283 



In the light of these volumetric data the conclusion is pat- 

 ent that plankton data from fluviatile environment contain on 

 the average a distribution error which approximates that in 

 plankton data from limnetic areas of similar extent selected 

 with reference to unity of environment as determined by local 

 factors. 



It should be noted in this connection that the conditions 

 prevailing when this plankton traverse of the Illinois River was 

 made, were most adverse to an equalized plankton in the fol- 

 lowing particulars. It was at a time of rapid seasonal change 

 in plankton during the decline of the vernal pulse, and it was at 

 a time of intercalation of flood water of local and recent ori- 

 gin, whose poverty in plankton is brought into contrast with 

 the larger content of the run-off of impounded backwaters else- 

 . where. Finally, the river stage, which was 9 feet at Kamps- 

 ville and 6.9 at La Grange, was such that the equalizing effect 

 of general overflow on plankton content in impounded back- 

 waters had ceased and local differences were emphasized, while 

 at the same time their discharge continued in considerable 

 volume. All of these factors, the last two of which are more impor- 

 tant in the river than in the lake, tend to diversify the plankton 

 content in the river at this season. It is reasonable to suppose 

 that under other conditions — such as general overflow, the 

 more stable features which attend falling levels above or below 

 9-7 feet, or in prolonged low water — we should find the uni- 

 formity of distribution of the plankton more pronounced than 

 it was on May 18-21, 1899, barring, however, the effect caused 

 by sewage contamination, which at all stages and seasons is 

 the most potent factor in the environment of the plankton of 

 the Illinois River. 



TRANSVERSE DISTRIBUTION AND RELATION OF SHORE TO PLANKTON. 



The shore is a factor of great importance in the aquatic 

 environment. It is here that land and water come into most 

 intimate relation; seepage and drainage waters enter here; 

 vegetation gains its foothold, affects the gaseous contents of 



