ANTARCTIC BOTTOM WATER 



i°5 



of silicate are removed from the Antarctic zone and are returned in the lower layers of 

 the warm deep water. 



The highest concentrations of silicate are found in the Antarctic zone, either at the 

 lowest observation, usually less than 500 m. from the bottom, or else at a higher level 

 in the bottom current. 



The silicate results from the bottom water of the Atlantic-Antarctic basin supply 

 some evidence for the existence of a cyclonic circulation in this current in this area. The 

 content of silicate in the bottom water appears to vary periodically, and this was specially 

 noticeable in two sections south-east of New Zealand and in the section in 8o° W. 



THE REGENERATION OF PHOSPHATE AND SILICATE 



As we have seen, the concentration of phytoplankton near the Antarctic convergence 

 suddenly increases enormously each spring and almost as quickly falls again under 

 the onslaught of the increasing concentration of zooplankton and also because of 

 possible limiting factors. The enormous increase of phytoplankton in spring is made at 

 the expense of the nutrient salts. Perhaps some idea of the very large concentrations of 

 both phytoplankton and nutrient salts in the Antarctic may be gathered from the 

 following figures for the average content of the 0-50 m. layer. For the sake of com- 

 parison, values from the English Channel have also been added. 



* Average at L. 4 in March 1933 and 1934. 



Even at the far south of the Antarctic zone the average value of the density of the 

 phytoplankton throughout the period from the beginning of the third week in December 

 to the end of the first week in March is nearly as great as that at the spring maximum in 

 the English Channel where the outburst lasts for approximately six weeks only. 



The huge amounts of phytoplankton found in the Antarctic are responsible for the 

 withdrawal from the photosynthetic zone of the surface layer of correspondingly large 

 amounts of nutrient salts, as the following data from St. 1529 in 64 547' S, 20°oo-6' E 

 conclusively show. 



The data in the above table are typical of a position where the concentration of 

 phytoplankton is high. In winter the 0-100 m. layer at such a station becomes com- 



