SECT. 1] THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA-WATER 33 



controlled by this exchange. In order to determine the quantity of oxygen 

 which has disappeared from a sample of water collected at depth, it is necessary 

 to estimate its oxygen content at the time it sank from the surface. The dif- 

 ference between the oxygen concentration of water in equilibrium with a 

 normal atmosphere at the temperature and salinity observed in situ and the 

 oxygen concentration as measured is referred to as the apparent oxygen utiliza- 

 tion, or A.O.U. The conditions specified are met only approximately in nature. 



Surface sea-water may be substantially supersaturated with oxygen under 

 conditions favorable for growth, or may be undersaturated under the turbulent 

 conditions obtaining in winter (Redfield, 1948). Some undersaturation is to be 

 expected in high latitudes at the time when deep water is formed. Seasonal 

 variations in atmospheric pressure may also influence the result (Carritt, 1954). 

 Finally, because the saturation values for oxygen are not linear functions of 

 temperature and salinity, the oxygen content of a mixture of sea-waters 

 equilibrated under different conditions would be different from that calculated 

 from the temperature and salinity of the mixture. These considerations intro- 

 duce errors into the estimation of the utilization of oxygen and its ratio to the 

 regeneration of nitrogen and phosphorus. Probably these errors are not greater 

 than the analytical errors in the determination of phosphate phosphorus or 

 nitrate nitrogen with which they are used, and are not large enough to account 

 for the discrepancy in the ratio as estimated from analyses of plankton and of 

 sea-water. 



A more significant source of error in the estimation of oxidative ratios from 

 sea-water analysis is the presence of nutrients in the water which have not been 

 derived from the oxidation of organic matter. The deep water of the oceans is 

 formed in high latitudes in winter where low light intensity restricts the growth 

 of plants. Consequently on sinking it may contain substantial quantities of 

 nutrients which are not in organic form. The presence of such preformed 

 nitrate in the deep water is indicated in Fig. 2B, in which samples from great 

 depth, indicated by solid circles, are seen to contain nitrate in much greater 

 quantity relative to the utilization of oxygen than obtains in the upper 1000 m 

 of water. Preformed nutrients are probably present in smaller quantities in 

 these upper layers. Their presence causes estimates of the oxidative ratio to be 

 too low and to be unreliable. Ratios obtained from the composition of plankton 

 are not subject to this error and consequently are to be preferred. 



The JO/JP ratio of — 276 will consequently be employed in the various 

 estimates to be discussed. The use of ratios derived from sea-water in certain 

 earlier studies (Redfield, 1942. 1948) led to estimates which require revision. 



b. Preformed nutrients and nutrients of oxidative origin 



The nutrients present in a sample of sea-water may be separated into two 

 fractions : ( 1 ) nutrients of oxidative origin which have been regenerated from 

 organic matter, and (2) preformed nutrients which were present as such in the 

 water at the time it sank from the surface. The quantity of a nutrient of oxida- 

 tive origin may be estimated from the apparent oxygen utilization by applying 



