217 



(PI. XLIII. and XLV. ). The effect of sudden floods, presuma- 

 bly those of tributaries but a short distance above Havana, 

 appears in February, 1896 (PL XLIIL), and in 1899 (PI. XLV.) 

 as a twelve- and two-fold increase respectively, which is re- 

 markably abrupt and is followed in both cases by a quick but 

 somewhat more gradual return to the previous condition. 



Owing to the complexity alike of the substances included 

 in these items of the analysis and of the plankton itself, no 

 uniform correlation of these factors can be discovered. Two 

 different and in a certain sense opposite tendencies can be de- 

 tected in the relationship of the movements of the plankton to 

 those of the substances under discussion. During the winter 

 season and the period of excess of nitrates, plankton pulses are 

 attended by increase in the albuminoid ammonia and organic 

 nitrogen. This appears in the Illinois with the pulses of April, 

 1896 (PL XLIIL), and December, 1898 (PL XLV.). During the 

 warmer months, when most of the plankton pulses occur, the 

 opposite tendency is seen in the movement of these substances. 

 They tend to decrease at times of plankton pulses, as may be 

 seen in August and October, 1896 (PL XLIII), in May, July, 

 September, and October, 1897 (PL XLIV.), and in June, 1898 

 (PL XLV.). With the pulses of December, 1895 (PL XLIIL), 

 and May, 1898 (PL XLV.), no marked effect in either direction 

 is apparent. 



In Spoon River any seasonal movement of the albuminoid 

 ammonia and organic nitrogen is quite thoroughly masked by 

 the disturbances due to floods. In 1898 (PL XLVII.) these 

 substances are a trifle lower in the warmer months than in the 

 colder, a condition which may result from the prevalence of 

 floods in the latter season. In 1897 (PL XLVI.) they increase 

 during the warm season and period of low water attending a 

 development of the plankton unusual in the water of this 

 stream. 



The effect of flood upon the quantity of these substances 

 in the water of this stream is well deflned, and seems to throw 

 light upon the relation which flood waters bear to the plankton 



