LABORATORY METHODS 25 



LABORATORY METHODS AND THEIR ACCURACY 



It will be convenient to describe the methods used in the analysis of the different 

 plankton samples separately under the headings of the different nets. 



Analysis of N 50 V samples 



The N 50 V net was used specially for the capture of diatoms and other phyto- and 

 microplanktonic organisms. Since the N 70 V net captures the medium-sized organisms 

 more efficiently, and was used through the same range of depth as the N 50 V at each 

 station, the method of analysis of samples caught by the N 50 V was not designed to 

 take proper account of the few Copepods, young Euphausians and Chaetognatha which 

 might also be taken. The proper estimation of these is left to the analysis of the N 70 V 

 samples. 



First of all any large organisms such as Salps, Ctenophores, Euphausians, Amphipods 

 and the larger Copepods were picked out. Then the volume of the sample was measured 

 after it had been allowed to settle for at least 24 hours in a measuring jar. The volumes 

 ranged from 1-5 cc. to as much as 290 cc. An estimate of the volume is, however, of 

 very little value, since some organisms with long spines such as Chaetoceros and 

 Corethron tend to settle together more loosely than do forms with not such long spines. 

 When the sample contained no large organisms the volume could be measured by noting 

 the height of the column of plankton in the tube on a scale placed at the side, and then 

 replacing the plankton by an equal quantity of water the volume of which could be 

 subsequently measured. This method avoided the unsettling and resettling of the 

 sample. 



By the addition of water the sample or a particular fraction of it was now diluted up 

 to a definite volume — 50 cc, 100 cc. or 150 cc, according to its bulk — placed in a 

 spherical flask and sampled by a 0-5 cc. stempel pipette. The fraction so taken for 

 examination usually ranged from 1 / 1 00 to 1 /300 according to the density of the sample, but 

 with much larger samples a further dilution was necessary, in extreme instances bringing 

 the fraction examined to but 1/3000 of the total sample. The contents of the stempel 

 pipette were now discharged upon a special slide ruled into small squares by lines 

 2 mm. apart and provided at its edges with a raised glass rim (see Fig. 15). This is 



Fig. 15. Counting slide used in the analysis of phytoplankton samples. 



now moved under the microscope by means of the Murray long-range mechanical 

 stage, 1 so that one row of squares after another can be examined until the whole area is 



1 Made by Messrs Watson and Sons, Holborn. 



