530 



a decline of 11 per cent. These figures are based on averages. 

 Naturally in tributary floods the increased volume and les- 

 sened plankton will cause a much greater dilution, though of 

 brief duration. In moderate and low-water stages the decline 

 will be less than the above average and of longer duration. 

 The relative discharge of the two streams also enters to fur- 

 ther complicate the problem. But whatever form the ratio of 

 production of the two streams may assume in a precise determi- 

 nation, it is safe to say that the general conclusion drawn 

 from the present data will be confirmed. 



QUALITATIVE COMPARISON OF PLANKTON OF SPOON AND ILLINOIS RIVERS. 



The details of this subject, in so far as they pertain to the 

 constituent species of the plankton of the two streams, will re- 

 ceive attention in the discussion of species in the second part 

 of this paper. For the present only the general phases and 

 most striking contrasts will be discussed. 



With a view to presenting in concrete form the contrast in 

 the relative population of the two streams I have prepared 

 Table XIV., which gives statistical data compiled from the 

 enumeration records. The table exhibits the number of indi- 

 viduals per cubic meter of water of the main groups of plank- 

 tonts in the thirty-five collections in Spoon River, and in simi- 

 lar collections made on the same date in the Illinois River. In 

 a few cases only, the Illinois River collections were made sev- 

 eral days prior or subsequent to the Spoon River collection. 

 The groups listed are the algae, diatoms, Rhizopoda, Masti- 

 (/ophotri, lnfi(s()ri<(. Rotatoria, EittotHostraca, insect larvae, and 

 miscellaneous. The total numbers of individuals and of species 

 are also given. These data are taken entirely from catches 

 with the silk net, and are subject to the errors arising from 

 leakage through the silk. The silt interferes in this case only 

 in so far as it obscures the planktonts to a greater extent in 

 Spoon River collections, and thus necessitates a greater dilution 

 in counting and, in consequence, a greater factor in computa- 

 tion. The margin of error is accordingly somewhat greater in 



