SECT. 2 J 



PRODUCTIVITY, DEFINITION AND MEASUREMENT 



145 



of the experiment, the same ratio is found between 14 C and 12 both in the 

 experimental water and in the algae. Under such conditions the method 

 resembles in fact an ordinary chemical technique, where in the light we measure 

 photosynthesis minus respiration, i.e. net-uptake of carbon dioxide. In experi- 

 ments of short duration on the other hand, the carbon- 14 technique measures 

 something between net and gross production. To obtain values of either of these 

 two quantities, it is necessary to make a correction. 



A proper correction is possible only if the rate of respiration is known. A 

 method for measuring the rate of phytoplankton respiration exclusively by 

 means of the carbon- 14 technique was described by Steemann Nielsen and 

 Hansen (1959). A curve showing the rate of 14 C-uptake as a function of the light 



O ■!= 



dark 



ight 



Hours after exposure in light to CO2 



Fig. 6. The release of previously assimilated 14 CC>2 in the light and in the dark. Chlorella. 

 (After Steemann Nielsen.) 



intensity is obtained by making experiments at different light intensities. By 

 extrapolation the apparent rate of respiration is deduced. It is necessary, how- 

 ever, to correct for the interaction of photosynthesis and respiration (Fig. 7). 

 The real respiration rate is about 10/6 of that obtained directly. The curve for 

 the net production is found by drawing a line parallel to the original curve but 

 intersecting the ordinate at a point representing the true respiration rate. 



It would be too complicated always to determine the rate of respiration 

 experimentally. Fortunately this is by no means necessary. Experiments with 

 plankton collected in natural environments of all kinds have shown that the 

 rate of respiration compared with the rate of light-saturated photosynthesis is 

 always low — on the average about 8% of it (Steemann Nielsen and Hansen, 

 1959). It is thus justified to correct for the interaction of photosynthesis and 

 respiration in measurements of photosynthesis at light saturation by arbitrarily 

 6 — s. n 



