SECT. 2] 



PRODUCTIVITY, DEFINITION AND MEASUREMENT 



i.r 



Therefore, we ordinarily find a reasonably high correlation between the poten- 

 tial rate of photosynthesis and the concentration of chlorophyll, if we restrict 

 ourselves to plankton samples from the same area, the same depth and the 

 same season (see Fig. 3). Unfortunately even then, due to difficulties in 

 measuring the concentration of chlorophyll, the results may at times turn out 

 to be poor. 



If we want to use chlorophyll as a general method for estimating primary 

 production it will be necessary to take into consideration the shape of the 

 different curves showing the rate of photosynthesis as a function of light (see 

 page 136). Krey (1958) has published a survey of the chemical methods for 

 the determination of the standing stock of phytoplankton. 



When comparing the standing stock of primary producers in the oceans, 

 the planktonic algae, with those in nearly all other biotypes, one striking 



3. Or 



2.0 



0.0 



Photosynthesis (75% of light saturated rate), mg C/m 3 /h 



Fig. 3. Rate of photosynthesis as a function of the concentration of chlorophyll a. Arctic 

 surface plankton, July. 



difference is seen. A stock of planktonic algae is often able to produce an 

 amount of organic matter of the same size as that originally found in the algae 

 in the course of a day. Even if we take our fastest growing agricultural crops 

 this is not possible. In maize during the month with maximum growth a daily 

 increase of organic matter is found which varies between 10 and 20% . The average 

 for the whole growth period is much less (Miiller, 1948). It is easy on infertile 

 land to find populations of perennial plants — for example lichens — for which it 

 takes years to produce as much organic matter as is found in the plants at a 

 given time. 



In oceanic phytoplankton the relation between organic matter present in 

 the stock and organic matter daily produced varies considerably. The light 

 conditions are of primary importance. During the winter at higher latitudes 

 very little, if any, organic matter is produced, thus causing a decrease in or 

 even the virtual disappearance of the phytoplankton. A stock of planktonic 



