205 



(1.292), all of which are in excess in its waters. The freedom 

 from sewage is evidenced by the low chlorine (3.8), while the 

 considerable amounts of free ammonia (.245), nitrites (.039), 

 and nitrates (1.01), indicate organic decomposition in progress 

 or completed. In the absence of any considerable contamina- 

 tion by sewage it seems probable that these substances have 

 their origin in the organic silt and the soil waters of the very 

 fertile catchment-basin of the stream. The water of Spoon 

 River, in so far as the nitrogenous substances (2.586) are con- 

 cerned, could support a much more abundant plankton than it 

 produces (.384). As in the case of the main stream, the ex- 

 planation of the slight production lies in the recent origin of 

 the tributary water. Impound Spoon River water in Thomp- 

 son's Lake, and it produces an abundant plankton. 



In food resources Quiver Lake is the poorest locality of the 

 four (1.456, total of nitrogenous substances), having 40 percent, 

 of the amount of the nitrogenous substances in the Illinois, 56 

 per cent, of that in Spoon River, and 67 per cent, of that in 

 Thompson's Lake. The suspended solids (268.9), the loss on 

 ignition (27. 5), and the oxygen consumed (5.9), are least here as a 

 result of slight access of silt-laden waters. The chlorine is low 

 (4.8), and would be much lower if contaminations from river 

 water at overflow could be eliminated ; and corroborative evi- 

 dence of the slight contamination of the waters of this lake by 

 sewage is seen in the amounts of free (.165) and albuminoid 

 (.251) ammonia, of organic nitrogen (.61) and nitrites (.023), all 

 of which exhibit minimum averages in this lake. Organic mat- 

 ter in decay is less abundant here than in the other localities, 

 being, for example, about 50 per cent, less than in Thompson's 

 Lake. The final products of decay, the nitrates, are greater 

 (.66) than the amounts of organic matter would lead us to ex- 

 pect, and are probably due in large part to soil waters from the 

 drainage basin. In the light of the production of Thompson's 

 Lake (6.68) the small amount of plankton produced in Quiver 

 Lake (1.62) finds no adequate explanation in a reduction of 33 

 per cent, in the total nitrogenous substances. Flushing by tribu- 



