522 



It is noticeable that Spoon River carries the largest amount 

 of matter in suspension, both absolutely and relatively, as 

 shown by the high total residue on evaporation (522.3 to 367.5 

 and 268.9) and by the smallest amount of the residue in solu- 

 tion (167.1 to 304.1 and 248.2). This is further shown by the 

 fact that the solids removed by the army filter in Spoon River 

 average 1,755 cubic centimeters per cubic meter to only 592 in 

 the same year in the Illinois. The large amount of organic 

 matter undecayed and undissolved, and therefore not available 

 for the support of the plankton, is partially indicated l)y the 

 high oxygen consumed (14.1 to 10.4 and 5.9), the high albumi- 

 noid ammonia (.60 to .48 and .25), and the high total organic 

 nitrogen (1.29 to 1.03 and .61), when considered in conjunction 

 with the small amount of residue in solution. On the other 

 hand, the waters of Spoon River are quite deficient in forms of 

 nitrogen more available for the phytoplankton, the free am- 

 monia (.24), nitrites (.039), and nitrates (1.01) being in each 

 case less than in the Illinois (.86, .147, and 1.58), while the 

 chlorine (3.8), an index of sewage contamination, is less than 

 a fifth of that in the channel (21.6). 



Spoon River has therefore great resources, in so far as or- 

 ganic matters and the products of their decay are concerned, 

 for the support of the plankton. Not all of the matter is in 

 solution for immediate utilization, but there is still sufficient 

 for a large plankton development, time for which has not been 

 allowed in the tributary stream. The immediate effect of the 

 access of the contributions of Spoon River to the channel is, 

 in the average, a dilution of its inorganic nitrogen per m.'' of 

 water, which is in some unknown measure made good by the 

 contributions of silt, in part of organic origin. The net result 

 is, of course, a large addition to the toial resources of the chan- 

 nel waters available for the present and future development of 

 the plankton. 



The amount of nitrogen in its several forms which Spoon 

 River carries is not small as rivers go, for this stream drains a 

 plain unsurpassed in fertility by any other part of the catch- 



