SECT. 2] PRODUCTIVITY, DEFINITION AND MEASUREMENT 139 



the oxygen content of the water between morning and afternoon has been 

 used as a means of estimating the organic production of marine areas. The 

 method can be employed only in very productive areas. Even here many 

 difficulties may present themselves (cf. Gessner, 1959). 



I). An Experimental Method using Oxygen Production 

 In photosynthesis oxygen is produced proportionally with the assimilation 

 of carbon dioxide. If only carbohydrates were produced we would have the 

 equation : 



co 2 + h 2 o .- e ^U (H 2 co) + o 2 . 



Equivalent amounts of CO2 and O2 should thus be assimilated and released. 

 However, proteins and fats are produced in addition to the carbohydrates. 

 Usually nitrate is the source of nitrogen used. It becomes reduced before being 

 converted into organic matter. In accordance with the basic composition of 

 planktonic algae, like Scenedesmus and Chlorella, most authors use a CO2/O2 

 exchange factor of about 0.75, expressing photosynthesis measurements 

 obtained by the oxygen technique in equivalents of carbon (Steemann Nielsen, 

 1952; Ryther, 1956a). 



An experimental oxygen method of measuring organic production in the sea 

 was introduced by Gaarder and Gran (1927). It has been used in slightly 

 different modifications. In the most recommendable form, samples are collected 

 from the various depths and siphoned into bottles with glass stoppers. Some of 

 the bottles are used for determining the concentration of oxygen before the 

 start of the experiment using the Winkler technique. The other bottles are 

 again lowered to the depths from where the samples came and kept there for 

 24 h fixed to a line hanging down from an anchored buoy. As some of the 

 bottles are wrapped in black material, it is possible to determine both the photo- 

 synthesis and the respiration taking place in the water samples. In respiration 

 approximately equivalent amounts of carbon dioxide and oxygen are produced 

 and consumed. 



The oxygen content in the black bottles minus that in the initial bottle 

 represents the combined rates of respiration of all of the organisms present, i.e. 

 planktonic algae, animals, bacteria. The oxygen content in the clear bottles 

 minus that in the black bottles represents the rate of photosynthesis by the 

 plankton. 



In principle the oxygen technique is very simple. However, in practice many 

 difficulties may be encountered. It is easy to understand why so many workers 

 have abandoned this technique after a first unsuccessful attempt ; to be used 

 properly it requires a great deal of skill and experience, and the water samples 

 analyzed must be fairly productive. Steemann Nielsen (1958) has described the 

 various difficulties and pitfalls, and the measures which may be taken. One of 

 the main difficulties is the production of air bubbles in the bottles during the 

 experiment, making the later determination of the oxygen production or uptake 

 illusory. This difficulty is of no importance for the bottles kept at a depth of 



