CLIMAX FRESH-WATER COMMUNITIES 



303 



the stoppage of current constitute a fundamental change in aquatic 

 climate, and the fishes are consequently unable to retard the process. 

 Biotic Development. Areas of sterile bottom are presented in small 

 rivers which" are dammed to make large artificial pools for the water 

 supplies of towns in central Illinois. Eddy (1934) has traced the de- 

 velopment of plankton in an artificial pool at Decatur, Illinois, mak- 

 ing collections from 1923 (one year after the reservoir was filled with 

 w^ater) to 1930 inclusive. The results showed an increase in constitu- 

 ent species throughout the period at the rate of three or four per year 

 from 1925 to 1930. There was a suggestion of one species dropping 

 out, but this may have been due to annuation. In general, the process 

 was additive development without succession. At the time the studies 

 ended, the plankton had reached a condition approaching that of the 

 Illinois River, which was regarded as essentially the climax plankton 

 of the stream (Table 11; of. Kofoid, 1908). 



TABLE 11 



Number of Plankton Organisms Averaged for June, July and August Col- 

 lections IN Lable Decatur, Showing Development from 1923 (Dam 

 Built in 1922) to 1928 



Key: Al. Alga; Co. Copepod; Pr. Protozoan; Ro. Rotifer; P. Perennial; 



S. Seasonal (from Eddy, 1934) 



Thousands per Cubic Meter 



1923 



1926 



1927 



1928 



P Lysigonium graiiulatuni Ehr. (Al). , . 

 S Euglena viridis Ehr. (Pr) 



Cyclops viridis Jurine (Co) 



S Eudorina elegans Ehr. (Pr) 



P Synchaela pedinata Ehr. (Ro.) 



S Closterium acerosum Schrk. (Al) 



S Tintinnidiuni fluviatile Stein (Pr) 



P Trachelomonas volvocina Ehr. (Pr) 



S Cyclops bicuspidatus Claus (Co) 



P Microcystis aeruginosa Ki'itz. (Al) 



P Synchaeta stylata Wierz. (Ro) 



Brachionus havanaensis Rous. (Ro) . . . . 

 S Euglena acus Ehr. (Al) 



Brachionus budapestinensis Daday (Ro) 

 S Conochiloides natans (Sel.) (Ro.) 



Anabaena circinalis Kutz. (Al) 



68,3 

 13.9 



533.7 

 11,033.8 



541.7 



891 



3 



3 



6 



21 



1 



133 



10 



10,667.4 



49.4 



.3 



8.6 



53.2 

 ,702 . 8 



9.3 

 0.2 

 1.7 



8.3 



,214.0 



982.1 



11.8 



4.5 



3.6 



.1 



,107.4 



,786.4 



16.3 



3.7 

 0.1 



0.1 

 400.0 



