Aquatic Plants 



239 



illumination close to the surface was excessive and depressed photo- 

 synthesis. The highest rate of oxygen production occurred at 5 m 

 in the middle of the day. No significant amount of photosynthesis 

 occurred at any depth before 0600 in the morning or after 2200 and 

 none below 35 m at 1400. 



Time- 



Fig. 6.28. Photosynthesis of a species of phytoplankton (Coscinodiscus) at the 



indicated depths during the course of the day off Stoke Point, England. (Jenkin, 



1937, Copyright, Council of Marine Biological Assoc, of United Kingdom.) 



If a plant is to grow, its photosynthesis during the day must build 

 up enough organic matter to more than make up for the material 

 lost by respiration not only during the day but also during the night. 

 In other words, the crucial value for the continued existence of the 

 plant is the compensation depth for the 24-hour period. This will 

 obviously occur at a shallower depth than the values reported for 

 experiments limited to the middle of the day. The compensation 

 depth for the complete day ranges between 20 and 30 m during the 

 summer in clear coastal water of the temperate oceans. In the winter 

 and in less transparent water the compensation depth occurs cor- 

 respondingly nearer the surface. No constructive growth is possible 

 for diatoms or other pelagic plants below these levels. When we 

 recall that the average depth of the ocean is more than 4000 m, it is 



