74 COMMUNITIES OF LARGE LAKES 
and bottom. Accordingly the conditions on the bottom at various 
depths are roughly shown in Table IX. 
TABLE Ix 
DEPTH 
PHYSICAL CONDITIONS rT Se VEGETATION 
Meters Feet 
Limit of sand-moving waves...... 8 26 
Limit of daily temperature fluctua- 
tions; limit of wave action; be- 
ginning of light decrease; pressure 
about 23 atmospheres.......... 25 82 Lowest record of Chara 
and (75) Cladophora 
Pressure 4 atmospheres; light re- 
GUCEMIEOE tei hctee sete ee te 390 128 Scanty filamentous algae 
(75) 
Seasonal temperature fluctuations 
less than 1°; light reduced to 3; 
pressure 54 atmospheres........ 54 177 Nostoc and diatoms (75) 
Light 4; pressure 7 atmospheres. . . 70 230 No bottom plants recorded 
No light; pressure 113 atmospheres; 
no change in temperature; uni- 
FORM CONGIEIONS: sctee). =a emee - II5 ar No plants recorded 
Greatest depth in the area con- 
sidered; pressure 15 atmospheres} 153 500 No plants recorded 
Greatest depth in lake; pressure 273 
ALMOSPHELES Meanie ios sees 274 goo 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. 
