198 
The similarity of the residual nitrates in the two lakes is strik- 
ing (.66 and .64), and it bears no apparent relation to their 
plankton production (1.62 and 6.68). The excess of other forms 
of nitrogen in Thompson’s Lake (roughly twice that in Quiver) 
would seem to indicate either that the decomposing nitroge- 
nous substances are utilized before they reach the form of ni- 
trates, or that they are abstracted from the water so promptly 
that they do not accumulate above a certain residual minimum 
which is apparent during the growing period of the phyto- 
plankton and of the coarser forms of aquatic vegetation. (See 
Plates XLIX. and L.) It is evident that the nitrates in the two 
lakes (.66 and .64) cannot adequately represent the nitrogenous 
resources of the two bodies of water; neither can they furnishany 
reliable clue to their actual productiveness in plankton. Other 
factors of the environment are equally or even more potent. 
The number of analyses and of plankton catches is so great 
(188 and 156 from Illinois River and 40 of each from Thomp- 
son’s Lake), and they are so distributed through the year, that 
the inference is justified that the nitrates shown by chemical 
analysis in the water of a lake or stream, especially during the 
growing period of vegetation, afford no reliable basis for judg- 
ment as to its plankton production. 
The sewage received by the Illinois River bears an impor- 
tant relation to the chemical condition of its water and thus to 
the plankton which it produces. No measurements are made 
by boards of public works of the amount of sewage which mu- 
nicipal systems discharge into the various streams which unite 
to form the Illinois River. Two sources of information are, 
however, available which throw some light on the extent of 
sewage pollution arising from these sources. They are the 
population of the cities in question and the pumpage of their 
water-works. Municipal engineers are accustomed to estimate 
the sewage discharged from a city with well-established sewage 
and water systems as approximately equivalent to the pump- 
age of the latter. I have accordingly prepared a table which 
includes practically all of the cities provided with these works 
