




108 
ey Stage of 
Turbidity Silt . 
Date Number per m.3 F . river above 
(in meters) | (in cm.?) eenenr a. 
AMOS ts 2AM i oe eee | 4,800 pod melts 1.8 
PNGGUSE Shoes aaoagoce sus] 112,000 aS) .19 1s 
September a. se sae | 1,240,000 as 5 ti 
September: (Ay ances | 106 , 000 | ao) 1.04 20 
September 21 tes ae et 800 | oS) trace 2,0) 


XI.) was only a fall of .1 and a rise of .2 of a foot—changes due to 
wind and the operation of the locks in the dams at either end of the 
pool. The estimated percentage of silt is near the minimum—from | 

a trace to 5 per cent.—and the turbidity was no greater than is 
customary (Pt. I., Table III.) in-our waters during periods of abun- 
dant plankton such as this (Pt. I., Pl. XI.). Beyond the presence 
of these rhizopods there was nothing in the plankton to suggest 
that the bottom had been stirred up any more than usual. No 
environmental factor is apparent to which we can attribute this 
wave of Difflugia in the plankton. It is due, I believe, to their own 
physiological condition. This was a time of prolonged low water 
and great sewage contamination, and of remarkable development of 
water-bloom, chlorophyll-bearing flagellates, unicellular alge, and 
some diatoms,—all elements in the food of Dzfflugta. In the open 
water Difjlugia could find abundant sustenance and thus maintain 
itself there. It is not strange, then, that we find it in these warm 
waters, richly charged with its food, assuming for the time a eulim- 
netic habit, perhaps as a result of rapid growth and lighter shells, and 
of increased metabolism—with reserve products which lighten the 
specific gravity and so facilitate flotation. 
This species is found throughout the whole range of temperatures. 
There are indications that its optimum lies above.60°, and perhaps 
near the maximum, 80°. This may, however, be the result of the 
effect of temperature upon the food supply of the organism. In any 
case the plankton data can not suffice to follow the complete seasonal 
cycle of an organism which is either an adventitious or but a tem- 
porary constituent. 
i 
