34 



DISCOVERY REPORTS 



greater than 2000 mg. in the autumn close to the Antarctic convergence are unusual as 

 far as our present data show. At no other station in a corresponding position were figures 

 of this magnitude obtained at any time, although no winter values are available. How- 

 ever, until more data are obtained of silicate content in and just north of the easterly 

 current of Weddell water, it is not possible to reject these values of over 2000 mg. 

 South of Australia in late May 1932 the surface content was only 460 mg. at the northern 

 boundary of the Antarctic zone. The value in this position when compared with that 

 of 2400 mg. in mid- April 1932 in a corresponding position north of Enderby Land is 



very small. 



In the South Pacific, north of the Ross Sea, two observations south of the Antarctic 

 convergence give summer and winter values. St. 1275, 59 29-2' S, 170 oi-8' W, in 

 late January 1934 had a surface value of less than 200 mg., whilst St. 950, 59 05-3' S, 

 163 46-5' W, in early September had a surface value of 800 mg. The very small amount 

 of silicate in the 0-60 m. layer at St. 1275 was corroborated by a similarly small value 

 found 3 farther south a day earlier, when in the 0-30 m. layer there was less than 200 mg. 

 present compared with over 2000 mg. in the bottom of the surface layer at 100 m. These 

 summer values north of the Ross Sea indicate that within the photosynthetic layer near 

 the Antarctic convergence the silicate content falls very considerably to a value which 

 may be limiting for phytoplankton requirements. 



Towards the eastern side of the South Pacific the surface values south of the con- 

 vergence varied in September between 800 and 1400 mg. In 8o° W five observations 

 were obtained as follows : 



The September, October and November values indicate that phytoplankton activities 

 had not begun to have much effect on the silicate content of the surface water by the 

 middle of November. We have no observations at the time of the peak concentration 

 of phytoplankton, but the silicate content at the surface must fall below 300 mg. in this 

 position. If we accept a value of about 1200 mg. as a pre-outburst value, the silicate 

 withdrawal at the peak concentration of phytoplankton must be of the order of 75 per 

 cent of the amount available. 



The silicate data at present available for the surface water at the northern part of 

 the Antarctic zone are relatively few, but they show that the percentage of silicate 

 withdrawn by the phytoplankton is large. In general it may be said that much less 

 silicate exists in the surface layer at the northern end of the zone than at the ice-edge. 

 It is estimated that in 1935 about 75 per cent of the available silicate was utilized 

 during the phytoplankton season near the Antarctic convergence north of the South 



