348 



[chap. 17 



photosynthesis of phytoplankton beneath a square meter of sea surface may be 

 expressed as a function of total daily incident radiation (Ryther, 1956). By 

 means of this relationship and the average radiation data of Kimball (loc. cit.) 

 the effects of seasonal and latitudinal changes in radiation on potential plant 

 growth may be demonstrated (Fig. 1). This picture is, of course, unrealistic, 



20 



> 15- 



10- 



10 



20 



30 



60 



70 



80 



90 



40 50 

 North latitude 



Fig. 1. Relative photosynthesis as a function of mean radiation at different latitudes and 

 seasons. 



for the effects of radiation must be considered in relation to other factors. It 

 does illustrate the seasonal and geographical variability of the radiation factor 

 per se, which is obviously critical at high latitudes and relatively unimportant 

 in temperate and tropical seas. 



3. Transparency 



Of more immediate significance to organic production than the radiation 

 falling on the sea surface is the amount of light available to the plants them- 

 selves. This is, of course, a function of the incident radiation, but is also in- 

 fluenced by the transparency of the water, the concentration of phytoplankton 

 and the presence of other particulate or dissolved substances which absorb light. 



Plant growth may occur whenever photosynthesis exceeds respiration ; the 

 light intensity at which the two processes are equal is the compensation in- 

 tensity. This is somewhat variable between species and even between differently 

 adapted members of the same species, but is of the general order of 100 ft- 

 candles. 



The compensation depth in the sea is the depth at which the compensation 

 intensity occurs, and the water layer above this, where plant growth may 

 occur, is known as the euphotic zone. For a given compensation intensity, the 

 compensation depth is a function of the incident radiation and the transparency 



