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. 
Siltand 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 Hntomostraca 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- 
