SECT. 2] 



PRODUCTIVITY, DEFINITION AND MEASUREMENT 



151 



Results obtained during the S.C.O.R. intercalibration trials in Honolulu, 

 September, 1961 (MS), indicate that the kind of neutral niters used in simulated 

 in situ experiments is important. Neutral filters made of stainless steel screen 

 or nylon net are very likely to be abandoned. It seems that the ultraviolet 

 light penetrating such filters — in contrast to neutral filters of glass — may be 

 harmful to the algae from the lower part of the photic zone. Inhibition of 

 photosynthesis, besides being caused by excess light of all wavelengths, is also 

 brought about by ultraviolet light (see Gessner, 1955). Owing to the thin walls 

 of the experimental bottles the protection against ultraviolet light is not 

 complete (unpublished data). 



With the "tank" method 14 C is added to water samples from the various 

 depths which are then exposed to a well-defined light intensity in a water-bath 

 at the same temperature as that found in the sea. After three to four hours the 



E 20 



40 



5 10 



Gross production, mqC/m day 



Fig. 11. A simulated in situ experiment in the North Atlantic (52° OO'N, 32° 42'W), 

 16 August, 1955. (After Steemann Nielsen.) 



plankton is filtered off and the rate of photosynthesis determined in the usual 

 way. It is the rate of potential photosynthesis which is directly measured by 

 this modification. As mentioned on p. 143, it is essential that the bottles are 

 agitated during the experiments. From the measurements of potential photo- 

 synthesis it is possible to estimate the rate of photosynthesis going on below a 

 surface unit of the sea. In order to make this calculation it is necessary briefly 

 to consider the influence of light on the photosynthesis in the algae found at 

 the different depths. 



Ryther (1959) has computed that, under ideal conditions (below saturation 

 intensity and with total absorption of the light by the algae), 17.5% of the 

 visible light ( = about 9% of the total light) may be converted in the photo- 

 synthesis of algae. However, photosynthesis is proportional to light intensity 

 only at low light intensities. At higher light intensities the overall process is 

 limited by the rates of enzymatic processes. Hence a curve showing the rate of 



