282 



we should find such irregularity in the (estimated) plankton. 



We now come to the section of the river dominated by the 

 Peoria-Pekin pulse of sewage, including 70 miles of channel — 

 from Holmes Landing to Peoria. The flood waters are still in 

 evidence, but in reduced volume, and there is marked increase 

 in the plankton content. The average departure from the mean 

 plankton is ±32 per cent., with arange of —64 to +48 — a total of 

 112 per cent. In the case of the total catches the average de- 

 parture from the mean is ±36 per cent., with a range of — 60 

 and +89— a total of 149 per cent. 



The upper section of the river, above Peoria, a stretch of 

 40 miles, was less disturbed by flood conditions, there being 

 only slight local invasions. This region is within the sphere 

 of influence of Chicago sewage, and not receiving any large 

 tributaries, we might expect but do not find conditions some- 

 what equalized here. The average departure from the mean 

 plankton is ± 76, with a range of —76 to +80 per cent. — a total of 

 156 per cent. The average departure of the total catch is ±34 

 per cent., with arange of— 27 to +66 per cent. — a total of 93 per 

 cent. These departures will be much reduced if we break this 

 section into an upper and lower region of two collections each, 

 the percentages falling from ±34 to ±2 and ±0 for plankton, 

 and to ±1 and ±39 per cent, for the total catch for the two 

 sections, each of which represents 20 miles approximately. 



The average departures from the mean plankton in the 

 four sections are respectively ±12, ±51, ±32, and ±76 per cent., 

 yielding a grand average of —43 per cent. ; while the corre- 

 sponding average departures for the total catches are ±44, ±5, 

 ±36, and ±34, with a grand average of ±29.7 per cent. These 

 four subordinate units of environment represent longitudinal 

 extensions of 20, 60, 70, and 40 miles. The area included in 

 Reighard's Lake St. Clair collections has a length of 32 miles 

 and a maximum width of 5^, and the average departure from 

 their mean (computed by similar methods for all localities) is ± 

 28.8 per cent. Similar methods of computation thus yield for 

 Lake St. Clair and these sections of the Illinois River almost 

 an identical ± error of distribution. 



