223 



amount present. During the long-extended periods of plank- 

 ton minimum it is apparent that the plankton must constitute 

 a very much smaller part of the total amount of albuminoid 

 ammonia in the water. 



That much of the albuminoid ammonia may be in the silt 

 is shown especially in the table (Table XL) and diagrams (PI. 

 XLVI. and XLVII.) of Spoon River at times of flood. In such 

 waters there is practically no plankton — as will be shown else 

 where from the examination of our plankton collections in 

 that stream — although the amount of albuminoid ammonia is 

 often very great. 



The average amounts of total organic nitrogen (Table X.) 

 in solution and in suspension during this same period are .69 

 and .34 parts per million respectively. The latter (.34), which 

 represents plankton and silt, thus constitutes about one third 

 of the total amount ( 1.03) of organic nitrogen in the water. 

 The proportion of this fraction which the plankton may consti- 

 tute under the most favorable conditions may be inferred from the 

 increase in the suspended organic nitrogen which attends the 

 spring pulse of 1898 (PL XL.). This rises from a previous level 

 of .12-16 parts per million to .24-. 64, the latter with the de- 

 cline of the plankton, and at its maximum (.64) it constitutes 

 46 per cent, of the total amount of organic nitrogen in the 

 water. On May 3, when the plankton is at its maximum, the 

 suspended organic nitrogen is but .24, or .25 per cent, of the 

 total. During the periods of plankton minimum the propor- 

 tion which the plankton forms of the total organic nitrogen 

 must be very much less than at times of plankton maximum, 

 since the amount in suspension shows no decrease at all pro- 

 portional to the fall in the amount of plankton. Here also 

 floods are quite as potent as plankton in causing marked in- 

 crease in the amount of total organic nitrogen in suspension, 

 as will be seen on comparison of the curves of this matter with 

 the hydrographs on Plates XLIII.-L. 



It is thus evident that the plankton does not form, even 

 under most favorable conditions, any large part of the total 



