comparison of results based on samples taken at 2- and h-meter 

 intervals. These data were derived from Table 67, which shows the 

 vertical distribution of zooplankters at Station 37A on several 

 dates in 1930. On five dates, samples were taken at 2-meter 

 intervals, giving a total of seven samples for a series. The mean 

 number per liter of each kind of plankter was determined first from 

 this series and then from the samples taken at 0.5, h, 8, and 12 

 meters only. The 25 pairs of means are shown in Table 63. 



In 12 of the 25 pairs of means the result was identical 

 for the 2-meter and U-meter series. In 7 pairs there was a 

 difference of one; in 3 there was a difference of 2j in 2 a dif- 

 ference of 3; and in one a difference of 6, Considering the 

 various sources of error involved in the determination of the number 

 of plankton organisms, it may be said that the two methods gave 

 essentially the same results. That is, the conclusions regarding mean 

 abundance at Station 37A on the dates in question would be the same 

 whether the mean was determined by a series of 7 samples or by a series 

 of h samples. The data presented in Table 63 establish the adequacy 

 of the series with U-meter intervals as compared with the series of 

 2-meter intervals, but not the adequacy of the series with 2-meter 

 intervals themselves. However, it may be argued that, if the vertical 

 distribution was such that means determined from samples taken every 

 k meters were essentially the same as means determined from samples 

 taken every 2 meters, it is highly improbable that a further decrease 

 in the distance between samples would have affected the results 

 materially. It is safe to conclude, then, that the routine procedure of 

 determining the mean number of plankters from samples taken at 

 intervals of 2 or 3 meters was adequate for the purposes of this report. 



Ordinarily the catch iras made up to hS cubic centimeters, and 

 since the volume of the trap was U5 liters, each cubic centimeter in 

 the bottle was equivalent to 1 liter of lake water. If the catch 

 happened to be very meager, it was concentrated to a smaller volume. 

 Very often it was desirable to combine two or more samples in one 

 bottle. In such cases 1 cubic centimeter would represent two or 

 more liters of lake water. Samples for counting were taken by means 

 of a 1 cubic centimeter piston (Stempel) pipette. The sample was 

 placed in a watch glass, transferred to a glass plate by means of a 

 medicine dropper, and counted under a binocular microscope. In 

 routine procedure two 1 cubic centimeter samples were counted separately, 

 and the number of organisms was computed from the mean. Whenever there 

 was pronounced lack of agreement in the counts of the two samples, one 

 or more additional samples were counted. 



There are several sources of error in the series of events 

 between taking the trap sample and recording the number of plankters and 



20lj 



