58 



REDFIELD, KETCHUM AND RICHARDS 



[CHAP. 2 



It should be emphasized that the procedures reveal only the net result of 

 the balance between absorption and regeneration. In the photic zone sub- 

 stantial quantities of nutrients may be absorbed and returned to the water by 

 regeneration in situ. These increments do not appear in the balance. They may 

 be determined by other methods such as those discussed in Chapter 7 which 

 measure absorption directly. 



-10 -5 5 



flxlO 8 

 -10 -5 5 10 15 20 



-10 -5 5 10 



Fig. 10. Rate of change of phosphate phosphorus concentration attributed to biological 

 activity, R, in layers of water at different depths and seasons in Long Island Sound. 

 Negative values represent excess of absorption over regeneration. Units : depths, z, 

 meters; i?, 10 -2 mg atom/m 3 /day. (Estimated from data of Riley, 1956a.) 



C. Differential Advection 



Water moves with velocities which vary with depth, usually decreasing 

 downward. Under conditions of differential advection the redistribution of 

 nonconservative properties with respect to depth due to the sinking of organic 

 matter leads to changes in the total quantity of the element present along the 

 path of flow. Differential advection thus provides a mechanism by which 

 geographical differences in distribution of nonconservative elements may be 

 produced. 



A simplified model will serve to indicate the principles and relations of the 

 several factors involved in this process. Consider two layers of water of equal 

 depth, h, moving with different velocities along the #-axis, separated by a 

 surface across which a nonconservative element N is transferred downward 

 by sinking of organic matter and upward by eddy diffusion. At any position 

 along the axis of flow, the concentration of the element is also subject to change 

 by horizontal advection. If — R represents the rate at which the mean con- 

 centration of N is being reduced in the upper layer by the synthesis, sinking 

 and decomposition of organic matter, R will represent its rate of increase in 

 the lower layer. If A is the coefficient of vertical diffusion in the surface of 

 separation and dN/dz is the gradient of concentration at this boundary 

 {A/h) dN/dz will represent the effect of eddy diffusion on N in the upper layer 

 and - {A/h) dN/dz its effect in the lower layer. The rate at which N is altered by 



