28 REDFIELD, KETCHUM AND RICHARDS [CHAP. 2 



JO/JP, is equal to -276. This appears to be the most appropriate estimate 

 for general use. 



These ratios provide a stoichiometric basis for evaluating the general pro- 

 portions in which the major nutrients present in sea-water may be expected to 

 change as the result of biological activity. The observed changes in the composi- 

 tion of sea-water in the ocean support this generalization. Their use depends 

 on the assumption that the composition of the plankton is statistically constant. 

 There are, however, a number of considerations which indicate that substantial 

 departures may be found under special circumstances. Riley (1951 ; 1956a) has 

 discussed the effects of the varying composition of the plankton with respect 

 to lipoid content, skeletal material, etc. He considers that the oxidation of 

 organic matter may lead to JO/zJP ratios which vary between — 250 and — 300 

 and that more extreme variations may be noted. 



The species composition of the biomass is observably variable both in time 

 and place, and each species may be expected to have a composition which 

 differs somewhat from others (Vinogradov, 1953). The chemical composition of 

 a given species may be expected to vary also with the general nutritional 

 conditions under which it grows. For these reasons statistical uniformity in 

 composition is probably approached only in large masses of water, where 

 deviations of this sort are averaged out. 



Analysis of plankton from Long Island Sound by Harris and Riley (1956) 

 indicates that on the average phytoplankton contains nitrogen and phosphorus 

 in the ratio of 16: 1 atoms, in agreement with Fleming's value. The zoo- 

 plankton in contrast yields an average N/P ratio of 24: 1. The individual 

 collections made at different times of year differ from the average by about 

 25%. In the case of the zooplankton the ratios have a seasonal trend, the 

 higher values being obtained in winter and spring. 



a. Effect of nutrient deficiencies on composition 



It has been demonstrated repeatedly in culture experiments that the ele- 

 mentary composition of unicellular algae can be varied by changing the 

 composition of the medium in which they grow. If one element is markedly 

 deficient in the medium, relative to its need by the organism, cell growth and 

 cell division can proceed for a limited period of time. The cells produced under 

 these conditions contain less of the deficient element than do normal cells. 

 When an element is provided in excess in the medium, luxury consumption 

 can increase its content in the cells. It has been shown with radiophosphorus 

 that the excess is readily exchangeable with the medium (Goldberg et al., 1951 ; 

 Rice, 1953). 



The data in Table II illustrate the extremes in phosphorus : carbon : nitrogen 

 ratios which can be produced when deficiencies in phosphorus and nitrogen 

 develop in culture experiments. Perhaps because of the composition of the 

 culture medium, the normal cells contained more than twice as much phos- 

 phorus as is observed in phytoplankton populations growing in the marine 

 environment. Deficiency of phosphorus in the medium reduced the ratio of 



