SECT. 1J THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA-WATER 29 



Table II 



Experimental Variation of the C : N : P Ratios (by Atoms) in Cultures of the 



Freshwater Alga, Chlorella pyrenoidosa (after data of 



Ketchum and Redfield, 1949) 



Conditions 



N 



Normal cells 



Phosphorus deficient cells 

 Nitrogen deficient cells 



phosphorus to carbon in the cells to about one-fifth and the deficiency of 

 nitrogen in the medium reduced the ratio of nitrogen to carbon to about one- 

 fourth of that of the normal cells. Relative to the phosphorus content the nitro- 

 gen content of the cells could be varied experimentally thirteen-fold. 



In natural waters the concentrations of available nitrogen and phosphorus 

 are greatly reduced during periods of active growth. One or other element 

 may be almost absent while an excess of the other may remain in the water. 

 In Long Island Sound, nitrogen appears to be the element available in minimum 

 proportions relative to the needs of the phytoplankton and its ratio to phos- 

 phorus varies from nearly zero to about 8: 1 (Riley and Conover, 1956). The 

 ratio of nitrogen to phosphorus in the plankton was found by Harris and Riley 

 (1956) to average 16.7: 1 with relatively small variations. Similarly, in the 

 coastal waters south of Long Island the ratio of nitrogen to phosphorus is 

 lower than that of the requirement of normal plankton. In spite of the ano- 

 malously low ratio of nitrogen to phosphorus in the water, the plants apparently 

 assimilate these elements in the normal ratio of 15 atoms to 1 until very low 

 concentrations are reached (see Fig. 1). However, when the nitrogen is nearly 

 depleted from the water the phytoplankton cells can apparently continue to 



N 5 



Fig. 1. Concentration of phosphate phosphorus and nitrate nitrogen in coastal waters 

 south of Long Island. Units: mg atoms/m 3 . Slope of envelopes, zJN/JP=15. (After 

 Ketchum et al., 1958.) 



