349 



level of production continues under the ice, which remains on 

 the stream for about three months (PI. XXIV.). There is con- 

 siderable fluctuation (PI. XLVII.) in the organic nitrogen, free 

 ammonia, and oxygen consumed, most if not all of which are 

 traceable to the access of storm waters rather than to any con- 

 siderable degree of stagnation. The catches are all full of silt, 

 though the turbidity of the stream is not great under the ice in 

 January and February. The silt at such times is mainly com- 

 minuted vegetation brought in by the storm waters. There is 

 a slight rise in the plankton production in March (.026), when 

 the river stands at 12.9 ft. and the plankton-rich waters of 

 Thompson's Lake (see Table V.) are brought into connection 

 slightly with Spoon River by overflow. 



During these three months the production in the tributary 

 is but a small fraction — never more than a tenth — of that in 

 the main stream. It continues to be a diluent of the channel 

 plankton. 



SUMMARY. 



The average plankton in all of the Spoon River collections 

 is .465 cm.^ per m.^ of water. In the Illinois it is 2.19, or over 

 4.7 times as much. If we omit the low-water period, Aug. 26 

 to the end of 1897, and compare only the remaining collections 

 between Aug. 18, 1896, and the close of operations, the ratio of 

 production in the two streams becomes .044 to 2.19, or 1 to 50. 

 As has been repeatedly pointed out in the preceding discussion, 

 this contrast in production is not explainable on any difference 

 in available chemical data. The tributary waters are fertile 

 enough to yield a large production. The explanation is rather 

 to be sought in the hydrographic conditions, in the recent ori- 

 gin, from rains or springs, of the tributary water, and in the more 

 rapid current, and consequently the less time for breeding a 

 plankton in the tributary environment. That this is the proba- 

 ble explanation is borne out by the large production in the only 

 period of prolonged low water in the tributary in the fall of 

 1897, when time for the growth of the plankton was afforded 

 in the slack waters of the tributary. 



