7 6 DISCOVERY REPORTS 



800 m. near New Zealand. The silicate at the nucleus of the current increased towards 

 the north from a value of 650 mg. to one of 2350 mg. near New Zealand, whereas in 

 the previous section the silicate content increased southwards in this current. At present 

 we know very little of the effect of seasonal and annual variations in nutrient salt content 

 in the deep currents, but unless the differing amounts of silicate present in the two 

 sections can be related to some such variation it is difficult to explain the observations 

 of 1932 and 1934. It will be remembered that the phosphate content at the depth of 

 minimum salinity of the intermediate current increased towards the south in September 

 1932, whilst in January 1934 the greatest value was found towards the north, near New 

 Zealand. The section must be repeated for a full understanding of the problem. 



In the sub-Antarctic region of the central part of the South Pacific Ocean the minimum 

 salinity was usually found at a depth of 800-1000 m., where a silicate content of 1000 

 1200 mg. was recorded in September 1932. 



In 8o° W our observations do not extend north of 55 S, but south of this latitude 

 the average silicate content at the salinity minimum in sub-Antarctic water was of the 

 order of 650-1100 mg. 



The amount of silicate data of the intermediate current at present available renders 

 the writing of a summary of this knowledge somewhat more difficult than for phosphate. 

 However, a very important fact emerges from a consideration of the vertical sections 

 of silicate content. Silicate leaves the Antarctic zone in two currents, the sinking 

 Antarctic surface water and the Antarctic bottom water. Both currents have northerly 

 components of movement, and the major portion of the silicate that leaves the Antarctic 

 zone does so in the bottom current. On account of the heavy mortality of diatoms in 

 the region of the Antarctic convergence and the fact that the silica skeletons of the 

 diatoms are excreted almost immediately after consumption of the plant organisms by 

 the zooplankton, the amount of silicate carried out of the Antarctic zone in the form 

 of plankton in the sinking surface water is relatively not as great as the amount of phosphate. 

 On the other hand, the Antarctic bottom water has a very large silicate content owing 

 to the solution of excreted and dead diatoms which sink into it within the Antarctic zone. 

 Also the silicate content of sub-Antarctic surface water, a component of the Antarctic 

 intermediate current, is small. These reasons show why the series of maxima and minima 

 obtained in the curve expressing the distribution from south to north of silicate content 

 at the depth of the salinity minimum of the intermediate current, is less marked than is 

 shown in the corresponding curve for phosphate content. The variation that does occur, 

 however, is correlated with that of the phosphate curve and is ascribed to the seasonal 

 variation of silicate in the constituent waters of the current. 



In the eastern part of the South Atlantic Ocean maximum silicate content at the depth 

 of minimum salinity occurs in the same latitude as does maximum phosphate. Apart 

 from a value of 2100 mg. at this position in 41 S, the silicate content at this depth in 

 the current varies between about 800 mg. and 1 100 mg. South of South Africa, where 

 the intermediate current lies at a greater depth in the Agulhas current region, a higher 

 content is found and up to 2750 mg. was recorded at the nucleus of the current. 



