62 



DISCOVERY REPORTS 



mated that there were 55 Corethron cells per litre at the surface, and 14 per litre at a 

 depth of 5 m. Below that depth the samples did not contain sufficient organisms to 

 obtain a count. 



At St. 501 a much better 

 series was obtained. This 

 station was worked at the 

 outer end of the Vakop line 

 in eastern type Weddell 

 Sea water. Large forms 

 were dominant, and as will 

 be seen from the analysis 

 of the net haul (Table XV), 

 the sample was not nearly 

 so rich either in species or 

 individuals as many of the 

 others obtained at this time, 

 particularly those from 

 western Weddell Sea water. 

 The chief results of the 

 centrifuge determinations 

 are shown in Fig. 31. It 

 will be seen that the phyto- 

 plankton as a whole reached 

 its maximum at a depth of 

 from 5 to ID m. and there- 

 after diminished steadily as 

 the depth increased. From 

 the estimations of the three 

 leading forms it appears 

 that while Thalassiosira 

 antarctica reached its maxi- 

 mum at the surface, Chaeto- 

 ceros criophilum was most abundant at a depth of 5 m., which is perhaps significant in 

 view of the fact that members of this genus have been shown by Marshall and Orr 

 (1928, p. 337) to be peculiarly susceptible to systrophe. Rhizosolenia styliformis showed 

 two well-defined maxima at 10 m. and at 30 m. It is probable that the lower of these 

 two was caused by older frustules sinking, as this species was not so abundant on the 

 Larsen line, and the surface current here was flowing to the north-west. On the other 

 hand this double maximum perhaps represented a parallel to that described by Gran 

 (19290, p. 18) for the same species in the northern hemisphere — off the south-west 

 coast of Greenland in August. 



Table 12 contains the full results of the centrifuge determinations from St. 501, 



Fig. 31. Diagram showing the distribution of the total phytoplankton 

 organisms, and of the dominant species with depth, from centrifuge 

 determinations at St. 501. 



