146 



NIELSEN 



[chap. 7 



estimating the rate of respiration to 8% of the rate of light-saturated photo- 

 synthesis, as was done by Steemann Nielsen (1952). The rate of the true net 

 production is thus obtained by multiplying the experimentally found value by 

 a factor of 96. In order to obtain a value of true gross production we must- 

 multiply by a factor of 106. 



ft 1,000 2,000 3,000 4,000 

 Lux 



Fig. 7. Experiments for measuring the rate of respiration (see text). (After Steemann 

 Nielsen and Hansen, 1959.) 



G. The Influence of Dark Fixation and Isotope Discrimination 



It is generally known that, in animals as well as in plants, a dark exchange of 

 CO 2 takes place which has nothing to do with photosynthesis. In this way 

 labelled CO 2 may be introduced into the organic matter of both plants and 

 animals. In a culture of algae the dark fixation of 14 C02 is negligible compared 

 to the fixation due to photosynthesis at light saturation, as was first shown by 

 Brown, Fager and Gaffron (1949). 



In ordinary 14 C-experiments for measuring organic production in the sea 

 other organisms are present in addition to the autotrophic algae. Nevertheless, 

 in experiments lasting 4 h, the dark fixation in water from the photic layer is 

 usually only a few (1-3) per cent of the fixation at light saturation. In water 

 with a very low production, preferably from the lower boundary of the photic 

 layer, dark fixation may be relatively higher (Fig. 8). The same is true in 

 polluted water with a high stock of bacteria. Whenever possible, dark bottles 

 should be employed in addition to the ordinary clear bottles and the dark 

 fixation rates should be subtracted from the ordinary measurements. 



