74 



COMMUNITIES OF LARGE LAKES 



and bottom. Accordingly the conditions on the bottom at various 

 depths are roughly shown in Table IX. 



TABLE IX 



Physical Conditions 



Limit of sand-moving waves 



Limit of daily temperature fluctua- 

 tions; limit of wave action; be- 

 ginning of light decrease; pressure 

 about 2\ atmospheres 



Pressure 4 atmospheres; light re- 

 duced to I 



Seasonal temperature fluctuations 

 less than i°; light reduced to f ; 

 pressure sf atmospheres 



Light \ ; pressure 7 atmospheres . . . 



No light; pressure \\\ atmospheres; 

 no change in temperature; uni- 

 form conditions 



Greatest depth in the area con- 

 sidered; pressure 15 atmospheres 



Greatest depth in lake; pressure 27I 

 atmospheres 



Vegetation 



Lowest record of Chara 

 and (75) Cladophora 



Scanty filamentous algae 

 (75) 



Nostoc and diatoms (75) 

 No bottom plants recorded 



No plants recorded 

 No plants recorded 

 No plants recorded 



I. THE LIMNETIC COMMUNITY 



(Station 1 ; List I) 

 Chicago is famous for its good water supply. However, if one fastens 

 a small sack of miller's bolting-cloth under an open water tap for an 

 hour in summer and examines the contents of the sack with the naked 

 eye and then with the microscope, he will be of the opinion that he has 

 not been straining drinking water but stagnant ditch water. He finds 

 small microscopic plants in great numbers (75), as well as large numbers 

 of small animals, most of the larger ones dead. Every person drinking 

 water from a lake or river drinks the small plants and animals. If 

 every one of the 2,000,000 persons in Chicago drank a quart of unfiltered 



by the United States Fish Commission, and Doctor Stimpson of the Academy pub- 

 lished a brief note on the invertebrate forms found in the lake, but never gave more 

 than a hint of the work, as the collections were all burned with the Academy's build- 

 ing. Subsequently, collections were made by the State Laboratory of Natural His- 

 tory, and later by the Fish Commissioners of Michigan. In the summer of 1902, the 

 University of Chicago and the Academy of Sciences made a single-day excursion, 

 but no report was ever published. 



