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 furnish any 

 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 



