40 REDFIELD, KETCHUM AND RICHARDS [CHAP. 2 



who observed the simultaneous exhaustion of nitrate and phosphate in the 

 waters of the English Channel during the growing season. Harvey's observa- 

 tions may be extended to the open ocean as shown in Fig. 2 and by other data 

 reviewed by Redfield (1934). 



In contrast, carbon is present in average sea-water in about ten times the 

 quantity which can be utilized if growth is limited by the phosphorus or 

 nitrogen available. This is illustrated in Fig. 3, which shows that the total 

 carbonate in ocean water is reduced by only 10% with the exhaustion of 

 the nitrate nitrogen. Clearly carbon does not become a limiting factor in the 

 growth of marine plants in the sea. In a similar way small residues of phosphate 

 or nitrate are frequently observed in surface waters from which the other 

 nutrient has become exhausted by phytoplankton growth. This may be ex- 

 plained by small variations in the ratios in which these elements are available 

 and are utilized by the population locally present. Thus, under the conditions 

 specified in Table V, a residue of phosphorus amounting to 0.15 mg atom/m 3 

 would remain in the water after the complete exhaustion of nitrogen. 



A number of elements known to be of biological importance, such as iron, 

 manganese, copper, zinc, cobalt and molybdenum, are available in sea-water 

 in very small concentrations and might act as limiting factors in the growth of 

 phytoplankton. By adding these elements to samples of surface water from 

 the Sargasso Sea, Menzel and Ryther (1961) have shown that the addition of 

 iron will stimulate the growth of the native plankton when it has come to an 

 end after the exhaustion of the nutrients naturally present. Additions of other 

 trace elements did not produce this effect. In similar experiments with water 

 from the English Channel, Harvey (1947) found that manganese might act 

 as a limiting factor in the growth of dnlamydomonas. Similar effects due to the 

 shortage of vitamins and other organic constituents are discussed in Chapters S 

 and 9. While it is clear that iron and manganese, and perhaps other 

 essential elements and compounds, are present in sea-water in critically small 

 quantities and may, under local conditions, become limiting, the fact that 

 nitrogen and phosphorus become exhausted so completely over wide areas of 

 the surface waters of the ocean is evidence that their concentrations are in 

 general the factors which control the quantity of phytoplankton produced. 



b. Potential fertility 



Harvey (1947a) has proposed that the total phosphorus or total combined 

 nitrogen in natural waters may be used to distinguish their potential fertility. 

 The potential fertility may be defined as the quantity of organic matter which 

 could be produced by photosynthesis from a unit volume of sea-water if it were 

 brought from depth to the surface and illuminated there until the limiting 

 nutrients were exhausted. Assuming the carbon of the organic matter to be 

 50% of its dry weight it would follow from the information in Table V that 

 the nitrogen would be exhausted when there had been produced organic matter 

 of dry weight equal to 5.48 g/m 3 and containing 2.74 g C/m 3 . Assuming a 



