590 
bulletin oe the bureau of fisheries. 
in this region. Whenever the quantity of either falls below a certain amount, it affects 
the distribution of the organisms. 
The second type of vertical distribution is characterized either by a very sparse 
population in a certain portion of the hypolimnion or by practically none at all. This 
is due either to a lack of dissolved oxygen or to a scarcity of food. In Conesus and 
Otisco Lakes, for example, the absence of organisms in the lower strata was caused by a 
lack of oxygen. (See fig. 20.) Results obtained on Wisconsin lakes show that the 
various zooplankton forms are capable of occupying water which has only a very small 
amount of dissolved oxygen, but a certain minimum quantity is necessary. The Clado- 
cera and Diaptomi are only rarely found in water which has less than 0.2 to 0.3 cc. per 
liter, while the minimum for Cyclops and nauplii is about o.i cc., and for rotifers from 
O T 
6J6 zie 
600 
L50 
air 
005 
006 
Fig. 21. — Vertical distribution of plankton organisms in Conesus Lake, Aug. 25, 1910. Scale, i vertical space=i meter; i horizon- 
tal space=io Crustacea, nauplii, and rotifers per liter of water and 100 algse and diatoms. Predominant forms: Cyclops, 
Polyarthra, Ceraiium, Coelosphcerium, Fragilaria. The column at the right marked O shows the quantity of dissolved 
oxygen at the various depths as indicated, and T represents the temperature. 
o.i to 0.2 CC. Several forms, however, such as Corethra larvae, an ostracod, and a number 
of protozoa are able to live in water which contains no trace of free oxygen; but all of the 
limnetic zooplanktonts, except larval Corethra, require at least a small amount of this 
gas in a free state. 
In Conesus Lake the maximum number of Diaptomi was found at 9 meters, where 
the water contained 1.5 cc. of oxygen per liter. Cyclops and the nauplii reached their 
maximum numbers at a depth of 10 meters, where this gas amounted to only 0.1 1 cc. ; 
only a few remained at 1 2 meters, where the quantity of oxygen was only 0.06 cc. , and 
none was found below this depth. 
In Cayuga and Seneca Lakes (fig. 21 and 22) by far the larger portion of the hypo- 
limnion had a very sparse population, being occupied by only a few Crustacea, repre- 
sentatives of Cyclops, Diaptomus, and Limnocalanus, between 50 meters and the bottom. 
