68 DISCOVERY REPORTS 



and troughs of the phosphate and silicate curves are due to seasonal variation in the com- 

 position of the constituent waters of the current. Thus by using phosphate or silicate 

 values we have other checks on the approximate values for the northward velocity of this 

 current which were obtained from salinity and oxygen results (Deacon, ibid.). With the 

 completion of the circumpolar cruise of R. R. S . ' Discovery II ' in 1 93 2 and from subsequent 

 longitudinal sections in 1933-7 there are available a number of sections from the pack- 

 ice northwards up the South Atlantic, Indian and Pacific Oceans. From these sections 

 it may be possible to estimate the approximate speed not only of the northerly com- 

 ponent of velocity of the intermediate current but also the southerly component of the 

 warm deep current. 



Until the upper and lower boundaries of the Antarctic intermediate current have been 

 determined more accurately no accurate summary can be made of the nutrient salt 

 content throughout the thickness of the layer. Since, however, the intermediate current 

 is characterized by a salinity minimum, a description of the phosphate content at this 

 depth will give some idea of the distribution of phosphate within the current. 



When the boundary between the layer above the intermediate current and the inter- 

 mediate water itself is crossed the phosphate content increases enormously, the places 

 of origin and the constitution of the two waters being widely different. 



Plate IV shows the phosphate content in the western part of the South Atlantic Ocean 

 in 30 W (Sts. 663-699). Maximum phosphate occurs in the area 38°-43° S which has 

 been described by Deacon (1933, p. 234) as the position where the greatest vertical 

 mixing occurs between the north-going Antarctic intermediate current and the south- 

 going warm deep water; phosphate is being added to the warm deep water in this 

 region. North of the Rio Grande ridge in about 35 S the maximum phosphate 

 content of the whole water column is not found in the warm deep water, but is 

 situated close to the depth of the salinity minimum of the Antarctic intermediate 

 water. Between 46 42^' S and 14 27 J' N the phosphate content of the intermediate 

 current at this position of minimum salinity varies in a series of crests and troughs 

 between 136 mg. and 98 mg., the average value being greater to the north of the 

 section. The fact that north of the Rio Grande ridge the maximum content of 

 phosphate is found in or close to the depth of the salinity minimum of the inter- 

 mediate current, and that the content in this position of the current increases slowly 

 northwards must be intimately connected with the transport of plankton (which probably 

 decomposes chiefly in this layer) by the Antarctic intermediate current. A study of the 

 numbers of organisms in the zooplankton population at the depth of the salinity minimum 

 of the intermediate water in this section will probably be of great interest in this connexion . 

 As we have seen, north of the Rio Grande ridge, phosphate is maximal at a depth close 

 to the salinity minimum of the intermediate water ; this maximum is sometimes greater 

 than the winter values of the constituent waters of the intermediate current, at 26 06Y S, 

 for example, it is 137 mg. at 1000 m. Hence it is reasonable to postulate that considerable 

 regeneration of phosphate is taking place in the intermediate current, which has an 

 initially high content, and this regeneration is due to the decomposition of plankton 



