144 



[chap. 7 



the influence of agitation in tank experiments. More or less independent of the 

 degree or the method of agitation, they found an increase of about 30 to 50% 

 in productivity in each case. These workers used an experimental time of about 

 4 h. If the duration of the experiments is not too long, the effect of non-agitation 

 may be without importance in some cases. 



Fig. 5. Water-bath with rotating disk. 



F. Measurements of the Rate of Respiration by Means of the Carbon-14 



Technique 



There has been some disagreement concerning the explanation of what is 

 really measured by the carbon-14 technique (see, for example, Ryther, 1954). 

 The disagreement was settled, however, when it was found out that the duration 

 of the experimental time was of importance (see Steemann Nielsen, 1958). 



In experiments of short duration 50-70% of the CO2 produced by the 

 respiration taking place in the algae is photosynthesized again before being 

 released by the cells (Steemann Nielsen, 1955). Fig. 6 presents experiments 

 showing that only about one-third of previously assimilated 14 C02 is released 

 in the dark as compared to simultaneous experiments in the light. In a pro- 

 ductivity experiment of long duration — 24 h or more — during the greater part 



