30 REDFIELD, KETCHUM AND RICHARDS [('HAP. 2 



assimilate phosphorus, which is then present in excessive proportions in the 

 water. 



The ability of phytoplankton to continue to form cells of normal elementary 

 composition, when growing in the sub-surface layers of the sea from which one 

 essential nutrient is practically exhausted, may be explained by the considera- 

 tion that mixing processes supply nutrients from deeper layers. Eddy diffusion 

 may deliver nitrogen and phosphorus to the euphoric zone in a higher ratio than 

 actually occurs in this layer, as Ketchum, Yaccaro and Corwin (1958) have 

 pointed out. The decomposition of cells in situ will also regenerate nitrogen and 

 phosphorus in the higher ratio characteristic of their composition. Under 

 steady-state conditions the net growth of the population is frequently limited 

 by the rate of diffusion of nutrients into the zone of active growth where the 

 phytoplankton assimilates the elements as fast as they are supplied. The 

 nutrients present in the water represent merely the residue of elements supplied 

 which are not required to form cells of normal composition. 



Thus, while culture experiments readily show the effects of nutrient de- 

 ficiencies in the medium on the composition of the organisms, it is by no means 

 certain that such deficiencies are developed by natural populations. 



B. The Changes in the Nutrient Content of Sea-Water Produced by Biological 



Activity 



The principal sources of the major elementary components of marine 

 organisms are the carbonate, phosphate and nitrate ions of sea-water. The 

 synthetic process by which these nutrients are removed from the water is 

 limited to the sub-surface layers of the sea into which adequate light may 

 penetrate. The regenerative processes being independent of light may take 

 place at any depth. Owing to the sinking of organized matter under the force 

 of gravity and the vertical migrations of organisms, significant quantities of 

 organic matter are carried downward to decompose at depth. Consequently 

 the cycle does not run to completion in the euphotic zone, where synthesis 

 exceeds regeneration sufficiently to produce the organic matter which de- 

 composes at depth. 



The result is a distribution of the biologically active elements in sea-water 

 which is distinct from that of the major salts which results from purely physical 

 motions. The nutrient elements are not only withdrawn from the euphotic zone 

 in quantities sufficient to form the biomass locally present, but are also trans- 

 ported to depth where the water is enriched by the regenerative process. The 

 distribution of oxygen is affected in a converse way. The excess synthesis in 

 the sub -surface layer produces oxygen which causes this layer to be super- 

 saturated, while the regeneration at depth reduces the oxygen content of the 

 water in proportion. The transition level where synthesis balances regeneration 

 is marked by the compensation point where the sea-water is exactly saturated 

 with oxygen. 



If the differences in the concentration of nutrient elements which are found 

 at different depths in the ocean are due to the decomposition of organic matter 



