lOO 



DISCOVERY REPORTS 



this Station represented the transition between two strongly contrasted types of plankton. 

 To begin with, the total number of organisms estimated here was very much smaller 

 than that at the preceding station, and besides Rhizosolenia the small species Nitzschia 

 seriata and Fragilaria antarctica were of considerable importance. This station also 

 marked the most southerly point at which Chaetoceros criophibim was taken in large 

 numbers, and as will be seen from the comparatively large percentage of "other forms " 

 in the table, certain species which were comparatively unimportant in considering this 

 series of observations as a whole, occurred at this station in some abundance. The more 

 noteworthy of these were Synedra spathulota (7 per cent of the total), Rhizosolenia alata 

 (i-i per cent) and R. tnincata (i-i per cent). 



Proceeding southwards the transformation was complete, the minute species Nitzschia 

 seriata being strongly dominant at each of the next three stations. At the first two of 

 these the catches were relatively small, but at the southernmost, though the constitution 

 of the phytoplankton remained much the same, it was appreciably richer. The only 

 other species present in any abundance at these stations were Fragilaria antarctica and 

 Chaetoceros neglectiis. 



The last series of observations to be considered here consisted of phytoplankton hauls 

 from three more stations to the south of those described above, and four hauls from a 

 twenty-four-hour station worked at the most southerly position of all on the edge of the 

 pack-ice. The full analyses of this material will be found in Table XXXI ; in addition, 

 the relative proportions of the principal species are shown in Table 19. 



Table 19 



* Excluding uncountable colonies. 



It will be seen that the phytoplankton at the first of these stations leads on naturally 

 from that encountered at St. WS 548, Nitzschia seriata still being markedly dominant, 

 and the only noticeable differences being a slight increase in the proportion of Chaeto- 



