284 DISCOVERY REPORTS 



ocean at all seasons. With these considerations in view, it is clear that the Haeckellian 

 terms cannot be applied rigidly. 



The term 'oceanic' has accordingly been used to describe all species whose time 

 distribution at great distances from land makes it improbable that they are necessarily 

 dependent on the ice in this way. Most are truly holoplanktonic though some have been 

 seen alive in pack-ice. 



Instead of 'neritic' one is constrained to use the expression ' neritic/ice-edge ', to 

 include with the forms that are not found far from land those almost certainly mero- 

 planktonic ones that seem able to use the ice-edge as a coast, and to flourish in the open 

 ocean for a short time after the dispersal of the pack. There are still a few important 

 species which future work may show to have been wrongly grouped here. Where any 

 doubt still exists full notes are given in the exposition of the scheme which follows. It 

 will be noted that as a general rule it is advisable to use the terms holoplanktonic and 

 meroplanktonic only with some such prefix as 'probably'. 



In the attempt to evolve a useful ecological scheme of subdivision, the concept of 

 'types of planktonic vegetation' as used by Gran and Braarud (1935, p. 332), but 

 applied in a rather more restricted way, has proved helpful. These authors define 

 ' types of planktonic vegetation ' as ' phytoplankton populations which have their main 

 occurrence quantitatively during the same season and whose dominant species all 

 belong to one group — diatoms, dinoflagellates . . ..' Since the Antarctic phytoplankton 

 is almost entirely diatomaceous, it is necessary to consider smaller taxonomic units than 

 those implied in Gran and Braarud's definition. Some genera and families lend 

 themselves to this, but some important species when classified on their time distri- 

 bution will only fall into taxonomically heterogeneous groups. Size distinctions are 

 helpful here, and have an obvious bearing on the food value of the phytoplankton for 

 diflterent zooplankton herbivores. In all it will be seen that a much higher degree of 

 arbitrary distinction than is necessary in northern waters has been found unavoidable. 

 Since our system is only intended to facilitate discussion of the qualitative data described 

 here, the point is of small moment, provided that its basis is clearly understood by the 

 reader. It is hoped, however, that the system will provide useful groundwork if future 

 work renders a more ' natural ' regrouping possible. 



Group I 



Fragilariopsis antarctica Small oceanic pennate diatoms with Distephanus. Numerically the most 

 Nitzschia seriata {} +N. important group at all seasons, except autumn. Most abundant at 

 delicatissima) peak of main increase in areas subject to neritic influence. Greatest 



Distephanus speculum relative importance before and just after the maximum. 



Group II 



Chaetoceros boreale 'Large diatom species' — the solenoids, large Chaetocerids, and two 



Ch. criophihim exceptionally elongated pennate forms. A very heterogeneous, essen- 



Rhizosolenia spp. tially oceanic, group with strong tendency to occur in local concen- 



Dactyliosolen antarctiais tration of from one to four of the categories mentioned. Abundance 



Corethron criophilum doubtless greatest at peak of main increase, and in neritic areas, but 



Synedra pelagica relative importance greatest during the post-maximal decrease, and 



Thalassiothrix antarctica more especially in autumn, in the Northern and Intermediate Regions. 



