Fractionation of Natural Phytoplankton Communities 



247 



toms was incorrect and that a significant fraction of the photo- 

 synthesizing biomass is contained in cells with one dimension 

 smaller than the net mesh size. Whichever explanation is cor- 

 rect, it can be stated with certainty that even tlie finest nets 

 should not be used for tlie collection of phytoplankton for studies 

 of primary production. 



The results obtained with the graded Millipore filter series 

 are presented in Figure 2. Again, a few points fall out of the 

 expected order; in six instances the retention by coarser filters 

 exceeded the retention by finer ones. These aberrant points 

 reflect the difficulty in obtaining representative aliquots of a sam- 

 ple containing an abundance of chain-forming organisms ( 6 ) . 



These data show that on the average, finer porosity filters 

 retain more activity than coarser ones even though the pores 

 of tlie finer filters are smaller in diameter than any described 

 photosynthetic organism (Figure 3). It is also apparent that 



140 

 130 

 120 

 I 10 



O 100% 



I- 



S 90 



I- 

 lu 

 0^ 80 



UJ 



I - 



_i 



UJ 60 



a: 



50 



40 



30 

 20 



FILTER OR NET PORE SIZE (/ji) 

 .2 3 .4 .5 6 7 8910 2 30 4 050 6 7 8910 



-1 1 — I I I M 



40 60 80 100 

 -i 1 — I — t r I I 



_i I I I I I I 



I III I I I I I 



AVERAGE RELATIVE RETENTION OF C'" LABELLED PHYTOPLANKTON COMMUNITIES 

 DURING THE SUMMER OF I960 



Fig. 3. Average relative retention of C^* labelled phytoplankton communi- 

 ties during the summer of 1960. 



