76 



DISCOVERY REPORTS 



200 



ESTIMATED 

 PHYTOPLANKTQN TOTALS 



MILLIONS 



100- 



been included, as the phytoplankton was essentially similar to that found outside the 

 South Shetlands at this time. Owing to the ice conditions this was the only station that 

 could be worked in the Bransfield Strait on this occasion, 

 so that the results from it are most logically considered 

 here. Towards the southern end of this line a rich phyto- 

 plankton was encountered, hence the working of the 

 extra stations, Sts. 379, 380 and 381, at closer intervals. 

 At these stations diatom nets and surface-water samples 

 only were worked. The rich phytoplankton in the 

 southern half of Drake Passage at this time evidently 

 represented an autumnal increase in water of Bellings- 

 hausen Sea origin. This increase led to a more normal 

 disposition of the phytoplankton totals with regard to 

 the position of the Antarctic convergence, than that 

 seen on the previous lines (Figs. 33-5). The Antarctic 

 phytoplankton encountered here was so much richer 

 than that taken in the sub-Antarctic surface water to 

 the northern end of the line, that the latter does not 

 show up at all when the totals are plotted diagram- 

 matically beneath the temperature curve (Fig. 36). 

 This is so despite the fact that at these more northerly 

 stations a distinct remnant of a Rhizosolenia plankton 

 was present, such as was found in the fairly rich hauls Fig. 36. Diagram showing the surface 

 near the Falkland Islands a month earlier. The relative t^P^-'f "res and total Diatomales on 



a hne 01 stations worked across Drake 



proportions of the leading forms on the Drake Passage paggggg from south to north, April 



line will be seen from Table 14, in which their numbers, 1930. 



and their percentage of the total estimated catch at each station are given. 



From the temperatures quoted in Table XX it is evident that the convergence was 

 crossed between Sts. 384 and 385 : this is also clearly shown by the rise in salinity and 

 fall in the phosphate values. The distribution of the principal forms, which is shown 

 in detail in Table 14, may therefore be briefly summarized as follows: 



South of the convergence in the comparatively rich hauls which represented an 

 autumnal increase in old Antarctic surface water of Bellingshausen Sea origin, the 

 dominant forms were small Chaetocerids, Nitzschia seriata and Fragilaria otitarctica, 

 and there was a falling off in the bulk of the catches as the convergence was approached 

 (see also Fig. 36). At St. 384, immediately to the south of the convergence, Nitzschia 

 seriata was very strongly dominant. 



North of the convergence Chaetoceros atlaniiciis was still important, but various 

 species of the genus Rhizosolenia accounted for a large proportion of the comparatively 

 scanty phytoplankton. At these four stations, 385-8, Ceratiiim pentagonum was present 

 in small numbers, various individuals showing all stages of variation between forms 

 grandis and longisetutn. The positions of all the stations on the lines crossing the con- 



