6 4 



DISCOVERY REPORTS 



any consideration of the nutrient salt concentration in the Antarctic intermediate current 

 which has within it a large proportion of water that was originally in the surface layer 

 in the Antarctic zone. The longitudinal section of phosphate content in the western 

 part of the South Atlantic Ocean (Plate IV) shows that the Antarctic intermediate 

 current has always a large phosphate content, and that north of the Rio Grande ridge 

 this current is the seat of the greatest concentration of phosphate. If we consider the 

 Antarctic intermediate layer as a large body of water which flows with a northerly 

 component and mixes at its upper and lower boundaries with other layers of lower 

 phosphate content, a slow and steady falling off of the phosphate content towards the 

 north would be expected. In practice this uniform fall of phosphate content is not found 

 and, because the Antarctic intermediate layer has a characteristic minimal salinity, we can, 

 by noting the phosphate content of the layer at this level, trace northwards the changes 

 of phosphate content in the nucleus of the current. As an example of the manner in which 

 the phosphate content at the depth of minimum salinity of the intermediate current 

 varies towards the north, the observations in section 3 in the eastern part of the South 

 Atlantic Ocean will be utilized, and the data are given below in Table V. The reason 



Table V 



why the phosphate content at the depth of minimum salinity of this layer does not fall 

 smoothly towards the north must be sought in the seasonal variation in phosphate of this 

 water at its place of origin and in the surface layer in the Antarctic zone. At the time 

 when the main outburst of phytoplankton is at its maximum, a large amount of phosphate 

 is withdrawn from the photosynthetic zone of the Antarctic surface water which itself 

 is travelling towards the north. The phosphate which has been thus withdrawn is held by 

 the phytoplankton and by the zooplankton which feeds on the plant life. Some of these 

 organisms travel northwards and sink at the Antarctic convergence, and it might be argued 

 that the sum total of phosphate, either free in the water or held by the plankton, which 

 arrives at the Antarctic convergence is the same as was originally in the water before 

 the main outburst of phytoplankton activity. The zooplankton organisms that can move 

 out of the northward going layer by vertical migration, are, however, responsible for the 



