n6 DISCOVERY REPORTS 



The phosphate and silicate concentrations at the level of minimum salinity in the 

 Antarctic intermediate current do not decrease regularly towards the North, and there 

 are regions of maximum phosphate and silicate which coincide with regions of maximum 

 salinity and minimum oxygen; there are also phosphate and silicate minima which 

 coincide with salinity minima and oxygen maxima. It is considered that these varia- 

 tions are related to the seasonal changes in the Antarctic surface layer and that they 

 may serve as a basis for an approximate estimation of the rate of the northward movement 

 of the intermediate current. 



To help in the analysis of the data from the Antarctic surface layer three areas, 

 distinguished by Dr T. J. Hart from the type of phytoplankton communities they 

 support, have been examined. A southern region extending 200 miles north of the 

 ice-edge, a northern region extending 300 miles south of the Antarctic convergence, 

 and an intermediate region are described. In winter the surface phosphate ranges from 

 about 125-130 mg. P 2 5 /m. 3 near the ice-edge to nomg. near the convergence; the 

 summer range is 80-85 m g- t0 75 m §- Smaller concentrations have been observed due 

 to dense local outbursts of phytoplankton, but it is thought that such conditions do 

 not last long and that the average content is soon restored. 



The winter silicate content of the Antarctic surface water at the ice-edge ranges from 

 3000-3500 mg. Si0 2 /m. 3 in the Atlantic Ocean to about 2000 mg. in the eastern part 

 of the Pacific Ocean ; the figures for the northern part of the zone are about half these 

 values. In summer the surface content in the Atlantic Ocean is reduced to 500-1000 mg. 

 near the ice-edge, and to a lower figure, sometimes to zero, in the northern part of the 

 zone. Several factors — date and extent of diatom outburst, currents and mixing — 

 make an accurate summary impossible. 



In the sub-Antarctic zone the winter surface phosphate content ranges from 100- 

 110 mg. P 2 O s /m. 3 in the south to about 70 mg. in the north, and in summer the corre- 

 sponding range is 60-70 mg. to 30-40 mg. The surface silicate content ranges from 

 about 700 mg. Si0 2 /m. 3 to 200 mg. in winter, and from 0-300 mg. to 0-200 mg. in 

 summer. 



In the southern part of the subtropical zone phosphate ranges from 30 mg. P 2 6 /m. 3 

 in winter to 5-10 mg. in summer, and silicate from 410 mg. SiO,/m. 3 to less than 250 mg. 



In the Antarctic zone lack of phosphate is not likely to be a limiting factor for the 

 growth of phytoplankton. Silicate may, on the other hand, be reduced to a very small 

 fraction of its original content, and in some stages of the phytoplankton production 

 a shortage of silicate must limit the growth. 



