236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



sonal breakdown. This is the explanation of the comparative absence 

 of good fishing grounds in the tropical parts of the world. But there 

 are other factors that promote inversion in particular sea areas; a 

 contoured shape of seabed coupled with the seasonal breakdown of 

 the thermocline may create rising bottom currents which produce a 

 more thorough and persistent mixing of the lower and surface waters. 

 More than one complex theory of this hydrodynamic nature has been 

 put forward to explain the exceptional amount of inversion that occurs 

 at the Dogger Bank in the North Sea. But whatever the detailed ex- 

 planations may be, all the deep-sea fishing grounds are places where 

 an exceptional uprising of bottom water takes place. With it, of 

 course, a supply of nutrients, particularly phosphate, is brought into 

 the sunlit zone of plankton growth. 



It is clear that only a limited recovery of deep-sea phosphate can be 

 expected. The inversion or sea-mixing influences are occasional rather 

 than perpetual; and they are strongly exerted locally rather than 

 generally. But the removal of phosphate from the sea by precipita- 

 tion can take place steadily. In its simplest form the marine cycle 

 for phosphorus may be summarized as follows : but for the steady flow 

 of fresh phosphate from igneous rock, the losses by deep-sea phosphate 

 precipitation would have prevented any lasting expansion of sea life. 

 Even with this constant land source of phosphate additions, it is clear 

 from modern fishery research that a shortage of phosphate in the sur- 

 face waters is the principal limitation to sea life. 



The land cycle of phosphorus began much later. First came the 

 invasion of dry land by plant life. Initially the medium of growth 

 was the layer of fine mineral particles derived from rock weathering, 

 the simple plant nutrients being provided by the rock-extracting 

 power of carbonic acid. Later, with the decay of dead plants, organic 

 matter and humus entered the medium and topsoils of the kind we 

 know today began to develop. As plant life increased its land in- 

 vasion a larger amount of the phosphate dissolved from apatite was 

 taken by plants. It is possible, however, that plant growth and soil 

 formation accelerated the rate of apatite extraction ; for the accumula- 

 tion of organic matter on the land surface with its concomitant produc- 

 tion of carbon dioxide by decomposition is likely to have increased 

 the amount of carbonic acid attacking surface rock and rock-derived 

 subsoil. 



In any case, some of the phosphorus needed for further plant 

 growth was provided by the decomposition of previous plant material. 

 The invasion of animal life enlarged and complicated the economy of 

 the rock-»plant— »soil-»plant cycle. Only a portion of the phos- 

 phorus taken by animals from plants was speedily returned to the 

 soil. The reason for this is found in the twofold function of phos- 



