CHEMICAL RESULTS OF LAST CRUISE OF CARNEGIE 



Extremely low concentrations of phosphate can occur 

 in the photosynthetic zone or in the convection layer only 

 when there is a marked thermal stratification of the 

 water layers below this zone. Thus, in regions where a 

 layer of water of low phosphate content occurs, there is 

 always a strong density gradient beneath this zone. This 

 prevents any great amount of mixing between the con- 

 vection zone and the layers below and thus isolates the 

 phosphate of lower levels from the photosynthetic zone. 

 In regions where a strong thermocline does not develop, 

 there is never an extreme reduction of the nutrient salts 

 in the surface layers. 



In the layers of water below the photosynthetic zone 

 phosphate is liberated by the decomposition of organic 

 debris and excretory products of holozoic organisms. 

 In our present state of knowledge of the bacteriology of 

 the sea we are unable to state the nature of all the bac- 

 teriological processes taking place at various depths 

 but, judging from the vertical distribution of chemical 

 substances in the sea, we are forced to conclude that one 

 series of results of the biological processes occurring 

 in the layers beneath the photosynthetic zone is a liber- 

 ation of nutrient salts and carbon dioxide and a con- 

 sumption of oxygen. Thus, the subphotic layers are 

 characterized by high concentrations of phosphate rela- 

 tive to that of the surface layer. In the Pacific the deep- 

 er water usually contains more than 250 mg PO4 per 

 cubic meter. 



Assuming there is no horizontal water movement, 

 the quantity of inorganic phosphorus occurring at any 

 given level below the phytosynthetic zone depends on the 

 rate at which it is being produced at that level and on 

 the rate at which it is being transported back to the sur- 

 face layer where the utilization of phosphate is taking 

 place. Both these factors are influenced by the stability 

 of the water column. 



The rate at which decomposition occurs in any layer 

 depends, among other things, on the amount of organic 

 matter available. This, in turn, depends on the abun- 

 dance of the plankton and on the rate of fall of the organ- 

 ic debris. As the density of the water increases, the 

 rate of fall decrease^ and consequently it may be expect- 

 ed that decomposition is more rapid in the thermocline 

 or the stratosphere than in the convection layer. In sup- 

 port of this we find that below the convection layer or 

 photosynthetic zone there is a rapid increase in the 

 quantity of dissolved phosphate with depth. 



The rate at which phosphate is returned to the photo- 

 synthetic zone will also depend on the stability of the water 

 since dissolved substances can be transported to any 

 important extent only by the water. The process of dif- 

 fusion of the dissolved substances themselves is too 

 slow to be of any practical significance. There are var- 

 ious forms of vertical water movements. Convection 

 currents are found practically everywhere. They are 

 caused by the settling of small, heavy particles of water 

 that have been cooled by radiation or contact with cold 

 air or have attained a high salinity because of evapora- 

 tion at the surface, and by the corresponding rising of 

 lighter particles. This type of circulation is probably 

 not effective below the thermocline, but in high lati- 

 tudes in the winter the density may become uniform 

 throughout all depths so that convective movements may 

 reach from the surface to the bottom. 



Another type of vertical circulation is caused by the 

 turbulent character of the movement in horizontal cur- 

 rents. The eddies transport dissolved substances from 



one layer to another. In the surface layers the eddies in 

 the wind currents are especially effective. The depth to 

 which this effect extends varies with the force of the 

 wind, the latitude, and the density gradient, but only ex- 

 ceptionally reaches below 200 meters. In addition one 

 has to consider the mixing by waves which also takes 

 place near the surface. Vertical components of currents 

 are in some places transporting water from greater 

 depths toward the surface or vice versa. Along the west 

 coast of the Americas and off Africa there is an upwell- 

 ing of water from intermediate depths (300 to 500 me- 

 ters) to the surface. 



Any type of vertical circulation is effective in re- 

 newing the phosphate content of the photosynthetic zone 

 and reducing that of the lower layers so long as it brings 

 water to the surface layer from the subphotic levels. 



Thus, we have two opposing phenomena tending to 

 establish the vertical distribution of phosphate in the 

 water column; the liberation of phosphate from sinking 

 organic debris, and transportation of this phosphate to 

 higher levels by vertical circulation. The exact depth at 

 which the greatest amount of phosphate is liberated can- 

 not be stated with any certainty on the basis of present 

 knowledge. One would expect decomposition to be more 

 rapid in warmer water and where the greatest amount of 

 organic material is available, namely, in or immediately 

 below the photosynthetic zone, but at these levels the 

 circulation is stronger than at greater depths and this 

 would tend to prevent the accumulation of phosphate. 

 According to the information obtained by the Carnegie 

 and others, however, it would appear that most of the 

 organic material is decomposed at the lower boundary 

 of, or below, the thermocline. 



The Carnegie data show that the maximum concen- 

 trations of phosphate and carbon dioxide, as indicated by 

 the pH, and a minimum concentration of oxygen occur 

 at a depth varying from 250 to 1500 meters below the 

 surface, depending on the locality. From this level to 

 the bottom the concentrations of phosphate and carbon 

 dioxide decrease with depth, whereas the concentration 

 of oxygen increases. These facts contradict the state- 

 ment often found in the literature, namely, that most of 

 the organic material decomposes at the bottom. It is 

 probable that where the water is sufficiently deep prac- 

 tically all this material has been reduced to inorganic 

 substances long before reaching the bottom. 



As to whether more decomposition takes place above 

 the level at which the maximum phosphate content and 

 minimum oxygen content are found, it is difficult to say 

 because above this level convective circulation is more 

 effective and it is possible that the accumulation of large 

 quantities of nutrients or a marked reduction in the oxy- 

 gen content is prevented by interchange of water with 

 the surface layer, where nutrients are consumed by the 

 phytoplankton, and oxygen is dissolved from the atmos- 

 phere. 



In discussing the distribution of phosphate we have 

 designated as the phosphate transition zone the layer of 

 water below the convection layer, and have defined it as 

 the layer in which the increase in the phosphate content 

 is greater than 0.1 mg PO4 per cubic meter per meter 

 of depth. In this zone the concentration of phosphate in- 

 creases from values near those at the surface to as much 

 as 250 to 300 mg per cubic meter at its lower boundary. 

 Since the rate of increase usually becomes less than 0.1 

 mg PO4 per cubic meter per meter before the maxi- 

 mum quantity is reached, thelayer of maximum phosphate 



