256 



The factors determining the decrease are the proportions 

 of silt and plankton and the character of each. When floccu- 

 lent debris is abundant, or when filamentous diatoms or algae, 

 Copepoda, or the Chtdocera with long antenna? are present in 

 numbers, the decrease upon centrifuging is greater. When the 

 silt is earthy or contains considerable quartz, and when the 

 plankton consists of Protozoa such as Synura, or Eotifera such 

 as Sijnchada, or Cladocera such as Clu/donis or Bosm'tno the de- 

 crease is less. The amount of plankton placed in the tubes of 

 the centrifuge also slightly affects the ratio of decrease in vol- 

 ume. For example, one of our largest planktons, measuring 

 11.15 cm.^ by the gravity method, fell but 8 per cent, when cen- 

 trifuged in a single tube. When divided among three tubes 

 the decrease became 14 per cent. This was a plankton largely 

 composed of Chi/dorus and Bosmhia. Another large plankton, 

 measuring 11.85 cm."^ by the gravity method, fell to 7.6 cm. upon 

 centrifuging in a single tube — a loss of 36 per cent. This con- 

 sisted very largely of Sj/iiura. As other large catches decreased 

 as much as 50, or even 60, per cent., it is clear that large vol- 

 umes do not necessarily yield only slight decreases. 



The instances in which the decrease exceeded 70 per cent, 

 are 5 in number. Of these, 3 contained Melosira or FragiUaria, 

 1 was rich in OscllUifia, and 2 contained considerable floccu- 

 lent debris from aquatic vegetation. All of the 12 whose de- 

 crease was less than 30 per cent, occur in April and May, when 

 Chydorus and Bosmina are at their maximum and constitute 

 a large, if not the greater, part of the plankton. In a few in- 

 stances these catches which showed slight reduction in volume 

 contained Hydro, insect larvte, and other adventitious forms 

 from surrounding vegetation. 



Both extremes contain numerous instances in which the 

 plankton catch is made up of typical plankton organisms, and 

 consequently the range in the decreases here recorded is normal 

 for the ranye in planktonaoccnrriiiy in our ivaters throughout the 

 year. It is therefore reasonable to assume that the cenfrifnyed voJ- 

 umeshere reported for jjhoikto/i in the Illinois must, onthe average, 



