Fractionation of Natural Phyto plankton Communities 



245 



of the values falling below 50 per cent retention. This scatter 

 may reflect the changing species composition of the phyto- 

 plankton although during the months of June and July 

 qualitative estimations of species abundance made with the in- 

 verted microscope led us to believe that large chain-forming and 

 non-chain-forming diatom species were dominant. If this im- 

 pression is correct, the passage of labelled organic matter through 

 the 35 /I net material could arise largely from fragmentation of 

 intact cells due to filtration. On the other hand, it is possible that 

 less conspicuous, small organisms made up an important fraction 

 of the photosynthesizing biomass and that these escaped notice 

 during the qualitative examination of the Lugol's iodine-presei'ved 

 material. The two samples in which the C^* activity of the net 

 sample exceeded that of the 0.45 /^ filter argues against the frag- 

 mentation hypothesis. It seems more likely that our first im- 

 pression of the dominance of the conspicuous chain-forming dia- 



FRIDAY HARBOR 



6 * 



EAST SOUND 



Fig. 2. The relative retention of C^ '-labelled phytoplankton by filters. 



compared with the retention of a 0.45 fi filter (A = 5 rt 1.2 /x; D = 0.80 



± 0.05 m; O = 0.30 ± 0.02 //; -M- = 0.22 fi (pore size varialMlity not 



given by manufacturer) ). 



