SECT. 1] THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA-WATER 4l 



dry-weight/wet-weight ratio of 0.2, the wet weight of plankton would be 28 

 g in 1 million g of sea-water or about 1 part in 36,000. 



This estimate serves to indicate the maximum standing crop that could occur 

 on the average in the photic zone under ideally efficient conditions. Actually, 

 the water becomes impoverished while it is at the surface by the sinking of 

 organic matter so that for this and other reasons such crops are not to be ex- 

 pected. Ryther (1960) has estimated that the plankton of oceans as a whole 

 contains 3 g/m 2 carbon. Assuming this to be concentrated in the upper 100 m, 

 the wet weight of plankton in the water would be equivalent to about 1 part 

 in 3 million. These estimates serve to emphasize the low- concentrations at 

 which organisms may be expected to occur in sea-water and the inefficiency 

 with which the potential fertility of the oceans is realized. 



c. Available oxygen 



The concentration of free oxygen in the water at the sea surface is determined 

 by the solubility of the gas when in equilibrium with the atmosphere. The 

 solubility is influenced greatly by temperature and to a less degree by salinity. 

 In water which has sunk below the euphotic zone, the dissolved oxygen is 

 removed by the oxidation of organic matter. The oxygen content of the deep 

 water of the oceans is consequently reduced in proportion to the quantity of 

 nutrients which has been regenerated within it. 



The composition of average sea-water, shown in Table V, indicates that such 

 water when equilibrated with the atmosphere at a temperature of 2°C, which 

 approximates that of the deep water of the oceans, will contain 735 mg atoms/ 

 m 3 of oxygen. This is only 16% more than the quantity required to oxidize all 

 of the organic matter that could be formed from the limiting amounts of 

 phosphorus and nitrogen present. 



The concentrations of phosphorus found in the waters of the Pacific and 

 Indian Oceans are more than twice those of the North Atlantic. The effect of 

 this fact on the demand which the oxidation of organic matter may make on 

 the available oxygen content of the water is illustrated in Table VI. The upper 

 part of this table shows the excess oxygen remaining after the oxidation of all 

 the organic matter which could be formed from the limiting quantity of phos- 

 phorus present. It may be seen that, in the Atlantic, oxygen is available in 

 large excess and the water would be 53% saturated after oxidation had gone 

 to completion. In the North Pacific, in contrast, more oxygen would be re- 

 quired than is available and the deep water would become anoxic under the 

 assumed conditions. 



As a matter of fact, the oxygen of the deep oceanic water is not as completely 

 utilized as these estimates indicate. The reason is that only a part of the 

 nutrients available in the surface waters of high latitudes, where these water 

 masses are formed, is used to form organic matter at the time when the water 

 sinks. The residue of preformed nutrients does not contribute to the utilization 

 of oxygen. In the lower part of Table VI allowance has been made for the pre- 

 formed phosphorus present in estimating the equivalent oxygen utilization. It 



