MEASURING THE PRODUCTIVITY OF THE SEA 



57 



Left, samples of water for production tests were collected in this specially designed sampler. 

 The water was in contact with glass only. The samples were placed on a rotating dial, right, 

 and illuminated. 



Briefly, the method adopted was as follows. Plankton algee cannot dist- 

 inguish between normal carbon dioxide and carbon dioxide which con- 

 tains radioactive carbon, C^*. Consequently, if a small quantity of C'^ 

 is added to the sea water the algae will assimilate it in the same propor- 

 tion. The amount can be determined by measuring the radioactivity in 

 the alga: with a Geiger-Miiller counter, the total amount of assimilated 

 carbon being then obtained by simple calculation. 



To measure the radioactivity the algae are filtered off by a collodion 

 filter with pores of about i /5,000th of a millimetre. 



The routine of measuring the production of organic matter below a 

 unit area is to take samples of water from the various depths, pour the 

 water into bottles, add radioactive carbon dioxide, and then lower the 



