522 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



the oxygen produced by photosynthesis as the difference between 

 the oxygen concentrations in a bottle exposed to the Hght and one 

 kept in the dark. Since this method is not very sensitive and since 

 it now appears that errors occur when sea water is kept in a bottle 

 for any length of time, it is confined to occasions where the pro- 

 ductive rate is fairly high, and significant results can be obtained 

 in experiment of, say, less than 12 hours. For this reason it is not 

 possible to use this technique to get values of gross production 

 in those areas which are of especial theoretical interest where 

 production is apparently low because of nutrient limitation. 



The other methods are less direct than the C'^ or the oxygen 

 technique. The rate of photosynthesis can be calculated from 

 chlorophyll, light and transparency data, assuming a general 

 relation between light and photosynthetic rate derived from 

 laboratory experiments (Ryther and Yentsch, 1957). This is 

 probably the simplest and best method of getting an estimate of 

 the gross production; the problem is that since the net production 

 is of most interest, a value for respiration is required, and this 

 raises the difficulties already mentioned. The net production can 

 be calculated from the rate of disappearance of a nutrient, usually 

 phosphate, from the water. This method, however, does not allow 

 for the rate at which phosphate is accumulating in the euphotic 

 zone owing to the breakdown of organic wastes, and this rate of 

 breakdown is still comparatively unknown and may be very 

 variable (Gushing, 1959a). 



In general, then, the problems of interpreting any one measure- 

 ment depend on the other factors being measured, and a main 

 result of this is to emphasize that in the sea no single sampling 

 method is sufficient to portray primary production. On the other 

 hand, the common properties of these techniques, the compara- 

 tively large number of obserx'ations that can be made, and the 

 use of carbon as a common unit enable easy comparison between 

 different areas and times. In this way it has been possible to go 

 to any part of the ocean and estimate a range of common para- 

 meters relevant to plant production. This ability to compare 

 directly areas with very different species composition has been 

 the greatest single advantage of these methods. Such descriptions 



