PHOSPHATE AND SILICATE AS LIMITING FACTORS 



113 



pletely removed from the upper part of the surface layer. In the Scotia Sea between 

 the South Orkney Islands and the Antarctic convergence our present evidence shows 

 that a minimum withdrawal of 50 per cent of the available silicate occurs during the 

 phytoplankton season at the southern end of the sea and approaching 80 per cent in 

 the north. 



The withdrawal of such a high proportion of available silicate must have a large effect 

 on the growth of phytoplankton, and silicate paucity is possibly a limiting factor at 

 one stage of the phytoplankton season. The more recent work of the last commission of 

 R.R.S. ' Discovery II ' has shown that the surface silicate may be reduced to 130-180 mg. 



Table VII 



The upper line at each station in the table represents the phosphate content in mg. P 2 5 /m. 3 at the 

 various depths, whilst the second line represents the corresponding silicate data in mg. Si0 2 /m. 3 



in an area near South Georgia where the concentration of phytoplankton was enormous. 

 If silicate then be possibly a limiting factor, the continued growth of phytoplankton is 

 checked at a time when the number of herbivorous zooplankton is increasing at a very 

 high rate. Harvey, Cooper, Lebour and Russell (1935, pp. 407-42) have shown that 



