SECT. 2] 



PRODUCTIVITY, DEFINITION AND MEASUREMENT 



155 



respiration and percentage of respiratory loss below a surface unit as a function 

 of incident radiation. 



The percentage of the respiratory loss ranges from 100 at radiation values 

 of 100 cal/cm 2 per day or less to 28 on extremely bright, long days. 



Taking into consideration the light adaptation of both the photosynthetic 

 and respiratory rates, the curves are oversimplified. At a total radiation of 

 e.g. 200 cal/cm 2 per day all values may vary considerably. The radiation in 

 question approaches the maximum one, e.g. during autumn at higher latitudes. 

 However, in the tropics, 200 cal/cm 2 per day will only be found when the 

 weather is strongly overcast. The curves will also vary considerably depending 



100 



200 300 4 00 500 600 700 

 Total incident radiation, g cal/cm 2 /day 



Fig. 16. Relative photosynthesis respiration, and percentage of respiratory loss as a 

 function of incident radiation. (After Ryther, 1959.) 



on whether the phytoplankton is distributed evenly within the photic layer or 

 concentrated, for example, near the surface or near the lower boundary of the 

 photic layer. 



Calculating reliable values for the rate of the net production can sometimes 

 be a rather time-consuming job which requires a great deal of expert knowledge. 

 In general it must therefore be recommended that measurements of primary 

 production are presented in terms of gross production. 



5. Transparency of the Water versus Organic Production 



A very simple method of estimating oceanic production is to measure the 

 transparency of the sea or even more simply to estimate the colour of the sea. 



