Fractionation of Natural Phytoplankton Communities 243 



Atomic Energy Commission, Contract No. AT (45-1) -1425. It 

 is a pleasure to acknowledge the cooperation of Dr. Robert L. 

 Fernald, Director, Friday Harbor Laboratories and the assistance 

 of Miss Rita Horner, Department of Oceanography. 



METHODS 



Water samples were collected with plastic buckets or modi- 

 fied plastic samplers (2, 15) and placed in clean 0.5 or 1 L 

 Pyrex glass stoppered bottles. A hypodermic syringe, containing 

 10-40 /'C of C^'*-labelled sodium carbonate solution (12) was 

 used for inoculation. After suitable agitation to insure complete 

 mixing, the sample was incubated in a water bath cooled with 

 circulating surface sea water, or in a plastic bucket in which 

 the water was frequently changed, under natural illumination 

 for two to six hours. 



At the end of the experimental period, the samples were 

 filtered in one of two ways. In the April-June series, the entire 

 sample was passed in succession through each piece of net and 

 filter, beginning with the coarsest and ending with the finest. 

 In the July- August series, the sample was shaken and an equal 

 volume ( 100 ml ) passed through each filter in the graded series 

 (Table 1). Filtration volumes were kept small to avoid progress- 

 ively increasing retention by the filters due to clogging. Dark 

 controls in which the C^^ concentration was quadrupled, were 

 run occasionally. The samples were filtered, using the second, or 

 aliquot filtration, procedure. After filtration, 10 ml of 0.001 N hy- 

 drochloric acid in 3 per cent sodium chloride solution was passed 

 through each filter to remove inorganic radioactive carbonate 

 solution retained on the filter. 



The radioactivity on the dried filters was measured with a 

 thin-window gas flow counter tube (Nuclear Chicago D-47) 

 mounted in a manual changer (Nuclear Chicago M-5) and an 

 associated scaler (Nuclear Chicago 161A). 



RESULTS AND DISCUSSION 



The results of 28 sets of observations obtained during the 

 spring and summer of 1960 are presented in Figures 1 and 2. In 

 both figures, the C^"* activity retained by the HA Millipore filter 



