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NIELSEN 



| CHAP. 7 



is much too large to retain any considerable part of the planktonic algae 

 found in the sea. 



Much to the disadvantage of plankton science, it took 50 years before the 

 use of net methods was abandoned. However, at the Plankton Symposium at 

 Bergen in 1957 convened by the International Council for the Exploration of 

 the Sea (I.C.E.S.) a recommendation was unanimously adopted: '"Net methods 

 should not be employed in quantitative phytoplankton studies." Braarud 

 (1958) has published a survey of the counting methods for the determination 

 of the standing stock of phytoplankton. Sedimentation combined with the use 

 of an inverted microscope is now used by most workers. 



30,000 



Fig. 2. Rate of gross photosynthesis per unit of chlorophyll as a function of light intensity ; 

 a = surface plankton, b = plankton from the lower part of the photic zone, 1 = tropical 

 plankton, 2-4 = temperate summer plankton, 5 and 6 = arctic summer plankton, 7 = 

 temperate winter plankton. (After Steemann Nielsen and Hansen.) 



In recent years the concentration of the pigments active in photosynthesis — 

 primarily chlorophyll a — has been employed, in addition to their use as an 

 index of the standing stock of plants as a means of estimating the rate of 

 potential photosynthesis and thus the rate of primary production (Ryther 

 and Yentsch, 1957). However, the rate of light-saturated photosynthesis at the 

 same concentration of chlorophyll varies within wide limits. Fig. 2 presents 

 some typical curves showing the rate of photosynthesis as a function of the 

 light intensity. Fortunately plankton from similar habitats gives similar curves. 



