238 Light 



limited to the littoral zone. Photosynthesis, the first step in the 

 ecological cycle of the sea upon which all marine life ultimately 

 depends, is carried forward primarily by the minute diatoms and 

 other microscopic planktonic plants. The growth of the phyto- 

 plankton is limited not only by the rapid diminution of light with 

 depth but also by the fact that vertical movements of the water sub- 

 ject the plants to continually changing light conditions. 



The greatest depth at which phytoplankton can grow successfully 

 in any body of water may be determined by suspending bottles 

 containing samples of the plant population at various levels beneath 

 the surface. Photosynthesis going on within the bottle adds oxygen 

 to the water, and respiration taking place simultaneously removes 

 oxygen from the water. The rate of respiration alone may be meas- 

 ured by placing at each level a blackened bottle from which all light 

 is excluded. The sum of the gain in oxygen in the clear bottles and 

 the loss in the dark bottles gives a measure of total photosynthesis. 

 The increment of oxygen in the clear bottles is the amount produced 

 by photosynthesis minus the amount consumed in respiration and 

 represents the actual increase in energy content or growth of the plants. 



With the diminution of light in any environment photosynthesis 

 is reduced but respiration remains approximately the same, pro- 

 vided, of course, that temperature and other factors are essentially 

 unchanged. For each species a point is reached in the reduction 

 of the illumination at which the rate of photosynthesis is just equal 

 to the rate of respiration; this is known as the compensation point or 

 better as the compensation intensity. At light intensities below this 

 value photosynthesis may still go on but the plant is fighting a losing 

 battle: it cannot survive indefinitely under these conditions because 

 the loss of energy due to the catabolic processes represented by 

 respiration exceed the gain in energy brought about by the anabolic 

 process of photosynthesis. With further light reduction a minimum 

 intensity is reached below which no photosynthesis at all can take 

 place. 



In the aquatic environment the level in the water at which the 

 compensation intensity is found is called the co7npematio7i depth. 

 Sample values for phytoplankton based on measurements in the 

 middle of the day are presented in Table 16. For phytoplankton in 

 general the compensation intensity has been found to be about 1 per 

 cent of the value of full sunlight at the surface. In Fig. 6.28 the 

 change in the amount of photosynthesis at various depths during the 

 course of the day is plotted for a type of diatom representative of the 

 marine plankton. It will be observed that during the noon hours the 



