154 NIELSEN [CHAP. 7 



By using the measurements of potential photosynthesis obtained with the 

 "tank" method, it is possible to calculate the production of matter below a 

 surface unit throughout a whole day. We must determine, first, the dependence 

 of photosynthesis on the light intensity for the plankton from the different 

 depths, and, secondly, the variation during the day of the light intensity at 

 the depths from which the samples originate. By measuring the variation of 

 light throughout the day by means of a deck-photometer and by making one 

 determination of the light penetration into the sea, it is possible to compute 

 exactly the relative light curves for all depths. 



In practice, however, such a procedure would be too time-consuming. 

 Formulae have therefore been developed for the calculation of primary produc- 

 tion below a surface unit by means of tank-experiments. In contrast to Stee- 

 mann Nielsen (1952) and Sorokin (1958) who tackled the problem empirically 

 by making simultaneous experiments in situ and in the tank, Ryther (1956) 

 and Rodhe et al. (1958) developed their formula on a more or less theoretical 

 basis. The latter authors showed that their formula agreed with the empirical 

 relationship used by Steemann Nielsen. Ryther (1959) has reconsidered the 

 problem. Where he originally used a standard curve describing photosynthesis 

 as a function of light intensity, he now takes the light adaptation into account. 

 It must be emphasized that all formulae to be used — both those empirically 

 found and those developed more or less theoretically — must take into con- 

 sideration the special conditions at the habitat of the plankton. 



Tailing (1957) has presented a widely applicable expression for the total or 

 integral photosynthesis of a population beneath a unit area of surface. The 

 integral photosynthesis was shown to be related to the logarithm of the surface 

 light intensity over a wide range of conditions. 



4. The Relation between the Rate of Gross Production and Net Production 



Below a Surface Unit 



When measuring primary production it must be emphasized that the fact 

 most important to general oceanography is the size of the net production 

 below a surface unit. The net production represents the potential source of 

 organic matter — or better energy — which can be transferred to the next 

 trophic level. 



At low latitudes and during the summer at higher latitudes it is usually 

 possible to calculate without much difficulty the approximate rates of net 

 production below a surface unit. 



By assuming the rate of respiration to be 8% of the light-saturated photo- 

 synthesis, Steemann Nielsen and Aabye Jensen (1957, p. 110) calculated the 

 respiratory rate per 24 h of all algae found in the photic layer in the tropical 

 ocean to be about 40% of the photosynthesis taking place daily. Ryther (1959) 

 has tackled the problem in a general way by making a somewhat simplified 

 assumption. Fig. 16 (after Ryther, 1959) presents relative photosynthesis, 



