524 



feet filtration and to check slightly the leakage of small 

 organisms through the silk. In Spoon River, on the other 

 hand, the plankton is scant and the silt mainly of fine loam 

 and clay in suspension, so that the silk net rarely clogs and 

 the escape of small planktonts is but slightly impeded. This 

 loss by leakage is far more significant in the case of Spoon 

 River than it is in the Illinois, for the escaping plankton con- 

 stitutes a relatively larger proporfion of Spoon River's total 

 product than an equally large or even greater loss would form 

 of the total product of the Illinois. Again, the quantity of 

 silt is both relatively and absolutely much greater in the waters 

 of Spoon River than it is in the Illinois, and it is of a different 

 nature. This greatly increases the difficulty of maintaining 

 any uniformity of standard in the estimation of the silt con- 

 tent of the plankton catches in the two streams. These sources 

 of error, although considerable, do not, however, invalidate the 

 conclusions here drawn regarding the relative productivity of 

 the two streams. They are still patent within any reasonable 

 limit of error. 



The plankton of Spoon River is very much less than that 

 of the Illinois. The average amount present in a cubic meter 

 of water in Spoon River, as shown by the average of the 

 amounts in thirty-six collections made between August 18, 1896, 

 and March 7, 1899, is .465 cubic centimeters. This average 

 amount is reduced to 0.191 cubic centimeters if the two collec- 

 tions of September 11 and 30, 1897, are omitted. This average is 

 still more reduced if we omit the collections from the last of 

 August, 1897, to the close of the year — a period of exceptional 

 and prolonged low water. The omission of these four months 

 lowers the average to 0.044 cm.'* per m.^ of water, an amount 

 which more truly represents the normal contributions of the 

 tributary to the main stream in years of rainfall normal both 

 in quantity and distribution. 



The average plankton content of a cubic meter of Illinois 

 River water, as shown by the average of 235 collections made 

 between June 12, 1894, and March 28, 1899, is 2.19 cubic centi- 



