342 



catches of the silk net is to some extent misleading, owing to 

 the relatively greater proportion which the escaping planktonts 

 form of the production in the tributary stream. Another factor 

 which prevents an equally accurate volumetric determination 

 of the plankton of the two streams is the presence in Spoon 

 River of a much greater proportion of silt. For example, in 1896 

 and 1898 the estimated ratios of silt and plankton in the average 

 of the catches (Table IV.) is .007 to .349 and .029 to .796. In 1897, 

 when low water and slight current and some probable invasion 

 of channel waters increased the plankton production, the ratios 

 are 1.257 to 1.173. The ratios of the first-named years are more 

 nearly normal for this tributary, and in such ratios it is quite 

 probable that the error in silt estimation to some undetermined 

 degree tends to prevent any precise determination of the actual 

 plankton production. Nevertheless, after a very wide margin 

 is allowed for probable error in the data, the comparison of 

 production in the two streams is instructive and significant, for 

 it is the direction of change or contrast in production which is 

 of greatest consequence, and this may be found even in the 

 presence of a large but distributed error. 



It should be noted that the plankton ordinants in the 

 Spoon River plates (XXII. — XXIV.) are plotted on a scale ten 

 times that of all other stations in order to give an appreciable 

 height to the plankton portion of the entry. 



PLANKTON PRODUCTION. 



1896. 



(Table IV.; PI. XXII., XLVI.) 



For purposes of comparison I introduce at this point a 

 table which gives in terms of monthly averages of plankton 

 in cm. 3 per m. 3 the relative production in the seven locali- 

 ties examined by us. The number of collections entering 

 into each average is stated, and the grand average of all collec- 

 tions and of the monthly averages are given for each station. 



In 1896, nine collections were made in Spoon River in 

 August-December, the average being only .007 cm. 3 per m. 3 



