517 



in the predominance of the diatoms, the Rhizopoda, and Chiron- 

 omus larvae, and in the character of the few rotifers present. 

 It is also evidenced by the erratic distribution and often small 

 numbers of many of the species occurring in the creek waters. 



There is little evidence in the data of marked seasonal 

 changes. The diminished number, both of species and individ- 

 uals, in the winter and spring months is due in part to the 

 flood conditions prevailing at that time. The disappearance 

 of the desmids in the colder months is apparent, as is also the 

 decline in the diatoms, the effect of which is heightened by the 

 greater proportion of dead and moribund individuals in the 

 winter months. There is also some slight evidence in the ta- 

 ble of a spring increase in March and April and of a late au- 

 tumn maximum in November. From conditions observed in 

 Spoon River and Quiver Lake during the summer months not 

 included in the period of our Quiver Creek collections it seems 

 probable that the plankton of this stream at that season of the 

 year is more diversified by the addition of flagellates and rotifers. 

 The flow of the stream is, however, much reduced at this sea- 

 son, and its contributions correspondingly small in quantity. 



The inter-relations of this creek plankton are very patent. 

 The diatoms are the predominant members, having one third 

 of the species and nine tenths of the individuals. The rhizo- 

 pods include one fifth of all the species and almost two thirds 

 of the animal individuals. We have here a rich diatom flora 

 supporting a rhizopod fauna. The remainder of the species, 

 about one half, belongs to diverse groups and is numerically, 

 at least, an insignificant element in the plankton assemblage, 

 constituting but 4 per cent, of the total population. 



A comparison of the plankton here delineated with that 

 of the river at the same seasons of the year leads to the follow- 

 ing conclusions : 



1. The creek organisms all occur in the river plankton. 



2. The facies of the river plankton is quite different from 

 that of the creek in that it has a greater abundance and variety 

 of organisms, a greater proportion of limnetic species, and a 



