SECT. 2] PRODUCTIVITY, DEFINITION AND MEASUREMENT 1 33 



on a purely theoretical basis. However, the disagreement must now be con- 

 sidered to be finally settled. During the first International Oceanographic 

 Congress in 1959, Menzel and Ryther (1960) were able to show that in 

 the very neighbourhood of the Bermudas, where most of the biological Sargasso 

 Sea work has been done by the Woods Hole Oceanographic Institute, the 

 thermocline breaks down during winter causing vertical mixing down to a 

 considerable depth. Nutrient salts are thus carried up into the photic zone 

 giving rise to a considerable primary production during winter. However, at 

 the same time they showed that the Bermudas (32°N) are situated at a border- 

 line south of which this breaking down of the thermocline in winter does not 

 take place. In the Sargasso Sea proper a low production rate is found throughout 

 the whole year. 



2. Definitions 



When discussing oceanic production it is very important to make Clear 

 exactly what is meant by the different expressions used. A study of the litera- 

 ture reveals that different authors often interpret identical terms in very 

 different ways; e.g. the word "production" has even been used by some 

 authors as synonymous with "standing stock". Such severe confusions must of 

 course be avoided. But even minor uncertainties may cause trouble. 



If we consider the autotrophic plants exclusively it is relatively simple to 

 present exact definitions, cf. the report from the "Committee on Terms and 

 Equivalents", appointed during the Plankton Symposium at Bergen, 1957 

 (Anon, 1958, Rapp. Cons. Explor. Mer, 144). By standing stock is meant the 

 quantity of autotrophic plants at a given time. It is synonymous with standing 

 crop. The term standing stock is used also for the quantity of organisms at 

 other trophic levels, for example, for the herbivorous zooplankton organisms. 

 The standing stock may be measured as biomass ( = live weight), as dry plank- 

 ton ( = plankton dried to a constant weight), as dry organic matter ( = dry 

 plankton less ash), as displacement volume (= volume of fluid displaced by 

 plankton which has been drained of water), as calculated volume (by counting 

 and measuring the algae) or as plasma volume ( = calculated volume less 

 skeleton and vacuoles). The standing stock of phytoplankton may also be given 

 in terms of the concentration of chlorophyll. The relationship between chloro- 

 phyll and total organic matter is variable, however. Chlorophyll should be used 

 only with caution as a measure of the phytoplankton standing stock. 



Since the term production is not unequivocal we prefer instead : gross primary 

 production = rate of real photosynthesis, and net primary production = rate of 

 real photosynthesis less rate of respiration by the algae. All of these quantities 

 are given as carbon fixed (or released) per square metre or per cubic metre per 

 unit of time. The time unit is usually 24 h. When measuring net primary 

 production per square metre, we ordinarily account for only the respiration 

 taking place in the photic layer ( = the layer between the surface and the com- 

 pensation depth, where, for a period of 24 h, the rates of gross production and 



