40 DISCOVERY REPORTS 



figures vary between 70 and 90 mg., the value depending entirely on the date of the 

 main outburst of phytoplankton. 



In late December ice-edge values of 95-100 mg. have been found, whilst just south 

 of the Antarctic convergence an average surface value of 75-80 mg. is usual. 



In January the ice-edge surface content is of the order of 85-90 mg. with northern 

 boundary values of 75-80 mg., although north of the Ross Sea in mid-January 1934 

 surface contents of 61 and 59 mg. were recorded at 67 33-8' S and 62°o8-i' S re- 

 spectively. These low values were separated by a content of 112 mg. at 65 05-3' S. 



For February an average ice-edge value of the order of 80-85 m §- ' s found, whilst 

 the northern boundary value approximates to 75 mg. 



In March ice-edge surface values are of the order 90-100 mg., but individual stations 

 give results which vary considerably. At the northern boundary of the Antarctic zone 

 the surface content is about 100 mg. 



The available April ice-edge values show that the position at the far south is com- 

 plicated either by very late phytoplankton outbursts or else secondary autumnal concen- 

 trations. At the end of April 1932 at the ice-edge north of Enderby Land the surface 

 value was 87 mg., whilst close to the South Orkneys in early April 1934 the value was 

 79 mg. On the other hand, the surface content steadily increased southwards in early 

 May 1934 towards the ice-edge north of Enderby Land, where a value of 130 mg. was 

 recorded. Northern boundary values are of the order of 100 mg. in April. 



In May at the few ice-edge stations the surface value is 130 mg. with northern 

 boundary figures of 1 15-120 mg. 



There are insufficient observations in June, July and August to offer any average 

 values, but it is of interest to note that on an average date of 30 August 1934, twelve 

 stations around South Georgia gave an average surface content of 120 mg. 



SEASONAL VARIATION OF SILICATE AT THE SURFACE 

 The following is a brief summary of our current knowledge of the silicate content in 

 different months. Silicate analyses were made a routine only during the third commission 

 of R.R.S. 'Discovery II', and therefore we cannot yet discuss the seasonal silicate 

 content all over the Southern Ocean. In certain areas such as the Pacific ice-edge, 8o° W, 

 Scotia Sea and South Georgia observations were made in several months. 



In September the data are confined to parts of the South Pacific Ocean and between 

 the Falkland Islands and Elephant Island. Those for the South Pacific agree with one 

 of the main conclusions drawn from the December-January and February-March ice- 

 edge cruises ; this conclusion was that the silicate content in the ice-edge region of the 

 south Pacific is greater in the west than in the east. Thus in the second week of September 

 1932 at the ice-edge north of the Ross Sea two surface observations were of the order 

 2000-2500 mg., whilst in late September 1932 a value of only 1000 mg. was found at 

 a position not far from the ice-edge in 98 W, a figure supported by another 

 observation in 8o° W of 1 150 mg. at the ice-edge in the second week of September 1934. 

 At the northern boundary of the zone in the South Pacific the available figures vary 



