude of 712 ft., covering an area of 5. 13 square miles, 

 and having (as far as surveyed) a maximum depth of 

 89 feet and an average depth of 22 feet. Its transpar- 

 ency by Secchi's disc as measured in August is about 

 four meters. 



