26 DISCOVERY REPORTS 



vergence the warm deep current climbs steeply and its upper layers are found in the 

 southern region of the Antarctic zone at a depth of about 100-150 m. from the surface. 

 Vertical mixing must then be responsible for enriching the Antarctic surface layer in the 

 far south. In this manner phosphate which originally left the Antarctic in the surface 

 layer and in its plankton is returned to the far south to begin the phosphate cycle again. 

 Obviously the amount of phosphate returned to the south must vary both seasonally and 

 annually, and if one considers the phosphate curves for both the southern and northern 

 regions it follows from what has been said of the cycle that the southern curve must 

 show changes corresponding to those in the northern curve. Earlier in this section the 

 fall in the southern curve during August, September and October was noted as being at 

 variance with the lack of phytoplankton activity, but the fall during this period may 

 be an echo of the low phosphate content of the northern region during November, 

 December and January. The rise in the southern curve of November may be related 

 to the rise in the northern curve for February, and thus the northern region mini- 

 mum at the end of January is reflected in the intermediate minimum at the end of 

 October in the southern curve. 



During December the southern curve shows a fall in phosphate content of 7 mg., 

 whereas the northern curve for March shows an increase of 14 mg. ; these values do 

 not appear to agree with the previous suggestion that the southern curve will mirror 

 the northern one. However, in the far south phytoplankton production is increasing 

 steadily in December until a maximum is reached in the third week of February. This 

 increase in concentration is accompanied by a withdrawal of phosphate which causes 

 the average of the 0-100 m. layer to fall. The extent of this fall is probably masked by 

 the arrival at the southern region of warm deep water ; this contains the regenerated 

 phosphate from Antarctic surface water ; this when it left the Antarctic zone in March 

 or later on the northern part of the phosphate cycle, was continuously increasing in 

 phosphate content. 



The date of the minimum value in the southern region towards the end of January 

 appears to be early when the date of the peak production of phytoplankton is considered ; 

 this occurs in the third week of February, and it would have been expected that the 

 phosphate minimum would have occurred at this time or even later if a time lag existed. 

 If, however, our supposition that an invasion of the area by water of high phosphate 

 content complicates and masks the extent of the fall of phosphate owing to production 

 in situ, it is possible that the position of the true minimum is also altered ; phytoplankton 

 requirements would cause this minimum to occur in late February, but prior to this 

 additions are made of water of high phosphate content which change the actual position 

 of the minimum. The water with a high phosphate content seems to arrive at a time 

 when the southern region phytoplankton is coming to a maximum and the changes 

 in the northern region appear to be reflected in the southern region after 9 months, 

 or possibly 1 year and 9 months. 



To summarize the above section it may be said that the 0-100 m. average phosphate 

 contents of the surface water have been plotted as 2-monthly means for two arbitrarily 



