2 8 DISCOVERY REPORTS 



by the phytoplankton owing to lack of data at the critical time. Thus at St. 1535 there 

 must have been a heavy phytoplankton concentration immediately previous to the time 

 when the station was made, i.e. the position of the station was in the "grazed down" 

 area which has been discussed on p. 17 of this report. At this station the surface value 

 of silicate was 2800 mg. at a time subsequent to a peak concentration of phytoplankton. 

 At the depth of the cold nucleus of the surface layer the value was 3000 mg. We know 

 from the character of the phytoplankton catches at this station that conditions for the re- 

 generation of silicate were ideal. The difference between the surface and cold nucleus 

 values of silicate content will only give a minimum amount of silicate withdrawn by a 

 previous phytoplankton maximum because most probably regeneration of silicate in situ 

 has been taking place. Examination of all the ice-edge data from stations made during 

 1933-5 has shown that the minimum net withdrawal of silicate by phytoplankton as 

 calculated from the difference between the surface and cold nucleus contents varies 

 from month to month. The minimum net withdrawal varied between about o and 

 1000 mg. of silicate in the months of September onwards to May, being greatest in the 

 summer months. 



North of the ice-edge, however, very much larger amounts of silicate are withdrawn 

 from the surface by phytoplankton requirements. At St. 1529 (64° 547' S, 2o u oo-6' E), 

 some 250 miles north of the pack-ice a value of only 750 mg. was found in the surface, 

 whilst at a depth of 100 m., in the bottom of the surface layer, about 3000 mg. were 

 recorded. This low surface value of silicate was accompanied by a very large concen- 

 tration of phytoplankton. Thus north of the ice-edge the silicate content at the surface 

 may be withdrawn to an extent of 75 per cent of the available silicate in exceptional 

 circumstances. It is interesting to note that at this station the surface phosphate was 

 also found to have been greatly reduced to an extent of 48 per cent of the available 

 phosphate. During this cruise of late February 1935 the average position of the ice-edge 

 between io° and 3d E was about 68° S ; at a latitude of 65 S in this sector the average 

 silicate content of the surface was 1550 mg. Thus, although exceptionally as much as 

 75 per cent of the available silicate may be withdrawn by phytoplankton activities in 

 65 S, a more usual value of the consumption is about 50 per cent. The fact that the 

 apparent net consumption at the actual ice-edge is lower than at 250 miles farther 

 north is interesting. According to Dr Hart it may be stated that the ice-edge does 

 not generally support such a large phytoplankton concentration as exists 250 miles 

 farther north. Consequently the silicate consumption at the ice-edge should be lower 

 than farther north. 



From Table III on p. 27 it might be inferred that the surface silicate content falls 

 from west to east in the high latitudes of the Southern Ocean from 30 W to 44 E. In 

 early May 1934 a line of stations was made from 58 43-3' S, 16 38-1' E towards the 

 ice-edge in 64 37-6' S, 44 16-3' E. This section cuts diagonally across the eastern side 

 of the area referred to above ; by inspection of the silicate content in the bottom of the 

 surface layer some slight evidence is obtained of a small fall in silicate content from 

 west to east. However, since our observations across this zone have not been made at 



