"5 



SUMMARY 



This report is based on the measurements of phosphate and silicate in the Southern 

 Ocean made by the Discovery Committee's scientific staff between 1929 and 1935; it 

 describes the cycles by which phosphate and silicate are removed from the Antarctic 

 by the northward-flowing surface and bottom currents and returned in the southward- 

 flowing deep current, and it gives an account of the concentrations and fluctuations of 

 phosphate and silicate in the surface, deep and bottom currents. 



Phosphate leaves the Antarctic as free phosphate or as organically combined phos- 

 phorus, and, in the surface current, combined in the plankton. After the surface 

 current sinks at the Antarctic convergence it becomes enriched by decomposition of 

 plankton, and in the South Atlantic Ocean, for example, the greatest concentrations of 

 phosphate are found between 38 S and 43 S in the west, and in 41 1° S in the east. 

 A large proportion of this phosphate is transferred to the upper part of the southward- 

 flowing deep current by vertical mixing ; before the deep current is enriched in this way 

 it is relatively poor in phosphate. The North Indian deep current is an exception, being 

 itself rich in phosphate. The lower part of the deep current also returns phosphate to 

 the South — taking it from the Antarctic bottom current. 



An examination of all the data available suggests that when the rich phytoplankton of 

 the Antarctic is consumed by the zooplankton, most of the silica is excreted while much 

 of the phosphorus is retained and not returned to the sea until the plankton decomposes. 

 As a result, the greatest concentrations of silica are found where the greatest concentra- 

 tions and mortality of diatoms occur— in the Antarctic zone, and the greatest concentra- 

 tions of phosphate where the decomposition of the zooplankton is greatest— between 

 the Antarctic intermediate current and the warm deep current in the sub-Antarctic 

 zone. The maximum phosphate content is generally about 160 mg. P 2 5 /m. 3 



Silicate is most abundant in the Antarctic bottom current. Although it is regenerated 

 to a large extent in the surface layer, there is a rain of debris including dead diatoms, 

 foraminifera, radiolaria, cast skins of Crustacea and zooplankton faeces, into the bottom 

 current. A silica content as much as 8600 mg. SiO a /m. 3 has been observed, but a 

 maximum of 6000-8000 mg. is more usual. A large proportion of the silica that is 

 carried northwards by the bottom current is transferred by vertical mixing to the lower 

 part of the deep current ; the smaller quantities carried northwards in the Antarctic 

 intermediate current enrich the upper part of the deep current. 



South of the Antarctic convergence the warm deep current has generally a higher 

 phosphate content than the surface and bottom currents. Observations repeated in the 

 same localities show that the content can alter appreciably in a short time, and it is 

 believed that such changes are related to those which occur in the water that sinks at the 

 Antarctic convergence : at the season when the sinking surface water is rich in decom- 

 posing plankton more phosphate will be transferred to the deep current, and a rich 

 patch will be produced in the deep current. 



15-2 



