520 



pear in large numbers in the creek waters after they mingle 

 with those of the impounding bottom-lands or the slowly mov- 

 ing current of the main stream. 



No chemical analyses have been made of the water in 

 Quiver Creek, but those made of samples taken at our plankton 

 station in Quiver Lake are in a measure applicable to the creek 

 itself, especially at low-water stages. In Table XIII. are data 

 derived from weekly analyses from September 24 to December 

 3, 1895, and fortnightly analyses from October 19, 1897, to 

 March 28, 1899, made by the Chemical Survey. A summary of 

 the averages will be found in the table on page 521. These 

 show a smaller amount of residue on evaporation in the lake 

 (268.9) than in the Illinois River (367.5), which may be due in 

 part to the deposition of suspended silt owing to the impound- 

 ing action of the lake and to the sandy nature of the catch- 

 ment-basin. The loss on ignition is somewhat less, and the 

 oxygen consumed very much less (5.9 to 10.4), in the lake than 

 in the river, indicating a smaller amount of organic matter 

 in the former. The small amount of chlorine (4.8 to 21.6) ex- 

 hibits the freedom of the lake from sewage contamination. 

 The albuminoid ammonia, representing undecomposed organic 

 matter, is also present in small quantity in the lake as com- 

 pared with the river (.25 to .48), while the free ammonia, indic- 

 ative of the first stages of decomposition, is still less (.165 to 

 .860). The nitrates, the final products of decomposition, are 

 not at all abundant in the lake (.66 to 1.58). 



The lake waters, and by inference the tributary creek 

 waters also, are thus deficient, as compared with the river, in 

 organic matters and the products of their decay. These prod- 

 ucts are fundamental constituents in the nutrition of the phy- 

 toplankton, which in turn supports that part of the zooplank- 

 ton which does not depend upon the organic detritus in sus- 

 pension for food. The chemical condition of the water of 

 Quiver Creek is thus unfavorable to the development of a 

 plankton quantitatively as great as that of the river. Further- 

 more, its waters, poorer in the plankton itself, not only dilute 



