549 



in 1895 and 1896 reach limits of + 118 per cent, and — 61 per 

 cent, respectively. This greater uniformity in total production 

 is brought about in part by the compensating effects of lessened 

 plankton content in years of high water, and increased con- 

 tent in years of low water. 



The mean total discharge is 67,750 cubic meters, or 149,- 

 050,000 pounds if we compute its weight as equal to that of the 

 same volume of water. This represents the mean annual loss 

 of living organic matter from the watershed of the Illinois- 

 The total production, including the plankton impounded in the 

 backwaters and that utilized there and in the channel as food 

 by other organisms, as well as that perishing within the water- 

 shed and contributing by its wastes and decay to the growth of 

 the coarser aquatic vegetation, — this total must indeed be much 

 greater than the amount indicated by these imperfect computa- 

 tions. If we add to this the undetermined but probably not in- 

 considerable volume which leaks through the meshes of the silk 

 net and is therefore not at all represented in our computations, 

 we reach a total annual production of still greater magnitude. 



TOTAL PRODUCTION AS AFFECTED BY LEAKAGE THROUGH 

 THE SILK NET. 



In an earlier paper ('97b) I have called attention to the 

 extent of leakage through the silk as determined by the enu- 

 meration of catches made by silk, hlter-paper, and the Berkef eld 

 filter. An attempt was also made later, in connection with 

 the enumeration of the organisms of the filter-paper catches, 

 to determine the volmnetric catch of plankton by this method. 

 It was my endeavor to estimate the proportions of silt and 

 plankton in these catches, as had previously been done in the 

 case of those of the silk net. There is, however, in these filter- 

 paper catches a silt of very different character. To that which 

 we find in the catches of the silk net there is added a large 

 amount of fine loam, clay, and sand which passes directly 

 through the silk, and a very considerable quantity of minute 

 flocculent particles, presumably bacterial zooglea. This floe- 



