185 



degree to the very abundant and minute plankton organisms. 

 In Spoon River the average of the monthly collections is 1,746 

 cu. cm,, three times as much as the main stream carries. The 

 heavy floods and rapid current of this tributary are responsible 

 for this large amount of earthy solids in suspension. 



In this matter of silt and turbidity the river as a unit of 

 environment stands in sharp contrast to the lake. Deposition 

 of solids and clear water are normal to the environment of the 

 lake, while solids in suspension and marked turbidity are the 

 rule with river waters. Owing to their varied occurrence these 

 elements, silt and turbidity, also add to the instability of 

 fluviatile, as contrasted with lacustrine, conditions. 



Silt and turbidity are usually attendant upon floods, so that 

 their unmodified effect upon the plankton is not easily deter- 

 mined. Some inferences and observations regarding the rela- 

 tion of these factors to the economy of the plankton may, how- 

 ever, be made. The silt affects the plankton indirectly by 

 hastening the solution of nutrient substances from the organic 

 detritus that forms a considerable portion of the unstable de- 

 posits which accumulate in shoal and in sheltered parts of the 

 stream. It hinders the penetration of light, thus checking the 

 development of the chlorophyll-bearing organisms while favor- 

 ing the multiplication of bacteria and hastening the decay of 

 organic matter in suspension. It also seems probable that it 

 produces a deleterious effect upon the Entoniostraca by ad- 

 hering to the hairs which clothe their various appendages, thus 

 hampering their movements and causing them to sink to the 

 bottom. Accessions of flood water are frequently followed by 

 an increase in the relative number of moribund and dead 

 Entomostraca, especially of the Copepoda. 



CHEMICAL CONDITIONS. 



The food supply is the most fundamental feature in the 

 environment of the plankton. Its abundance or scarcity de- 

 termines to a large degree the growth and reproduction of or- 

 ganisms, and its fluctuations are important factors in deter- 



