557 



are able to compute the mean annual production on the basis 

 of 235 observations extending over a period of five years. At 

 present writing no series of observations of like, or even approx- 

 imately like, extent has been published concerning any other 

 body of water. Comparisons upon this basis are therefore not 

 possible. 



The scientific or economic value of comparisons upon other 

 bases than the mean annual production or the full seasonal 

 course of production cannot be great, unless it be for coinci- 

 dent seasons. Furthermore, data of production without com- 

 parable environmental data lose much of their significance. 



The volumetric determination of the plankton of streams 

 elsewhere has not been carried on to any considerable extent. 

 Steuer ('01), in his paper upon the entomostracan fauna of the 

 backwaters of the Danube at Vienna, gives a brief list of organ- 

 isms observed in the plankton of the Danube itself, but no vol- 

 umetric determinations. His conclusion regarding the potamo- 

 plankton — a term whose very validity he contests — is : "Das 

 einzige wichtige Ergebniss der ' Potamoplanktonforschung ' 

 scheint mir bis jetzt die Feststellung der grossen Armuth un- 

 serer fliessenden Gewasser an Mikroorganismen zu sein." It 

 is in a similar vein that Whipple ('99) notes the paucity of 

 plankton organisms in rivers, resulting presumably from sani- 

 tary examinations of streams of New England. The results of 

 our investigation upon the Illinois River place limitations on 

 these conclusions. It is a question of time and nutrition and 

 the absence of deleterious industrial wastes. Given ordinary 

 stream water free from poisonous industrial wastes and suffi- 

 cient time for breeding, a typical plankton may be expected, it 

 seems, in every river, especially the larger ones, and in the 

 lower reaches of those of the smaller type. 



Steuer's volumetric work on the two backwaters of the 

 Danube, covering, it seems, nineteen collections in a period of 

 fifteen months, from June, 1898, to August, 1899, indicates a 

 planktograph somewhat similar to our backwater plankto- 

 graphs in that he finds a vernal pulse in May and a midsum- 



