ANTARCTIC INTERMEDIATE WATER 69 



which sinks mainly at the Antarctic convergence and in the area of mixing 100-200 miles 

 farther north. Maximum phosphate is found, however, in the region 38°-43° S, i.e. well to 

 the north of the Antarctic convergence where an enormous mortality of phytoplankton 

 occurs, as is evident from the position of the belt of diatom ooze. Phosphate must 

 be carried northwards in the current either free or held by the plankton. Organically 

 combined phosphorus may also be present in the sinking water. Phytoplankton in the 

 south consists mainly of diatoms which have a siliceous skeleton. If at the Antarctic 

 convergence and in the area of mixing 100-200 miles north of it there is a heavy 

 mortality of diatoms, the skeletons will tend to sink, and some of the descending debris 

 will pass through the south-going warm deep water and eventually arrive at the bottom 

 to form the diatom ooze and the high silicate concentration in the bottom water. The 

 phosphorus which was originally withdrawn by the phytoplankton will now be distributed 

 between the zooplankton and its excretions, and the dying phytoplankton. It is probable 

 that phosphorus is excreted as organically combined phosphorus and as such will be 

 unavailable for other plankton until after decomposition to phosphate, a process which 

 may take months for completion. During this time the Antarctic surface water is 

 travelling northwards and, north of the Antarctic convergence, becomes part of the 

 Antarctic intermediate current. 



The distribution of zooplankton individuals caught in closing nets in the longitudinal 

 section in 30 W showed that although the numbers decreased towards the north across the 

 Antarctic convergence the bulk of the zooplankton sank into deeper layers. That is, an 

 increase in numbers occurred in the layer between 500 and 750 m. at 46 42I' S and 

 also in 43 S in the layer between 750 and 1000 m. It does not necessarily follow 

 that this always happens, as the zooplankton has considerable powers of vertical migra- 

 tion, but it may be expected that any dying organisms would be carried into the 

 Antarctic intermediate layer. We know that phosphate is at a maximum in the region 

 38°-43° S in the mixed water, andsince oxygen is minimal and hydrogen-ion concentration 

 is greatest in this mixed water it can be argued that decomposition and regeneration 

 must be greatest in this region. It is probable that decomposition of plankton and 

 regeneration of phosphate is proceeding everywhere in the area between the Antarctic 

 convergence and 38°-43° S, but the process may be more intense in these latitudes 

 with the result that maximum phosphate is found in this position. 



In March- April 1933 in section 3 in the eastern part of the South Atlantic Ocean 

 between 46°47' S and 3°46' N (Sts. 1162-1184), the phosphate content at the salinity 

 minimum of the Antarctic intermediate current again progressed in a series of crests 

 and troughs from south to north. The section is shown in Plate V. The actual limits of 

 the phosphate content at this depth are between 76 mg. and 1 57 mg., but the latter figure 

 stands alone and the general variation is between 81 mg. and 128 mg. The high value of 

 157 mg. occurs in the subtropical zone at a depth of 970 m. at St. 1165 in 41 01' S, 

 9 34-3' E; this position corresponds in latitude to that of maximum phosphate content 

 in the western part of the same ocean. It seems reasonable to suggest that maximum 

 decomposition in this layer is occurring either at or immediately south of St. 1165. 



