24 



DISCOVERY REPORTS 



The mean value between o-ioo m. has been selected because this layer with very few 

 exceptions consists of true Antarctic surface water unmixed with the upper layers of the 

 warm deep water. The mean values so obtained were averaged over two-monthly periods 

 and plotted against the various months of the year. Having regard to the considerable 

 differences in the actual phosphate content across such a large area as the Antarctic 

 zone which stretches from the ice-edge to the Antarctic convergence, together with the 

 differences in the concentrations of phytoplankton throughout the zone, it was decided 

 to draw two seasonal curves : one, the curve for the northern region constructed from the 

 results of stations within a distance of 300 miles south of the Antarctic convergence; the 

 second, a curve for the southern region made from values at stations within 200 miles 

 of the ice-edge. These curves are shown in Fig. 7 ; the numbers opposite the points on 

 the curves give the numbers of observations from which each two-monthly average is 

 calculated. No curve has been constructed for the area between the southern and 

 northern regions of the Antarctic zone because at present our observations in this area 

 are not sufficiently numerous. 





2..IA 120 



\4l HO 



o 



2 

 2 



|.T<t 100- 



90 



NORTHERN REGION 

 SOUTHERN REGION 



SEPT OCT NOV DEC JAN FEB MAR APR MAY 



Fig. 7. The average phosphate contents of the o-ioom. layer in the southern and northern 

 regions in the Antarctic zone of the Southern Ocean. 



In the curve for the southern region it will be noticed that the average phosphate 

 content of the 0-100 m. layer falls from August to the end of October, then rises slightly 

 during November, falls again to mid-January and then rises until the end of May. 

 We have no results for July, and consequently the June-July and July-August averages 

 cannot be calculated. The January-February average is very slightly lower than that 

 for December-January, so that the true position of the minimum is reached about the 

 end of January. 



