138 NIELSEN [CHAP. 7 



algae must grow if the stock is not to be decimated due to sinking and grazing. 

 This is, for example, in contrast to a population of terrestrial lichens. This type 

 of population is able to survive during a long winter season without being much 

 reduced with regard to the content of organic matter. 



Our knowledge of the influence of the nutrient conditions — supply of P and 

 N — on the growth rate of the planktonic algae is still scant. Probably minor 

 variations in the growth rate have a considerable influence on the balance 

 found in the sea, for example, between phytoplankton and zooplankton. 

 According to the material from the Galathea Expedition, Steemann Nielsen and 

 Aabye Jensen (1957) arrived at a phytoplankton growth rate of about 25% per 

 day for the relatively unproductive tropical seas. Such a growth rate would 

 compensate for the losses due to grazing and sinking and thus condition a 

 constant, but relatively small, standing stock of planktonic algae. 



C. Consumption of CO2 or Nutrient Salts as a Means of Measuring Primary 



Production 



This method was first used by Atkins (1922) in the western English Channel. 

 He proceeded originally by determining the loss of carbon dioxide under 1 m 2 

 of surface from the end of winter to the height of summer. As the loss in carbon 

 dioxide equals the amount used in photosynthesis, a minimum value of the 

 organic production in six months could be calculated. It was not possible for 

 example to consider the exchange of CO 2 with the atmosphere or regeneration 

 of CO2 due to the respiration of all organisms, including also the planktonic 

 algae. Atkins (1923) later used the loss of phosphate under 1 m 2 of surface in 

 the same way and Cooper (1933) based his estimate — also for the English 

 Channel — on the change in the content of oxygen and nitrate. The agreement 

 between the different techniques was rather good. 



These methods have been used relatively little outside the English Channel. 

 Many difficulties may turn up. At best it is possible to obtain the order of 

 magnitude of the value of the minimum production in a given sea area. Even 

 for the English Channel a detailed investigation will turn out to be difficult 

 (Cooper, 1958). Recently Steele (1958) has used the method in the northern 

 part of the North Sea. The loss of phosphate in the photic layer during the 

 production season was measured and a correction for the regeneration of 

 phosphate was made by assuming the regeneration rate to be the same in the 

 photic layer and in the water layers below, where no uptake of phosphate takes 

 place. Simultaneous — but unfortunately rather few — measurements by means 

 of the carbon- 14 technique have shown a good agreement. 



The method of using the consumption of a nutrient as a means of measuring- 

 primary production cannot be used universally. Certain conditions are strictly 

 compulsory. First, definite seasons must exist. Winter conditions without any 

 primary production are necessary. Secondly, no real interchange must take 

 place with either the bottom or other water-masses. A definite thermocline thus 

 must be found. 



In Russia and in the U.S.A. (cf., e.g. Fedosov, 1958) the daily variation in 



