242 Marine Microbiology 



a low but somewhat variable percentage of C^'^-labelled phyto- 

 plankton passes through the meshes of a No. 25 plankton net 

 (mesh size about 70 /x). More recently, Yentsch and Ryther (16) 

 working in Vineyard Sound, showed that the net portion (aper- 

 ture size: 65 /') of the ph\toplankton made up on the average 

 a small portion of the total population: 9 per cent of total cell 

 numbers, 8 per cent of the chlorophyll and 2 per cent of the 

 photosynthesis. 



Carbon dioxide uptake by phytoplankton in the water sam- 

 ple, after inoculation with C^^ and suitable incubation in the 

 light, is measured by collection of the plankton on a fine filter 

 and the radioactivity retained measured with a suitable counter. 

 Most investigators employ membrane or molecular filters with 

 pore sizes falling between 1 and about 0.4 /x. Goldberg et al. ( 3 ) 

 examined microscopically the size distribution of all preserved 

 organisms on an HA Millipore filter in two sea water samples 

 and concluded that the filter gave complete retention of all 

 plankton above a size of 0.5 /'.. Lasker and Holmes ( 7 ) , however, 

 pointed out that there may be some leakage of radioactive ma- 

 terial through filters in this pore-size range, even though the 

 experimental organism, Dunaliello prirnolecta, is considerably 

 larger than the filter pore size. A similar phenomenon was also 

 observed by Guillard and Wangersky (4). 



During the spring and summer of 1960, the authors at- 

 tempted to assess the approximate size distribution of photo- 

 synthetically active planktonic organisms in the waters around 

 San Juan Island, Washington, by inoculating sea water samples 

 with C^*, and after incubation, measuring the C^^ activity of the 

 organisms retained on a size-graded series of nets and membrane 

 filters. Data are reported from two locations: Friday Harbor, 

 48°32.7' N, 123°00.6' W, an area of vigorous tidal mixing; and 

 East Sound, 48°39.7' N, 122°53.9 W, a sheltered arm extending 

 into Orcas Island with little tidal mixing and pronounced stratifi- 

 cation during summer. 



Financial support was provided by the Friday Harbor Lab- 

 oratories, by which the senior author was employed as instructor 

 and investigator during the summer session of 1960, and by the 



