ii2 DISCOVERY REPORTS 



rate at which the phytoplankton is eaten (dependent on the number and kind of herbi- 

 vorous animals). Here we are concerned with the effect of the nutrient salt concentra- 

 tion. 



Data from various parts of the Antarctic zone in 1933-5 have been included in 

 Table VII so that the extent of the withdrawal of nutrient salts by phytoplankton may 

 be seen. These stations have been selected from a large number of observations as 

 examples of large withdrawals of nutrient salts. It must be remembered that particularly 

 in the arbitrarily defined northern region of the Antarctic zone the duration of the main 

 outburst of phytoplankton is fairly short, and consequently the amount of data obtained 

 at these times is bound to be relatively small. Table VII represents a fair number of 

 the appropriate data. From this table it is clear that the phosphate content in the 

 photosynthetic zone of the Antarctic surface layer is never 1 reduced to less than about 

 55 mg. P 2 5 /m. 3 The winter maximum of the English Channel may reach 40 mg., and 

 during the spring outburst of phytoplankton this is almost entirely withdrawn from the 

 surface. The Antarctic phytoplankton is of course very much richer than that of the 

 English Channel but even so, there still remains after the main outburst more phosphate 

 in the Antarctic surface water than is present before the spring outburst in the English 

 Channel. Thus it may be reasonably assumed that lack of phosphate is not likely to be 

 a limiting factor for the growth of phytoplankton in Antarctic water unless Antarctic 

 phytoplankton requires a higher minimum initial amount than does northern temperate 

 phytoplankton. 



On the other hand, the withdrawal of silicate from the upper layers of the Antarctic 

 surface water during the phytoplankton outburst is enormous. St. 1198 was situated 

 almost at the northern boundary of the Antarctic zone in the South Atlantic Ocean 

 and was taken in late November which is probably the time of maximum production 

 for this position. The actual phytoplankton density in the 0-50 m. layer expressed as 

 arbitrary colour units was 15,940 with which may be contrasted the spring maximum 

 of the English Channel which is of the order of 2970. As may be seen from Table VII 

 silicate was completely withdrawn from the 0-50 m. layer at St. 1 198. In the Antarctic 

 zone of the western part of the South Pacific Ocean the silicate content was less than 

 200 mg. in January 1934, whilst in the Scotia Sea and eastwards across the south Atlantic 

 Ocean in January and February 1935 the silicate was reduced to the order of 500-700 mg. 

 from a pre-outburst value of 3000-3500 mg. During the South Georgia survey in 

 November 1933 surface values of silicate content ranged from < 400 mg. to 1 000 mg. at a 

 time just after the maximum concentration of phytoplankton. Possibly had the observa- 

 tions been made a little earlier at South Georgia we might have found the silicate com- 



1 Since the above was written R.R.S. 'Discovery II' has been investigating the nutrient salt content of 

 the surface water north of Prince Olaf harbour at South Georgia at a time when the concentration of phyto- 

 plankton was very high. In these circumstances it was found that the surface content of phosphate and 

 silicate had been reduced to values of about 28-33 m g- ar *d 130-180 mg. respectively. These values are very 

 much lower than have hitherto been recorded near South Georgia especially for phosphate. The observations 

 were made in a localized patch where production had been extremely high and this may account for the 

 very low values of the nutrient salts. 



