8 DISCOVERY REPORTS 



convergence and more particularly of the ice-edge vary considerably both with the season 

 and with the year ; but the arbitrary regions have been found to agree so well with the 

 types of phytoplankton communities encountered that there is little doubt that they 

 have a real biological significance. There is a very strong possibility that the southern 

 region comprises the area of the East Wind Drift of the Southern Ocean, unless the ice 

 is very far north. 



Fig. 5. Positions of stations where observations for phosphate or silicate were made around South Georgia. 



Dr Hart further informs me that in the northern region the phytoplankton does not 

 begin to increase very rapidly until the first week in November, when the gradient 

 becomes extremely sharp. By the middle of December the numbers are falling off and 

 the downward gradient of the concentration becomes as sharp as the upward gradient 

 of early November. At the beginning of April the numbers are comparable with the 

 concentration of early November before the large increase ; a slight increase was recorded 

 in May. In the southern region, on the other hand, the increase in phytoplankton con- 

 centration is never rapid and the maximum is far below that of the northern region. 

 Moreover, the increase starts in the first week of November and carries on until a 

 maximum is reached in the third week of February. A slow decline then sets in to the 

 third week of April, followed by a small autumnal increase in May, after which it declines 

 again. All the dates mentioned above are subject to an annual variation of ± 2 weeks. 



