SECT. 2] 



PRODUCTIVITY, DEFINITION AND MEASUREMENT 



143 



prefers the intermediate curve. The loss in counts is much less and the curve 

 approaches a straight line sufficiently. 



The difficulty in obtaining a reliable curve when using the windowless 

 counter is due to the difficulty in preparing very thin, even samples of barium 

 carbonate crystals. If the curve approaches a straight line, it is unnecessary to 

 prepare such very thin samples. By means of scintillation counting Jiits and 

 Scott (1961) have overcome the difficulty. 



Another detail in the carbon- 14 technique to be discussed here is the removal 

 of any carbon in inorganic form before measuring the activity of the niters. 

 After drying the niters, the present writer places them for 20 min in a closed 

 container above concentrated hydrochloric acid. Experiments have shown that 

 this procedure removes all inorganic carbon completely. Some workers instead 

 pass a volume of very dilute hydrochloric acid through the filters by suction. 



5 10 



mg BoC0 3 per cm 



Fig. 4. Self-absorption correction curves. A = end-window 3.0 nig/cm 2 , B = end-window 

 1.5 mg/cm 2 , C = windowless counter. 



When making experiments on photosynthesis with planktonic algae, it is 

 important that no settling of the algae takes place. This is important regardless 

 of the technique used for measuring photosynthesis. When making in situ 

 measurements, where the bottles are placed on a wire hanging down from an 

 anchored buoy, it would be very difficult to make any special arrangement for 

 stirring the experimental water. In marine areas this is usually unnecessary 

 anyway. The waves, the swell and the wind keep the buoy in constant motion 

 and consequently the bottles are never at rest. Some stirring will take place. 



In tank experiments on the other hand, stirring is advisable. The simplest 

 method is to place the bottles on a rotating wheel in a water bath (see Fig. 5). 

 The rotating speed is without importance. In addition to the stirring an even 

 illumination of all the bottles is obtained, also an important condition neglected 

 by some workers. Doty and Oguri (1957) have reported experiments showing 



