288 Nutrients 



sion as insufficient for the production of the siHceous shells of diatoms. 

 The importance of manganese for the growth of phytoplankton is 

 discussed by Harvey (1949). In general, however, we know alto- 

 gether too little about the occurrence of trace elements in natural 

 waters in relation to the amounts needed for the healthy growth of 

 aquatic organisms. Since artificial sea water prepared from the 

 list of common constituents will not support life indefinitely, we re- 

 alize that other materials in minor quantities are essential for con- 

 tinued growth. 



In both the fresh-water and marine environments particular in- 

 terest has centered on the availability of phosphate and nitrate not 

 only because these nutrients commonly become critically scarce but 

 also because their depletion is often brought about by the growth 

 of the plant population itself. We have little precise information on 

 the rate of use of nutrients by phytoplankton under natural condi- 

 tions or their minimum requirements, but a certain amount of data is 

 available from laboratory experiments. In one carefully controlled 

 set of tests a population of Chlorella vulgaris of 70 million cells per 

 liter reduced the phosphorus content of its culture medium from 1.6 

 microgram-atoms ( /Ag-atoms ) per liter to 0.4 jug-atom per liter and in 

 another test from 0.8 ;u,g-atom per liter to zero in 24 hours. The 

 diatom Nitzschia frustrulum in a population of 40 million cells per 

 liter reduced its supply of phosphorus from 1.6 ju,g-atoms per liter to 

 zero in a similar length of time (Rice, 1949). The growth rate of 

 these fresh-water species was sharply reduced at the lower concentra- 

 tions of phosphorus. The multiplication of the marine diatom 

 Nitzschia closterium was found to be independent of phosphorus 

 concentration at values above 0.5 jttg-atom per liter, but below 0.3 

 and especially below 0.15 /^g-atom per liter division rate was dras- 

 tically curtailed. By summer the concentration of phosphorus in 

 many natural waters has been reduced to critically low values— for 

 example to less than 0.1 in the English Channel and to less than 0.25 

 yug-atom per liter in western Lake Erie. 



A classical illustration of the seasonal exhaustion of nutrients by 

 the marine phytoplankton is shown in Fig. 8.1 that represents the 

 seasonal changes in phosphate and nitrate in the English Channel. 

 The considerable supply of these nutrients present during the winter 

 is rapidly reduced each spring by the growth of the phytoplankton— 

 sometimes to indeterminably small quantities. The concentration of 

 nitrate and phosphate commonly remains very low during the sum- 

 mer when no effective stirring of the water takes place. Meanwhile 



