Chapter 25 



Size Fractionation of C^*-Labeled Natural 

 Phytoplankton Communities* 



Robert W. Holmes and George C. Anderson 

 INTRODUCTION 



Xlankton nets of the finest available mesh are much too 

 coarse to pennit quantitative assessment of the abundance and 

 species composition of standing stock of marine phytoplankton. 

 This is concluded from comparisons of species abundance ob- 

 tained with phytoplankton net catches and with water samples 

 concentrated by centrifugation or sedimentation techniques (8, 

 11). Furthennore, estimates of species abundance obtained by 

 centrifugation of water samples have been found to be consider- 

 ably lower than estimates obtained with sedimentation techniques 

 (11, 14). Species recovery by centrifugation becomes as large as 

 by gravity settling when inorganic precipitates are deliberately 

 formed in the water during centrifugation (13). Serial dilution 

 techniques have been found to give even lower estimates of nan- 

 noplankton abundance in sea water samples than did centrifuge 

 methods (1). 



The results of the above investigators apply only to the 

 standing crop of phytoplankton. It might also be asked how, 

 under natural conditions, the rate of carbon dioxide assimilation 

 is distributed among various size categories of the standing crop? 

 Preliminaiy observations by Holmes et al. (5) in the eastern 

 tropical Pacific were not conclusive, but the data suggested that 

 organisms responsible for most of the primary production pass 

 through a fine nylon net (mesh size 30 x 30 /'), In lakes, Rodhe 

 et al. ( 10 ) and Rodhe ( 9 ) have convincingly demonstrated that 



* Contribution No. 246, from the Department of Oceanography, University of 

 Washington. 



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