74 BEDFIELD, KETCHUM AND BICHARDS [CHAI\ 2 



which shows the distribution of phosphorus at a depth of 2000 m in the oceans 

 of the world. This depth lies below the intermediate layer in which nutrient 

 concentrations are maximal, and the concentrations represent approximately 

 the mean in the great mass of deep water. 



In the antarctic circumpolar sea which connects the several oceanic basins 

 the mean concentration of phosphorus is relatively uniform and varies between 

 2 and 2.5 mg atoms/m 3 . Proceeding northward in the Pacific and Indian Oceans, 

 it increases gradually to more than 3 mg atoms/m 3 . In the Atlantic it decreases 

 to less than 1.25 mg atoms/m 3 and to about 1 mg atom/m 3 in the Arctic basin. 

 Thus the Pacific basin on the whole contains phosphate at about twice the con- 

 centration of the Atlantic. If extreme variation is considered, the Arctic waters 

 of the Atlantic contain phosphate in the ratio of 1 : 3.5 when compared to the 

 maximum concentrations present in the North Pacific, and this ratio is in- 

 creased to 1 : 7 when deep Mediterranean water is compared with the richest 

 part of the Pacific or Indian Oceans. 



The differences in the nutrient content of the oceanic basins must depend on 

 the composition of the deep water masses at the time of their origin and their 

 subsequent modification by the balance of biological and physical factors in 

 the course of their drift. This we have seen to be the case for the Antarctic 

 Intermediate Water with its associated minimum oxygen layer, and for the 

 distribution of phosphorus fractions in the North Atlantic Deep Water. The 

 primary cause of the differences found in the deep water of the Pacific and 

 Indian Oceans, in contrast to the Atlantic, quite clearly relates to their respec- 

 tive sources. The deep water of the former originates in the antarctic circumpolar 

 sea (Southern Ocean) and flows northward with an initial concentration of 

 phosphorus of 2 to 2.5 mg atoms/m 3 . In contrast the North Atlantic Deep 

 Water is formed in the Arctic, where the concentration of phosphorus is 1- 

 1.25 mg atoms/m 3 . Thus, at their origins, the deep waters of the Pacific and 

 Indian Oceans contain twice as much phosphorus as the Atlantic. To these basic 

 concentrations relatively small additions are made by biological effects. 



Superficially, these oceans may be likened to estuaries opening in common 

 on the antarctic circumpolar sea. In the Pacific and Indian Oceans little or no 

 deep water is formed at the northern extremity. In consequence the circulation 

 is estuarine, the surface waters flowing southward to be replaced by an influx 

 of deep water drawn from the Southern Ocean. In the Atlantic the deep water 

 is formed by sinking from the surface in high northern latitudes and is replaced 

 by the flow of surface water northward. From the principles discussed in 

 connection with estuaries, it might be expected that the differential advection. 

 combined with the sinking and decomposition of organic matter at depth, 

 would lead to the accumulation of decomposition products northward in the 

 deep water of the estuarine Pacific and Indian Oceans and southward in that 

 of the anti-estuarine Atlantic. Fig. 18 shows that in the direction of flow in 

 the former oceans the increase in phosphorus concentration, which may be 

 attributed to biological factors, amounts to about 1 mg atom/m 3 while in the 

 Atlantic the increase is about half this amount. 



