A LIMNOIvOGICAI^ STUDY OF THE FINGER LAKES. 
591 
But the bottom stratum was more densely populated, possessing a larger number of 
Limnocalanus macrurus, as well as a small population of Mysis relicta. A similar dis- 
tribution of the Crustacea in the lower water was found also in Canandaigua and Skane- 
ateles Lakes. 
The limiting factor in these lakes was not the lack of dissolved oxygen in the lower 
water, since there was an abundance of it even at the bottom, but it was a scarcity of 
food. The chlorophyl-bearing portion of the plankton is the primary source of food for 
the rotifers and the Crustacea, either directly or indirectly, and, as noted above, these 
organisms are confined chiefly to the epilimnion. This means substantially that the 
Fig. 22. — Vertical distribution of plankton organisms in Cayuga Lake, Aug. 12, 1910. Scale, r vertical space=5 meters; i hori- 
zontal space=io Crustacea, nauplii, and rotifers per liter of water and 600 algae and diatoms. Predominant forms; Bosmina, 
Polyarthra, Ceratium, and Asterionella. The column at the right marked O shows the quantity of dissolved oxygen at the 
various depths as indicated, and T represents the temperature. 
zooplankton, not only of the upper water, but also at all other depths, is dependent upon 
the food supply of the epilimnion. The zooplanktonts which occupy the epilimnion have 
the first choice of this food, and those in the hypolimnion must be content with that 
which reaches them from the upper water. 
Granting that the lower water has an abundance of dissolved oxygen, the density 
of its population depends upon the food supply, which, in turn, is dependent upon the 
excess produced by the epilimnion and upon the volume of the hypolimnion. The 
excess of food produced by the upper water depends upon the productivity of that stratum 
and upon the amount consumed by the zooplankton therein. The largest excess of 
