MEASURING THE PRODUCTIVITY OF THE SEA 55 



oxygen content before and after suspension, it is possible by means of a 

 simple calculation to find out the production of organic matter per volume 

 unit at each depth. By adding together the production at the various depths 

 we get a measurement of 24 hours of production under a given surface. 



During the war fortnightly tests of this kind were made in the Isef jord, 

 in Denmark, over a period of two years. The annual production under 

 a surface of one square metre proved to be 600 grams, or about the equi- 

 valent of a season's production in a Danish cornfield. 



Why cannot the same method be employed in the ocean? Mainly 

 because production in the open ocean is spread over a much greater 

 depth. In the Isefjord the productive layer is between five and seven 

 metres thick; in the ocean it is about 100 metres thick. As the measure- 

 ment can only be made per volume unit the same effort would give only 

 between one-tenth and one-twentieth of the same result, even supposing 

 that productivity area for area were the same. But as production per 

 area is also considerably less in the ocean than it is in the Isefjord, its 

 measurement by amount of oxygen produced is quite impossible. The 

 method is altogether too insensitive. 



The elaboration of new methods based on chemical analyses and sensi- 

 tive enough for measuring the production of the ocean must be conside- 

 red impossible. 



A heaven-sent opportunity to devise a new method was provided by 

 the arrival in Denmark, just as the final plans of the expedition were 

 being laid, of the first consignment of radioactive carbon 14. By concen- 

 trated effort a new method had been worked out by the spring of 1950; 

 the equipment was obtained and a supply of C^* purchased from America. 

 It is expensive, a quarter of a gram of barium carbonate, of which only 

 about 0.5 per cent, is carbon 14, costing about £100. But it goes a long 

 way, as only a very small quantity is used at a time. Let us take a look 

 at this material. 



The carbon normally occurring in nature is carbon 12, or C*", the 

 figure indicating the number of particles in the nucleus of the atom. An 

 atom of C*" is made up of a nucleus round which rotate six negatively 

 charged particles, the electrons. There are always six electrons to a car- 

 bon atom, whether this is C^" or C^*, for the simple reason that there 

 are six positively charged particles, the protons, in the nucleus and in an 

 atom there must be equilibrium. It is the fact; that there are six protons in 

 the nucleus which makes it a carbon atom. Every element has its own 

 particular number of protons in the nucleus. 



In the nucleus there are also a number of particles, called neutrons, 



