DISCUSSION OF RESULTS. 47 



CHAPTER III. 



DISCUSSION OF RESULTS- 



VALUE OF PLANKTON COLLECTIONS. 



It is a little difficult to state just what value should be placed 

 on the measurement of plankton collections in limnology. 



It is, of course, evident that such collections give material 

 for the qualitative determination of the fauna and flora, so that 

 lakes can be compared with each other with reference to the 

 distribution of forms. Care must be taken in this, however, 

 unless the lakes are under examination for a long period of time, 

 for, while in general the annual appearance of any form will be 

 at about the same time in all lakes, nevertheless this appear- 

 ance is subject to considerable variation, partly from differences 

 in local conditions, and partly from differences in other condi- 

 tions of environment ; I think there is no doubt that the algae of 

 the "bloom:" may differ in their maximum periods not only days, 

 but perhaps weeks, when the lakes appear to have similar condi- 

 tions. Thus it may happen that a form may be abundant 

 in one lake, and absent or present in small numbers in another 

 at any given time, but later or earlier it may be abundant in the 

 second lake. The absence of a form at a particular time is not 

 always proof that the form is never present. For example, 

 Diaptomus sicilis might easily be overlooked in the fauna of 

 Green lake if we were to depend entirely on summer collec- 

 tions, or Cyclops pulcliellus in Lake Winnebago if no winter col- 

 lectons were made. As has been indicated already, also, 



