30 



DISCOVERY REPORTS 



taken place and the concentration was very small. On the other side of the Pacific, 

 in 78 W, at St. 141 5 in September 1934, the silicate value at the ice-edge in 63 40' S 

 was 1 1 50 mg., a figure the order of which is corroborated by one of 1000 mg. for the 

 surface silicate at St. 974 in late September 1932 in 63 57' S, 101 16' W. At none of 

 the above stations was there any evidence of the main outburst of phytoplankton having 

 begun. It will be seen that the silicate content of the surface water diminishes from west 

 to east. This fact was amply confirmed by the ice-edge cruises of December-January 

 1933/4 and February-March 1934 across the Southern Ocean south of the Pacific. 

 During both these cruises the surface content of silicate at the ice-edge diminished from 

 west to east as shown by the following table : 



W longitude 

 Surface silicate 



W longitude 

 Surface silicate 



77 °S 

 195° 



79" 34 

 !75° 



77 5i 

 2100 



97 °4 



1 150 



Westzvard Cruise 



98 10' 114 46' 

 950 2250 



Eastward Cruise 

 129 16' 



2550 



HS 52 

 1400 



129" 27 

 2050 



I 7 I°26' 



2275 



135" 54 

 2050 



153 4i 

 2400 



I59 u i3' 

 2600 



It is not only the ice-edge stations which show that the silicate content of the surface 

 water increases from east to west in this area. The 0-100 m. averages have been 

 calculated for all stations within 250 miles of the ice-edge and are shown plotted 

 against longitude in the graph below in Fig. 8. It may be seen that apart from several 

 fluctuations the silicate content of this layer rises from east to west in both the 

 westward and eastward cruises. In the west the silicate content in mid-January is 

 greater than in late February, a fact which is directly connected with the time of the 

 phytoplankton maximum in this area. In mid- January the phytoplankton had not yet 

 attained its maximum concentration in the west, whereas in late February the observa- 

 tions were made during the time of the maximum. Also in the east near 8o° W the 

 time of the main phytoplankton outburst has been found to be earlier than in the far 

 west. At the end of the eastern cruise in the second week of March the phytoplankton 

 maximum was passed in 8o°-ioo° W, and regeneration of silicate had set in with the 

 result that the values in March were greater than those in December. The reason 

 why the surface layer is richer in silicate in the west than in the east in the far south 

 of this part of the Southern Ocean lies in a difference in level of the warm deep water 

 on the two sides of the ocean and also in the fact that the warm deep water in its 

 passage across the ocean from west to east loses silicate by mixing with other layers. 

 In the west the warm deep water climbs steeply towards the surface when near the 

 Antarctic convergence, and in the east more gradually ; the result is that in the Antarctic 

 zone the maximum temperature of the warm deep water lies at a depth of about 300 m. 

 in the west and about 600-800 m. in the east. Silicate returns to the Antarctic mainly 

 in the lower layers of the warm deep water, and thus if this layer is closer to the surface 

 in the west than in the east more silicate will be transferred to the surface layer by 

 boundary mixing in the west than in the east. Also the warm deep water as it passes 



