349 
level of production continues under the ice, which remains on 
the stream for about three months (Pl. XXIV.). There is con- 
siderable fluctuation (Pl. 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 em.’ 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. 
