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NIELSKN 



[chap. 7 



bright day. The plankton is presumed to be identical at all depths. A curve for 

 a day with a continuous light intensity of 50% of that on a bright day is 

 identical with the one which would be obtained were the curve as such moved 

 upwards 15 m. The rate of photosynthesis, measured per surface unit, would 

 now be about 80% of that measured on the bright day. By reducing the 

 average light intensity at the surface to one -third, the rate of photosynthesis 

 would become about 65% of that registered during a bright day. 



It is interesting to note that, according to Bauer (1957), who during the 

 summer in a Bavarian lake made daily measurements of the incident light and 

 of the rate of primary production below a surface unit, the rate of production 

 decreased to about 60% on days when the incident light was about one-third 



Gross production 



Fig. 20. Light intensity and rate of gross production at different depths. Tropical 

 ocean. (After Steemann Nielsen and Aabye Jensen, 1957.) 



of that on a completely bright day. This is nearly the same as the value obtained 

 by theoretical calculations. 



The relatively trivial effect of variations in the light intensity on the rate of 

 photosynthesis below a surface unit is due to the fact that light saturation in 

 planktonic algae is reached at a relatively low light intensity and that light 

 inhibition of photosynthesis is found at very high light intensities. At the very 

 surface in the summer the rate of photosynthesis per day is even higher when 

 the average light intensity is only half or one-third of that found during a 

 bright day (see Fig. 20). Days with the average light intensity reduced to one- 

 third are rare, as stated above. If we avoid making measurements during the 

 few extreme dark days, the variation in the incident light does not ordinarily 



