40 THE LIFE OF DEVILS LAKE 



a higher speed and for longer periods the results would doubtless 

 have been different. 



Examinations have been made of the filtrate from the Sedg- 

 wiek-Rafter tubes which show a considerable loss of such forms as 

 Coelosphaerium, while, vice versa, filter paper, even if carefully 

 washed, retains a considerable quantity of material, so that in 

 either case the results are too low. 



The usual planeton series includes readings at the shore, the 

 surface, some distance from shore, and at depths of 0.6, 2.1 and 3.0 

 to 4.3 m., the latter reading depending on the depth of the lake at 

 the point and time of collection. There was some variation from 

 this general plan, especially in the earlier years of the work, when 

 the collections were preliminary to the main series. Some of the 

 collections were lost so that the series is not quite complete. It is 

 sufficiently so, however, to give a good idea of the planeton abund- 

 ance and distribution thruout the periods of observation. 



The counts have been made mostly by my former assistant, 

 Mr. E. G. Moberg. Some of them are my own, however, but these 

 have been cheeked with his bj^ comparative counts on the same 

 material. Whenever possible it is desirable that readings should 

 be made by the same observer, in order to remove the personal 

 equation, which cannot otherwise be eliminated entirely from work 

 of this character. 



The series prior to August, 1913, are more or less fragmentary. 

 Beginning at this time, however, readings were taken at intervals 

 of usually a week to ten days until September, 1914. In some cases 

 readings were taken at intervals of one to two days, while there 

 are one or two intervals of two weeks duration. During winter 

 and early spring the interval was lengthened to from three to six 

 weeks. An additional series of readings with two to three week 

 intervals* was taken from Dec. 3, 1922, to July 23, 1923. 



Readings were made at several levels instead of a single one 

 to ascertain the distribution of organisms at various depths. In 

 several instances these readings were taken in parallel series for 

 different hours of the day and night in an effort to determine the 

 vertical migration of the zooplancton. The results of these readings, 

 which were very indefinite, will be discussed later. The series of 

 readings from top to bottom gives a much better record of the 

 planeton abundance than would readings at only one level. 



The accompanying charts give a graphic representation of the 

 occurrence of the various selected species and groups, during the 

 periods of observation fi'om 1911 to 1923.** The chart of one year 

 is not strictly comparable with that of another, in respect to total 

 numbers, at le ast for some of the algae and bacteria ; since the period 



•In one caHe six weeks. 



"♦I'.asi'Cl I'll oiii" liti'c samples. 



