Limitation by Nutrients in Nature 289 



the growth of the diatoms is curtailed by the very depletion the 

 diatoms have caused. Further growth cannot take place until the 

 autumnal breakdown of thermal stratification allows a new supply of 

 nutrients to be brought up from the deeper water layers by vertical 

 turbulence. Sometimes a second period of diatom growth occurs 

 during the autumn, temporarily reducing the concentration of nutri- 

 ents again. Eventually reduced light and the more effective stirring 

 caused by lower temperatures and stronger winds bring this autumnal 

 growth to an end, and allow the surface layers to regain the higher 

 concentration of nutrients characteristic of the winter period. 



^ 00 CL *-^ > O 

 3 3 0) U O O) 

 — > < CO O Z Q 



1925 1926 



Fig. 8.1. Seasonal changes in availability of nutrients for marine phytoplankton as 



represented by annual cycle of phosphate and nitrate in the EngUsh Channel. 



( Harvey, 1928, Copyright, Cambridge Univ. Press. ) 



In bodies of water that are suitable in regard to size and drainage, 

 such as many inland ponds, the seasonal depletion of nutrients may 

 be prevented artificially by adding fertilizer to the water and thus 

 removing this limiting factor for the growth of the plants. Organic 



