134 NIELSEN [CHAP. 7 



respiration are identical). Ordinarily the respiration in the algae found below 

 the photic layer must be disregarded. In the oceans this is usually relatively un- 

 important, because the bulk of the algae are found in the photic layer. However, 

 in some very productive shallow localities we may find the main bulk of algae 

 below the photic layer (see page 131). In other areas the vertical daily mixing 

 of the water-masses extends below the lower limit of the photic zone. In the 

 latter case the respiration in the whole mixed layer must be taken into account 

 when calculating net primary production per square metre. 



In the case of experimental methods, such as the oxygen technique to be 

 described on page 139, it is not possible to measure the respiration of the auto- 

 trophic algae separately. The respiration of all organisms found in the experi- 

 mental bottles is included. In fact, we measure instead community respiration, 

 but unfortunately not the total community respiration. All bigger organisms 

 ordinarily escape during the sampling of the water for the experiments. 



Studies of community metabolism in the sea must be considered complicated. 

 It is rather significant that the only detailed studies of aquatic community 

 metabolism based entirely on observations have been made in two freshwater 

 springs (Odum, 1957; Teal, 1957). In such localities the turnover takes place 

 in one direction only, in contrast to ordinary marine and freshwater localities 

 where it is cyclic, making investigations much more complicated. For in- 

 vestigating the community metabolism in the springs, Odum's "up-stream- 

 down-stream" method can be used, whereby all water running through the 

 springs is used for a compound experiment. 



The only generally reliable technique by which it is possible at present to 

 measure exclusively the respiration of the photoautotrophic planktonic algae 

 found in the sea is a specific, rather complicated modification of the carbon- 14 

 method (see page 145). 



3. Methods for Measuring Primary Production 



A . General Statements 



The rate of primary production may be measured either directly or in- 

 directly by estimating the standing stock of phytoplankton and using a con- 

 version factor. In the first case the production is either measured experimentally 

 by enclosing water samples in bottles or by utilizing differences in the water- 

 masses during a certain period by measuring some property at the start and at 

 the end of this period. All of the methods have their advantages and dis- 

 advantages. They have all contributed to our present knowledge of marine 

 production. However, the use of several of these methods is restricted. The size 

 of the organic production is of importance for the applicability of most of them. 

 The existence of seasonal variations is also of importance. Some of the methods 

 are applicable only if a typical sequence exists between a winter season without 

 production and a summer season with production. 



Finally, it must be mentioned that it is possible in broad outline to estimate 



